EGS 2101 Set II.docx

7/29/2019 EGS 2101 Set II.docx

1/20

EGS 2101 GEOPHYSICAL ENVIRONMENT Dr. David Kuria

Page 1 GEGIS Department: Kimathi University College of Technology

EARTHS SHAPE, INTERNAL STRUCTURE AND COMPOSITIONEarly in the twentieth century it became evident from the study of seismic waves that theinterior of the Earth has a radially layered structure, like that of an onion. These layers canbe defined by either their chemical or their rheological properties. The Earth has an outersilicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous thanthe mantle, and a solid inner core. The boundaries between the layers are marked by abrupt

changes in seismic velocity or velocity gradient. Each layer is characterized by a specific setof physical properties determined by the composition, pressure and temperature in the layer.

Scientific understandingof Earth's internalstructure is based onobservations oftopography andbathymetry, observationsof rock in outcrop,samples brought to thesurface from greaterdepths by volcanic

activity, analysis of theseismic waves that passthrough the Earth,measurements of thegravity field of the Earth,and experiments withcrystalline solids atpressures and

temperaturescharacteristic of theEarth's deep interior.

The structure of Earthcan be defined in twoways: by mechanicalproperties such as

rheology, or chemically. Mechanically, it can be divided into lithosphere, asthenosphere,mesosphere, outer core, and the inner core. The interior of the earth is divided into 5important layers. Chemically, Earth can be divided into the crust, upper mantle, lowermantle, outer core, and inner core. The geologic component layers of Earth are at thefollowing depths below the surface:

Depth (km) Layer060 Lithosphere (locally varies between 5 and 200 km)035 Crust (locally varies between 5 and 70 km)3560 Uppermost part of mantle352,890 Mantle100200 Asthenosphere35660 Upper mantle6602,890 Lower mantle2,8905,150 Outer core5,1506,360 Inner core


2/20

Dr. David Kuria EGS 2101 GEOPHYSICAL ENVIRONMENT

GEGIS Department: Kimathi University College of Technology Page 2

The layering of Earth has been inferred indirectly using the time of travel of refracted andreflected seismic waves created by earthquakes. The core does not allow shear waves to passthrough it, while the speed of travel (seismic velocity) is different in other layers. Thechanges in seismic velocity between different layers causes refraction owing to Snell's law.Reflections are caused by a large increase in seismic velocity and are similar to light

reflecting from a mirror.

CoreThe average density of Earth is 5,515 kg/m3. Since the average density of surface material isonly around 3,000 kg/m3, we must conclude that denser materials exist within Earth's core.Further evidence for the high density core comes from the study of seismology.Seismic measurements show that the core is divided into two parts, a solid inner core with aradius of ~1,220 km and a liquid outer core extending beyond it to a radius of ~3,400 km. Thesolid inner core was discovered in 1936 by Inge Lehmann and is generally believed to becomposed primarily of iron and some nickel. In early stages of Earth's formation about 4.5billion (4.5109) years ago, melting would have caused denser substances to sink toward thecenter in a process called planetary differentiation, while less-dense materials would have

migrated to the crust. The core is thus believed to largely be composed of iron (80%), alongwith nickel and one or more light elements, whereas other dense elements, such as lead anduranium, either are too rare to be significant or tend to bind to lighter elements and thusremain in the crust (see felsic materials). Some have argued that the inner core may be in theform of a single iron crystal.The liquid outer core surrounds the inner core and is believed to be composed of iron mixedwith nickel and trace amounts of lighter elements. Recent speculation suggests that theinnermost part of the core is enriched in gold, platinum and other siderophile elements.The matter that comprises Earth is connected in fundamental ways to matter of certainchondrite meteorites, and to matter of outer portion of the Sun. There is good reason tobelieve that Earth is, in the main, like a chondrite meteorite. Beginning as early as 1940,scientists, including Francis Birch, built geophysics upon the premise that Earth is like

ordinary chondrites, the most common type of meteorite observed impacting Earth, whiletotally ignoring another, albeit less abundant type, called enstatite chondrites. The principaldifference between the two meteorite types is that enstatite chondrites formed undercircumstances of extremely limited available oxygen, leading to certain normally oxyphileelements existing either partially or wholly in the alloy portion that corresponds to the coreof Earth.Dynamo theory suggests that convection in the outer core, combined with the Coriolis Effect,gives rise to Earth's magnetic field. The solid inner core is too hot to hold a permanentmagnetic field but probably acts to stabilize the magnetic field generated by the liquid outercore. The average magnetic field strength in the Earth's outer core was measured to be 25Gauss, 50 times stronger than the magnetic field at the surface.Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the

rest of the planet. In August 2005 a team of geophysicists announced in the journal Sciencethat, according to their estimates, Earth's inner core rotates approximately 0.3 to 0.5 degreesper year relative to the rotation of the surface.The current scientific explanation for the Earth's temperature gradient is a combination ofheat left over from the planet's initial formation, decay of radioactive elements, and freezingof the inner core.


3/20



The Inner CoreThe inner core of the Earth, its innermosthottest part as detected by seismologicalstudies, is a primarily solid sphere about1,216 km in radius, or about 70% that of theMoon. It is believed to consist of an iron-

nickel alloy, and may have a temperaturesimilar to the Sun's surface, approximately5778 K (5505 C).The existence of an inner core distinct fromthe liquid outer core was discovered in 1936by seismologist Inge Lehmann usingobservations of earthquake-generated seismicwaves that partly reflect from its boundaryand can be detected by sensitiveseismographs on the Earth's surface.

The outer core was believed to be liquid due

to its inability to transmit elastic shearwaves; only compressional waves are observed to pass through it. The solidity of the innercore has been difficult to establish because the elastic shear waves that are expected to passthrough it are very weak and difficult to detect.Based on the abundance of chemical elements in the solar system, their physical properties,and other chemical constraints regarding the remainder of Earth's volume, the inner core isbelieved to be composed primarily of a nickel-iron alloy, with very small amounts of someother elements. Hence it is referred to as 'Nife': 'Ni' for nickel, and 'Fe' for ferrum or iron.The temperature of the inner core can be estimated using experimental and theoreticalconstraints on the melting temperature of impure iron at the pressure (about 330 GPa) of theinner core boundary, yielding estimates of 5,700 K (5,430 C; 9,800 F). The range of pressurein Earth's inner core is about 330 to 360 gigapascals (3,300,000 to 3,600,000 atm), and iron

can only be solid at such high temperatures because its melting temperature increasesdramatically at these high pressures.

J. A. Jacobs was the first to suggest that the inner core is freezing and growing out of theliquid outer core due to the gradual cooling of Earth's interior (about 100 degrees Celsius perbillion years). Prior to the inner core's formation, the entire core was molten, and the age ofthe inner core is thought to lie between 2-4 billion years. Little is known about how the innercore grows. Because it is slowly cooling, many scientists expected that the inner core wouldbe homogeneous. It was even suggested that Earth's inner core may be a single crystal of iron;however, this is at odds with the observed degree of disorder inside the inner core.Seismologists have revealed that the inner core is not completely uniform and contains large-scale structures that seismic waves pass more rapidly through than others. The surface of the

inner core exhibits rapid variations in properties at scales at least as small as 1 km. This ispuzzling, since lateral temperature variations along the inner core boundary are known to beextremely small (this conclusion is confidently constrained by magnetic field observations).Recent discoveries suggest that the solid inner core itself is composed of layers, separated bya transition zone about 250 to 400 km thick. If the inner core grows by small frozen sedimentsfalling onto its surface, then some liquid can also be trapped in the pore spaces and some ofthis residual fluid may still persist to some small degree in much of its interior.

Figure 1 Mapping the structure of the Earth with

seismic waves


4/20



Because the inner core is not rigidly connected to Earth's solid mantle, the possibility that itrotates slightly faster or slower than the rest of Earth has long been entertained. In the1990s, seismologists made various claims about detecting this kind of super-rotation byobserving changes in the characteristics of seismic waves passing through the inner core overseveral decades, using the aforementioned property that it transmits waves faster in somedirections. Estimates of this super-rotation are around one degree of extra rotation per year,

although others have concluded it is rotating more slowly than the rest of Earth by a similaramount.

Growth of the inner core is thought to play an important role in the generation of Earth'smagnetic field by dynamo action in the liquid outer core. This occurs mostly because itcannot dissolve the same amount of light elements as the outer core and therefore freezingat the inner core boundary produces a residual liquid that contains more light elements thanthe overlying liquid. This causes it to become buoyant and helps drive convection of the outercore. The existence of the inner core also changes the dynamic motions of liquid in the outercore as it grows and may help fix the magnetic field since it is expected to be a great dealmore resistant to flow than the outer core liquid (which is expected to be turbulent).

Speculation also continues that the inner core might have exhibited a variety of internaldeformation patterns. This may be necessary to explain why seismic waves pass more rapidlyin some directions than in others. Because thermal convection alone appears to beimprobable, any buoyant convection motions will have to be driven by variations incomposition or abundance of liquid in its interior. There is an East-West asymmetry in theinner core seismological data. There is a model which explains this by differences at thesurface of the inner core - melting in one hemisphere and crystallization in the other.

The Outer CoreThe outer core of the Earth is a liquid layer about 2,266 kilometers thick composed of ironand nickel which lies above the Earth's solid inner core and below its mantle. Its outerboundary lies 2,890 km beneath the Earth's surface. The transition between the inner core

and outer core is located approximately 5,150 km beneath the Earth's surface.

The temperature of the outer core ranges from 4400 C in the outer regions to 6100 C nearthe inner core. Because of its high temperature, modeling work has shown that the outer coreis a low viscosity fluid (about ten times the viscosity of liquid metals at the surface) thatconvects turbulently. Eddy currents in the nickel iron fluid of the outer core are believed toinfluence the Earth's magnetic field. The average magnetic field strength in the Earth's outercore was measured to be 25 Gauss, 50 times stronger than the magnetic field at the surface.The outer core is not under enough pressure to be solid, so it is liquid even though it has acomposition similar to that of the inner core. Sulfur and oxygen could also be present in theouter core.

Without the outer core, life on Earth would be very different. Convection of liquid metals inthe outer core creates the Earth's magnetic field. This magnetic field extends outward fromthe Earth for several thousand kilometers, and creates a protective bubble around the Earththat deflects the Sun's solar wind. Without this field, the solar wind would directly strike theEarth's atmosphere. This could potentially have slowly removed the Earth's atmosphere,rendering it nearly lifeless, as is hypothesized for Mars.


5/20



The low viscosity of the outer core is important in seismology because low-viscosity fluidscannot sustain shear stresses: their rapid deformation in response to shear stresses causes thestresses to go to zero. Therefore, s-waves attenuate completely in the outer core, and theonly s-waves that appear after a wave exits the outer core do so due to splitting of p-wavesinto an s-wave component.

The MantleThe mantle is divided into sections which are based upon results from seismology. Theselayers (and their depths) are the following: the upper mantle (starting at the Moho, or base of the crust around 7 to 35 km, downward

to 410 km), the transition zone (410660 km), the lower mantle (6602891 km), the anomalous D" layer with a variable thickness (on average ~200 km thick).[4][9][10][11]The top of the mantle is defined by a sudden increase in seismic velocity, which was firstnoted by Andrija Mohorovii in 1909; this boundary is now referred to as the "Mohoroviidiscontinuity" or "Moho". The uppermost mantle plus overlying crust are relatively rigid andform the lithosphere, an irregular layer with a maximum thickness of perhaps 200 km. Belowthe lithosphere the upper mantle becomes notably more plastic in its rheology. In someregions below the lithosphere, the seismic velocity is reduced; this so-called low-velocityzone (LVZ) extends down to a depth of several hundred km. Inge Lehmann discovered aseismic discontinuity at about 220 km depth; although this discontinuity has been found inother studies, it is not known whether the discontinuity is ubiquitous. The transition zone isan area of great complexity; it physically separates the upper and lower mantle. Very little isknown about the lower mantle apart from that it appears to be relatively seismicallyhomogeneous. The D" layer at the coremantle boundary separates the mantle from the core.

The mantle differs substantially from the crust in its mechanical characteristics and itschemical composition. The distinction between crust and mantle is based on chemistry, rocktypes, rheology and seismic characteristics. The crust is, in fact, a product of mantle melting.Partial melting of mantle material is believed to cause incompatible elements to separatefrom the mantle rock, with less dense material floating upward through pore spaces, cracks,or fissures, to cool and freeze at the surface. Typical mantle rocks have a higher magnesiumto iron ratio, and a smaller proportion of silicon and aluminium than the crust. This behavioris also predicted by experiments that partly melt rocks thought to be representative of Earth'smantle.

Mantle rocks shallower than about 410 km depth consist mostly of olivine, pyroxenes, spinel-structure minerals, and garnet; typical rock types are thought to be peridotite, dunite(olivine-rich peridotite), and eclogite. Between about 400 km and 650 km depth, olivine is notstable and is replaced by high pressure polymorphs with approximately the same composition:one polymorph is wadsleyite (also called beta-spinel type), and the other is ringwoodite (amineral with the gamma-spinel structure). Below about 650 km, all of the minerals of theupper mantle begin to become unstable. The most abundant minerals present have structures(but not compositions) like that of the mineral perovskite followed by the magnesium/ironoxide ferropericlase. The changes in mineralogy at about 400 and 650 km yield distinctivesignatures in seismic records of the Earth's interior, and like the moho, are readily detectedusing seismic waves. These changes in mineralogy may influence mantle convection, as theyresult in density changes and they may absorb or release latent heat as well as depress orelevate the depth of the polymorphic phase transitions for regions of different temperatures.


6/20



The changes in mineralogy with depth have been investigated by laboratory experiments thatduplicate high mantle pressures, such as those using the diamond anvil.

The mantle solid/plastic. This is because of the relative melting points of the different layers(nickel-iron core, silicate crust and mantle) and the increase in temperature and pressure asone moves deeper into the Earth. At the surface both nickel-iron alloys and silicates are

sufficiently cool to be solid. In the upper mantle, the silicates are generally solid (localisedregions with small amounts of melt exist); however, as the upper mantle is both hot andunder relatively little pressure, the rock in the upper mantle has a relatively low viscosity,i.e. it is relatively fluid. In contrast, the lower mantle is under tremendous pressure andtherefore has a higher viscosity than the upper mantle. The metallic nickel-iron outer core isliquid despite the enormous pressure as it has a melting point that is lower than the mantlesilicates. The inner core is solid due to the overwhelming pressure found at the center of theplanet.

In the mantle, temperatures range between 500 to 900 C at the upper boundary with thecrust; to over 4,000 C at the boundary with the core. Although the higher temperatures farexceed the melting points of the mantle rocks at the surface (about 1200 C for

representative peridotite), the mantle is almost exclusively solid. The enormous lithostaticpressure exerted on the mantle prevents melting, because the temperature at which meltingbegins (the solidus) increases with pressure.Due to the temperature difference between the Earth's surface and outer core, and theability of the crystalline rocks at high pressure and temperature to undergo slow, creeping,viscous-like deformation over millions of years, there is a convective material circulation inthe mantle. Hot material upwells, while cooler (and heavier) material sinks downward.Downward motion of material occurs at convergent plate boundaries called subduction zones.Locations on the surface that lie over plumes are predicted to have high elevation (due to thebuoyancy of the hotter, less-dense plume beneath) and to exhibit hot spot volcanism. Thevolcanism often attributed to deep mantle plumes is alternatively explained by passiveextension of the crust, permitting magma to leak to the surface (the "Plate" hypothesis).

The convection of the Earth's mantle is a chaotic process (in the sense of fluid dynamics),which is thought to be an integral part of the motion of plates. Plate motion should not beconfused with the older term continental drift which applies purely to the movement of thecrustal components of the continents. The movements of the lithosphere and the underlyingmantle are coupled since descending lithosphere is an essential component of convection inthe mantle. The observed continental drift is a complicated relationship between the forcescausing oceanic lithosphere to sink and the movements within Earth's mantle.

Although there is a tendency to larger viscosity at greater depth, this relation is far fromlinear, and shows layers with dramatically decreased viscosity, in particular in the uppermantle and at the boundary with the core. The mantle within about 200 km above the core-

mantle boundary appears to have distinctly different seismic properties than the mantle atslightly shallower depths; this unusual mantle region just above the core is called D, anomenclature introduced over 50 years ago by the geophysicist Keith Bullen. D may consist ofmaterial from subducted slabs that descended and came to rest at the core-mantle boundaryand/or from a new mineral polymorph discovered in perovskite called post-perovskite.Earthquakes at shallow depths are a result of stick-slip faulting, however, below about 50 kmthe hot, high pressure conditions ought to inhibit further seismicity. The mantle is alsoconsidered to be viscous, and so incapable of brittle faulting. However, in subduction zones,


7/20



earthquakes are observed down to 670 km. A number of mechanisms have been proposed toexplain this phenomenon, including dehydration, thermal runaway, and phase change.The geothermal gradient can be lowered where cool material from the surface sinksdownward, increasing the strength of the surrounding mantle, and allowing earthquakes tooccur down to a depth of 400 km and 670 km.

The pressure at the bottom of the mantle is ~136 GPa (1.4 million atm). There existsincreasing pressure as one travels deeper into the mantle, since the material beneath has tosupport the weight of all the material above it. The entire mantle, however, is still thought todeform like a fluid on long timescales, with permanent plastic deformation accommodated bythe movement of point, line, and/or planar defects through the solid crystals comprising themantle. Estimates for the viscosity of the upper mantle range between 1019 and 1024 Pas,depending on depth, temperature, composition, state of stress, and numerous other factors.Thus, the upper mantle can only flow very slowly. However, when large forces are applied tothe uppermost mantle it can become weaker, and this effect is thought to be important inallowing the formation of tectonic plate boundaries.

The Crust

In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, whichis chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury,Venus, Mars, Io, and other planetary bodies have been generated largely by igneousprocesses, and these crusts are richer in incompatible elements than their respectivemantles.The crust of the Earth is composed of a great variety of igneous, metamorphic, andsedimentary rocks. The crust is underlain by the mantle. The upper part of the mantle iscomposed mostly of peridotite, a rock denser than rocks common in the overlying crust. Theboundary between the crust and mantle is conventionally placed at the Mohoroviidiscontinuity, a boundary defined by a contrast in seismic velocity. Earth's crust occupies lessthan 1% of Earth's volume.

The oceanic crust of the sheet is different from its continental crust. The oceanic crust is 5km to 10 km thick and is composed primarily of basalt, diabase, and gabbro. The continentalcrust is typically from 30 km to 50 km thick, and is mostly composed of slightly less denserocks than those of the oceanic crust. Some of these less dense rocks, such as granite, arecommon in the continental crust but rare to absent in the oceanic crust. Both the continentaland oceanic crust "float" on the mantle. Because the continental crust is thicker, it extendsboth above and below the oceanic crust, much like a large iceberg floating next to smallerone. (The slightly lighter density of felsic continental rock compared to basaltic ocean rockalso contributes to the higher relative elevation of the top of the continental crust.) Becausethe top of the continental crust is above that of the oceanic, water runs off the continentsand collects above the oceanic crust. The continental crust and the oceanic crust aresometimes called sial and sima respectively. Due to the change in velocity of seismic waves it

is believed that on continents at a certain depth sial becomes close in its physical propertiesto sima and the dividing line is called The Conrad Discontinuity.

The temperature of the crust increases with depth, reaching values typically in the rangefrom about 200C to 400C at the boundary with the underlying mantle. The crust andunderlying relatively rigid uppermost mantle make up the lithosphere. Because of convectionin the underlying plastic (although non-molten) upper mantle and asthenosphere, thelithosphere is broken into tectonic plates that move. The temperature increases by as much


8/20



as 30C for every kilometer locally in the upper part of the crust, but the geothermalgradient is smaller in deeper crust.

The average age of the current Earth's continental crust has been estimated to be about 2.0billion years. Most crustal rocks formed before 2.5 billion years ago are located in cratons.Such old continental crust and the underlying mantle asthenosphere are less dense than

elsewhere in the earth and so are not readily destroyed by subduction. Formation of newcontinental crust is linked to periods of intense orogeny or mountain building; these periodscoincide with the formation of the supercontinents such as Rodinia, Pangaea and Gondwana.The crust forms in part by aggregation of island arcs including granite and metamorphic foldbelts, and it is preserved in part by depletion of the underlying mantle to form buoyantlithospheric mantle.

Lithospheric plates

Figure 2 The major and minor lithospheric plates. The arrows indicate relative velocities in mm yr-1

at active

plate margins of current plate motions

The radially layered model of the Earths interior assumes spherical symmetry. This is notvalid for the crust and upper mantle. These outer layers of the Earth show important lateralvariations. The crust and uppermost mantle down to a depth of about 70100 km under deep

ocean basins and 100150 km under continents are rigid, forming a hard outer shell called thelithosphere. Beneath the lithosphere lies the asthenosphere, a layer in which seismicvelocities often decrease, suggesting lower rigidity. It is about 150 km thick, although itsupper and lower boundaries are not sharply defined. This weaker layer is thought to bepartially molten; it may be able to flow over long periods of time like a viscous liquid orplastic solid, in a way that depends on temperature and composition. The asthenosphereplays an important role in plate tectonics, because it makes possible the relative motions ofthe overlying lithospheric plates.


9/20



The brittle condition of the lithosphere causes it to fracture when strongly stressed. Therupture produces an earthquake, which is the violent release of elastic energy due to suddendisplacement on a fault plane.Earthquakes are not distributed evenly over the surface of the globe, but occurpredominantly in well-defined narrow seismic zones that are often associated with volcanic

activity. These are:(a) the circum-Pacific ring of fire;(b) a sinuous belt running from the Azores through North Africa and the AlpineDinarideHimalayan mountain chain as far as S.E. Asia; and(c) the world-circling system of oceanic ridges and rises.The seismic zones subdivide the lithosphere laterally into tectonic plates. A plate may be asbroad as 10,000 km (e.g., the Pacific plate) or as small as a few 1000 km (e.g., thePhilippines plate). There are twelve major plates (Antarctica, Africa, Eurasia, India,Australia, Arabia, Philippines, North America, South America, Pacific, Nazca, and Cocos) andseveral minor plates (e.g., Scotia, Caribbean, Juan de Fuca). The positions of the boundariesbetween the North American and South American plates and between the North American andEurasian plates are uncertain. The boundary between the Indian and Australian plates is not

sharplydefined, but may be a broad region of diffuse deformation.

Types of plate marginAn important factor in the evolution of modern plate tectonic theory was the development ofoceanography in the years following World War II, when technology designed for warfare wasturned to peaceful purposes. The bathymetry of the oceans was charted extensively by echo-sounding and within a few years several striking features became evident. Deep trenches,more than twice the depth of the ocean basins, were discovered close to island arcs and somecontinental margins; the Marianas Trench is more than 11 km deep. A prominent submarinemountain chain called an oceanic ridge was found in each ocean. The oceanic ridges rise to

as much as 3000 m above theadjacent basins and form a

continuous system, more than60,000 km in length, thatgirdles the globe. Unlikecontinental mountain belts,which are usually less thanseveral hundred kilometersacross, the oceanic ridges are20004000 km in width. Theridge system is offset atintervals by long horizontalfaults forming fracture zones.

These three features trenches, ridges and fracturezones originate from differentplate tectonic processes. Thelithospheric plates are very thinin comparison to their breadth.

Most earthquakes occur at plate margins, and are associated with interactions betweenplates. Apart from rare intraplate earthquakes, which can be as large and disastrous as the


10/20



earthquakes at plate boundaries, the plate interiors are aseismic. This suggests that theplates behave rigidly. Analysis of earthquakes allows the direction of displacement to bedetermined and permits interpretation of the relative motions between plates.

There are three types of plate margin, distinguished by different tectonic processes. Theworld-wide pattern of earthquakes shows that the plates are presently moving apart at

oceanic ridges. Magnetic evidence confirms that the separation has been going on for millionsof years. New lithosphere is being formed at these spreading centers, so the ridges can beregarded as constructive plate margins. The seismic zones related to deep-sea trenches,island arcs and mountain belts mark places where lithospheric plates are converging. Oneplate is forced under another there in a so-called subduction zone. Because its thin inrelation to its breadth the lower plate bends sharply before descending to depths of severalhundred kilometers, where it is absorbed. The subduction zone marks a destructive platemargin. Constructive and destructive plate margins may consist of many segments linked byhorizontal faults. A crucial step in the development of plate tectonic theory was made in 1965by a Canadian geologist, J. Tuzo Wilson, who recognized that these faults are notconventional transcurrent faults. They belong to a new class of faults, which Wilson calledtransform faults. The relative motion on a transform fault is opposite to what might be

inferred from the offsets of bordering ridge segments. At the point where a transform faultmeets an oceanic ridge it transforms the spreading on the ridge to horizontal shear on thefault.

Likewise, where such a fault meets a destructive plate margin it transforms subduction tohorizontal shear. The transform faults form a conservative plate margin, where lithosphere isneither created nor destroyed; the boundary separates plates that move past each otherhorizontally. This interpretation was documented in 1967 by L. Sykes, an Americanseismologist. He showed that earthquake activity on an oceanic ridge system was confinedalmost entirely to the transform fault between ridge crests, where the neighboring plates rubpast each other. Most importantly, Sykes found that the mechanisms of earthquakes on thetransform faults agreed with the predicted sense of strikeslip motion.

Transform faults play a key role in determining plate motions. Spreading and subduction areoften assumed to be perpendicular to the strike of a ridge or trench, as is the case for ridge Xin the figure above. This is not necessarily the case. Oblique motion with a component alongstrike is possible at each of these margins, as on ridge Y. However, because lithosphere isneither created nor destroyed at a conservative margin, the relative motion betweenadjacent plates must be parallel to the strike of a shared transform fault.

The water bodiesA water or water body is any significant accumulation of water, usually covering the Earth oranother planet. The term body of water most often refers to large accumulations of water,such as oceans, seas, and lakes, but it may also include smaller pools of water such as ponds,

puddles or wetlands. Rivers, streams, canals, and other geographical features where watermoves from one place to another are not always considered bodies of water.

Some bodies of water are man-made (artificial), such as reservoirs or harbors, but most arenaturally occurring geographical features. Bodies of water that are navigable are known aswaterways. Some bodies of water collect and move water, such as rivers and streams, andothers primarily hold water, such as lakes and oceans.


11/20



The term body of water can also refer to a reservoir of water held by a plant, technicallyknown as a phytotelma.

There are some geographical features involving water that are not bodies of water, forexample waterfalls and geysers.Here follows an exhaustive list of water bodies: Arm of the sea - also sea arm, used to describe a sea loch. Arroyo (creek) - a usually dry creek bed or gulch that temporarily fills with water after a

heavy rain, or seasonally. Barachois - a lagoon separated from the ocean by a sand bar Basin - a region of land where water from rain or snowmelt drains downhill into another

body of water, such as a river, lake, or dam. Bay - an area of water bordered by land on three sides. Bayou - a slow-moving stream or a marshy lake. Beck - a small stream. Bight - a large and often only slightly receding bay, or a bend in any geographical feature. Billabong - a pond or still body of water created when a river changes course and some

water becomes trapped. (Australian usage). Boil - a body of water formed by a spring. Brook - a small stream. Burn - a small stream. Canal - a man-made waterway, usually connected to (and sometimes connecting) existing

lakes, rivers, or oceans. Channel - the physical confine of a river, slough or ocean strait consisting of a bed and

banks. See also stream bed and strait. Cove - a coastal landform. Earth scientists generally use the term to describe a circular or

round inlet with a narrow entrance, though colloquially the term is sometimes used todescribe any sheltered bay.

Creek - a small stream. Creek (tidal) - an inlet of the sea, narrower than a cove. Dam - a barrier across flowing water that obstructs, directs or slows down the flow, often

creating a reservoir, lake or impoundment. The word "dam" can also refer to the reservoirrather than the structure.

Draw - a usually dry creek bed or gulch that temporarily fills with water after a heavyrain, or seasonally.

Estuary - a semi-enclosed coastal body of water with one or more rivers or streamsflowing into it, and with a free connection to the open sea

Firth - the Scots word used to denote various coastal waters in Scotland. It is usually alarge sea bay, estuary, inlet, or strait.

Fjord (fiord) - a submergent landform which has occurred due to glacial activity.

Glacier - A large collection of ice or a frozen river that moves slowly down a mountain. Gulf - a part of a lake or ocean that extends so that it is surrounded by land on three

sides, similar to, but larger than a bay. Harbor - a man-made or naturally occurring body of water where ships are stored or may

shelter from the ocean's weather and currents. Inlet - a body of water, usually seawater, which has characteristics of one or more of the

following: bay, cove, estuary, firth, fjord, geo, sea loch, or sound. Kettle - a shallow, sediment-filled body of water formed by retreating glaciers or draining

floodwaters.


12/20



Lagoon - a body of comparatively shallow salt or brackish water separated from thedeeper sea by a shallow or exposed sandbank, coral reef, or similar feature.

Lake - a body of water or other liquid, but usually freshwater, of considerable sizecontained on a body of land.

Loch - a body of water such as a lake, sea inlet, firth, fjord, estuary or bay. Mangrove swamp - Saline costal habitat of mangrove trees and shrubs. Marsh - a wetland featuring grasses, rushes, reeds, typhas, sedges, and other herbaceous

plants (possibly with low-growing woody plants) in a context of shallow water. See alsoSalt marsh.

Mediterranean sea - a mostly enclosed sea that has limited exchange of deep water withouter oceans and where the water circulation is dominated by salinity and temperaturedifferences rather than winds

Mere - a lake or body of water that is broad in relation to its depth. Millpond - a reservoir built to provide flowing water to a watermill Moat - a deep, broad trench, filled with water, surrounding a structure, installation, or

town. Ocean - a major body of saline water that, in totality, covers about 71% of the Earth's

surface. Oxbow lake - a U-shaped lake formed when a wide meander from the main stem of a river

is cut off to create a lake. Phytotelma - a small, discrete body of water held by some plants. Pool - various small bodies of water such as a swimming pool, reflecting pool, pond, or

puddle. Pond - a body of water smaller than a lake, especially those of man-made origin. Puddle - a small accumulation of water on a surface, usually the ground. Rapid - a fast moving part of a river Reservoir - an artificial lake, used to store water for various uses. River - a natural waterway usually formed by water derived from either precipitation or

glacial meltwater, and flows from higher ground to lower ground. Roadstead - a place outside a harbor where a ship can lie at anchor; it is an enclosed area

with an opening to the sea, narrower than a bay or gulf (often called a "roads"). Run - a small stream or part thereof, especially a smoothly flowing part of a stream. Salt marsh - a type of marsh that is a transitional zone between land and an area, such as

a slough, bay, or estuary, with salty or brackish water. Sea - a large expanse of saline water connected with an ocean, or a large, usually saline,

lake that lacks a natural outlet such as the Caspian Sea and the Dead Sea. In commonusage, often synonymous with ocean.

Sea loch - a sea inlet loch. Sea lough - a fjord, estuary, bay or sea inlet. Slough (wetland) - the word slough has several meanings related to wetland or aquatic

features. Source (river or stream) - the original point from which the river or stream flows. A river's

source is sometimes a spring. Sound - a large sea or ocean inlet larger than a bay, deeper than a bight, wider than a

fjord, or it may identify a narrow sea or ocean channel between two bodies of land. Spring - a point where groundwater flows out of the ground, and is thus where the aquifer

surface meets the ground surface Strait - a narrow channel of water that connects two larger bodies of water, and thus lies

between two land masses.


13/20



Stream - a body of water with a detectable current, confined within a bed and banks. Subglacial lake - a lake that is permanently covered by ice and whose water remains liquid

by the pressure of the ice sheet and geothermal heating. They often occur under glaciersor ice caps. Lake Vostok in Antarctica is an example.

Swamp - a wetland that features permanent inundation of large areas of land by shallowbodies of water, generally with a substantial number of hummocks, or dry-landprotrusions.

Tarn - a mountain lake or pool formed in a cirque excavated by a glacier. Tide pool - a rocky pool adjacent to an ocean and filled with seawater. Vernal pool - a shallow, natural depression in level ground, with no permanent above-

ground outlet, that holds water seasonally. Wash - a usually dry creek bed or gulch that temporarily fills with water after a heavy

rain, or seasonally. Wetland - an environment "at the interface between truly terrestrial ecosystems and truly

aquatic systems making them different from each yet highly dependent on both.

Earths atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained byEarth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation,warming the surface through heat retention (greenhouse effect), and reducing temperatureextremes between day and night (the diurnal temperature variations).

Atmospheric stratificationdescribes the structure of theatmosphere, dividing it intodistinct layers, each withspecific characteristics such astemperature or composition.The atmosphere has a mass of

about 510

18

kg, three quartersof which is within about 11 kmof the surface. The atmospherebecomes thinner and thinnerwith increasing altitude, withno definite boundary betweenthe atmosphere and outerspace. An altitude of 120 km iswhere atmospheric effectsbecome noticeable duringatmospheric re-entry ofspacecraft. The Krmn line, at

100 km, also is often regardedas the boundary betweenatmosphere and outer space.

Air is the name given toatmosphere used in breathing and photosynthesis. Dry air contains roughly (by volume) 78.09%nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of othergases. Air also contains a variable amount of water vapour, on average around 1%. While aircontent and atmospheric pressure varies at different layers, air suitable for the survival of


14/20



terrestrial plants and terrestrial animals is currently only known to be found in Earth'stroposphere and artificial atmospheres.

In general, air pressure and density decrease in the atmosphere as height increases. However,temperature has a more complicated profile with altitude. Because the general pattern ofthis profile is constant and recognizable through means such as balloon soundings,

temperature provides a useful metric to distinguish between atmospheric layers. In this way,Earth's atmosphere can be divided into five main layers: exosphere, thermosphere,mesosphere, stratosphere and troposphere

ExosphereThe exosphere is the uppermost layer of the atmosphere. In the exosphere, an upwardtravelling molecule moving fast enough to attain escape velocity can escape to space with alow chance of collisions; if it is moving below escape velocity it will be prevented fromescaping from the celestial body by gravity. In either case, such a molecule is unlikely tocollide with another molecule due to the exosphere's low density.

The main gases within the Earth's exosphere are the lightest gases, mainly hydrogen, with

some helium, carbon dioxide, and atomic oxygen near the exobase. The exosphere is the lastlayer before outer space. Since there is no clear boundary between outer space and theexosphere, the exosphere is sometimes considered a part of outer space. The altitude of itslower boundary, known as the thermopause and exobase, ranges from about 250 to 500kilometres depending on solar activity. Its lower boundary at the edge of the thermospherehas sometimes been estimated to be 500 to 1,000 km above the Earth's surface. The exobaseis also called the critical level, the lowest altitude of the exosphere, and is typically definedin one of two ways: The height above which there are the negligible atomic collisions between the particles

and The height above which constituent atoms are on purely ballistic trajectories.The fluctuation in the height of the exobase is important because this provides atmosphericdrag on satellites, eventually causing them to fall from orbit if no action is taken to maintainthe orbit.

The upper boundary of the exosphere can be defined theoretically by the altitude about190,000 kilometres, half the distance to the Moon, at which the influence of solar radiationpressure on atomic hydrogen velocities exceeds that of the Earths gravitational pull. Theexosphere observable from space as the geocorona (luminous part of the exosphere) is seen toextend to at least 100,000 kilometres from the surface of the Earth. The exosphere is atransitional zone between Earths atmosphere and interplanetary space.

ThermosphereThe thermosphere is the biggest of all the layers of the earth's atmosphere directly above themesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causesionization. The International Space Station has a stable orbit within the middle of thethermosphere, between 320 and 380 kilometres. Auroras also occur in the thermosphere.Named from the Greek (thermos) for heat, the thermosphere begins about 80kilometres above the earth. At these high altitudes, the residual atmospheric gases sort intostrata according to molecular mass. Thermospheric temperatures increase with altitude dueto absorption of highly energetic solar radiation by the small amount of residual oxygen still


15/20



present. Temperatures are highly dependent on solar activity, and can rise to 1,500 C.Radiation causes the atmosphere particles in this layer to become electrically charged,enabling radio waves to bounce off and be received beyond the horizon. At the exosphere,beginning at 500 to 1,000 kilometres above the Earth's surface, the atmosphere turns intospace.The highly diluted gas in this layer can reach 2,500 C during the day. Even though the

temperature is so high, one would not feel warm in the thermosphere, because it is so nearvacuum that there is not enough contact with the few atoms of gas to transfer much heat(conduction and convection). A normal thermometer would read significantly below 0 C, dueto the energy lost by thermal radiation overtaking the energy acquired from the atmosphericgas by direct contact. Above 160 kilometres, the anacoustic zone (where there are stillenough gas molecules to produce substantial drag, but they aren't close enough to transmitsound because the probability of molecular collisions is so low) prevents the transmission ofsound.The dynamics of the lower thermosphere (below approximately 120 kilometres) aredominated by atmospheric tide, which is driven, in part, by the very significant diurnalheating. The atmospheric tide dissipates above this level since molecular concentrations donot support the coherent motion needed for fluid flow.

MesosphereThe mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphereand directly below the thermosphere. In the mesosphere temperature decreases withincreasing height. The upper boundary of the mesosphere is the mesopause, which can be thecoldest naturally-occurring place on Earth with temperatures below 130 K. The exact upperand lower boundaries of the mesosphere vary with latitude and with season, but the lowerboundary of the mesosphere is usually located at heights of about 50 km above the Earth'ssurface and the mesopause is usually at heights near 100 km, except at middle and highlatitudes in summer where it descends to heights of about 85 km.The stratosphere, mesosphere and lowest part of the thermosphere are collectively referredto as the "middle atmosphere", which spans heights from approximately 10 to 100 km. The

mesopause, at an altitude of 8090 km, separates the mesosphere from the thermospherethe second-outermost layer of the Earth's atmosphere. This is also around the same altitudeas the turbopause, below which different chemical species are well mixed due to turbulenteddies. Above this level the atmosphere becomes non-uniform; the scale heights of differentchemical species differ by their molecular masses.Within the mesosphere, temperature decreases with increasing altitude. This is due todecreasing solar heating and increasing cooling by CO2 radiative emission. The top of themesosphere, called the mesopause, is the coldest place on Earth.

The main dynamical features in this region are strong zonal (East-West) winds, atmospherictides, internal atmospheric gravity waves (commonly called "gravity waves") and planetarywaves. Most of these tides and waves are excited in the troposphere and lower stratosphere,

and propagate upward to the mesosphere. In the mesosphere, gravity-wave amplitudes canbecome so large that the waves become unstable and dissipate. This dissipation depositsmomentum into the mesosphere and largely drives global circulation.Noctilucent clouds are located in the mesosphere. The mesosphere is also the region of theionosphere known as the D layer. The D layer is only present during the day, when someionization occurs with nitric oxide being ionized by Lyman series-alpha hydrogen radiation.The ionization is so weak that when night falls, and the source of ionization is removed, thefree electron and ion fall back into a neutral molecule.


16/20



A 5 km deep sodium layer is located between 80105 km. Made of unbound, non-ionizedatoms of sodium; the sodium layer radiates weakly to contribute to the airglow.The mesosphere lies above the maximum altitude for aircraft and below the minimumaltitude for orbital spacecraft. It has only been accessed through the use of sounding rockets.As a result, it is not a well understood part of the atmosphere. The presence of red spritesand blue jets (electrical discharges or lightning within the lower mesosphere), noctilucent

clouds and density shears within the poorly understood layer are of current scientific interest.

Millions of meteors enter the atmosphere, an average of 40 tons per day. Within themesosphere most melt or vaporize as a result of collisions with the gas particles containedthere.

StratosphereThe stratosphere is the second major layer of Earth's atmosphere, just above the troposphere,and below the mesosphere. It is stratified in temperature, with warmer layers higher up andcooler layers farther down. This is in contrast to the troposphere near the Earth's surface,which is cooler higher up and warmer farther down. The border of the troposphere andstratosphere, the tropopause, is marked by where this inversion begins, which in terms of

atmospheric thermodynamics is the equilibrium level. The stratosphere is situated betweenabout 10 km and 50 km altitude above the surface at moderate latitudes, while at the poles itstarts at about 8 km altitude.

Within this layer, temperature increases as altitude increases, the top of the stratosphere hasa temperature of about 270 K, just slightly below the freezing point of water. Thestratosphere is layered in temperature because ozone (O3) here absorbs high energy UVB andUVC energy waves from the Sun and is broken down into atomic oxygen (O) and diatomicoxygen (O2). Atomic oxygen is found prevalent in the upper stratosphere due to thebombardment of UV light and the destruction of both ozone and diatomic oxygen. The midstratosphere has less UV light passing through it, O and O 2 are able to combine, and is wherethe majority of natural ozone is produced. It is when these two forms of oxygen recombine to

form ozone that they release the heat found in the stratosphere. The lower stratospherereceives very low amounts of UVC, thus atomic oxygen is not found here and ozone is notformed (with heat as the by-product). This vertical stratification, with warmer layers aboveand cooler layers below, makes the stratosphere dynamically stable: there is no regularconvection and associated turbulence in this part of the atmosphere. The top of thestratosphere is called the stratopause, above which the temperature decreases with height.Methane (CH4) while it is not a direct cause of ozone destruction in the stratosphere, doeslead to the formation of compounds that do destroy ozone. Monoatomic oxygen (O), in theupper stratosphere, reacts with methane (CH4) to form a hydroxyl radical (OH). This hydroxylradical is then able to interact with non-soluble compounds like chlorofluorocarbons and UVlight break off chlorine radicals (Cl). These chlorine radicals break off an oxygen atom fromthe ozone molecule, creating an oxygen molecule (O2) and a hypochlorite radical (ClO). The

hypochlorite radical then reacts with an atomic oxygen creating another oxygen molecule andanother chlorine radical, thereby preventing the reaction of a monoatomic oxygen with O 2 tocreate natural ozone.

Commercial airliners typically cruise at altitudes of 912 km in temperate latitudes (in thelower reaches of the stratosphere). They do this to optimize fuel burn, mostly thanks to thelow temperatures encountered near the tropopause and the low air density that reduces


17/20



parasitic drag on the airframe. It also allows them to stay above any hard weather (extremeturbulence).Because the temperature in the tropopause and lower stratosphere remains constant (orslightly increases) with increasing altitude, there is very little convective turbulence at thesealtitudes. Though most of the turbulence at this altitude is caused by variations in the jetstream and other local wind shears, areas of significant convective activity (thunderstorms) in

the troposphere below may produce convective overshoot.

The stratosphere is a region of intense interactions among radiative, dynamical, and chemicalprocesses, in which horizontal mixing of gaseous components proceeds much more rapidlythan vertical mixing. An interesting feature of stratospheric circulation is the quasi-BiennialOscillation (QBO) in the tropical latitudes, which is driven by gravity waves that areconvectively generated in the troposphere. The QBO induces a secondary circulation that isimportant for the global stratospheric transport of tracers such as ozone or water vapor. Innorthern hemispheric winter, sudden stratospheric warmings can often be observed which arecaused by the absorption of Rossby waves in the stratosphere.Bacterial life survives in the stratosphere, making it a part of the biosphere. Also, some birdspecies have been reported to fly at the lower levels of the stratosphere.

TroposphereThe troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% ofthe atmosphere's mass and 99% of its water vapor and aerosols. The average depth of thetroposphere is approximately 17 km in the middle latitudes. It is deeper in the tropicalregions, up to 20 km, and shallower near the poles, at 7 km in summer, and indistinct inwinter. The lowest part of the troposphere, where friction with the Earth's surface influencesair flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 kmdeep depending on the landform and time of day. The border between the troposphere andstratosphere, called the tropopause, is a temperature inversion.

The word troposphere derives from the Greek: tropos for "turning" or "mixing," reflecting the

fact that turbulent mixing plays an important role in the troposphere's structure andbehavior. Most of the phenomena we associate with day-to-day weather occur in thetroposphere.

The chemical composition of the troposphere is essentially uniform, with the notableexception of water vapour. The source of water vapour is at the surface through theprocesses of evaporation and transpiration. Furthermore the temperature of the tropospheredecreases with height, and saturation vapour pressure decreases strongly as temperaturedrops, so the amount of water vapour that can exist in the atmosphere decreases stronglywith height. Thus the proportion of water vapour is normally greatest near the surface anddecreases with height.The pressure of the atmosphere is maximum at sea level and decreases with higher altitude.

This is because the atmosphere is very nearly in hydrostatic equilibrium, so that the pressureis equal to the weight of air above a given point. The change in pressure with height,therefore can be equated to the density with this hydrostatic equation.


18/20



where: g = standard gravity; = density; z = height; p = pressure; R = gas constant; T =temperature (Kelvin); m = molar mass.

The temperature of the troposphere generally decreases as altitude increases. The rate atwhich the temperature decreases, dT/dz, is called the environmental lapse rate (ELR). TheELR represents the difference in temperature between the surface and the tropopause

divided by the height. The reason for this temperature difference is the absorption of thesun's energy occurring at the ground which heats the lower levels of the atmosphere, and theradiation of heat occurs at the top of the atmosphere cooling the earth, this processmaintaining the overall heat balance of the earth.

As parcels of air in the atmosphere rise and fall, they also undergo changes in temperaturefor reasons described below. The rate of change of the temperature in the parcel may be lessthan or more than the ELR. When a parcel of air rises, it expands, because the pressure islower at higher altitudes. As the air parcel expands, it pushes on the air around it, doingwork; but generally it does not gain heat in exchange from its environment, because itsthermal conductivity is low (such a process is called adiabatic). Since the parcel does workand gains no heat, it loses energy, and so its temperature decreases. (The reverse, of course,

will be true for a sinking parcel of air.)

Since the heat exchanged dQis related to the entropy change dS by dQ= TdS, the equationgoverning the temperature as a function of height for a thoroughly mixed atmosphere is,

where S is the entropy. The rate at which temperature decreases with height under suchconditions is called the adiabatic lapse rate.For dry air, which is approximately an ideal gas, the dry adiabatic lapse rate is given as,

If the air contains water vapour, then cooling of the air can cause the water to condense, andthe behaviour is no longer that of an ideal gas. If the air is at the saturated vapour pressure,then the rate at which temperature drops with height is called the saturated adiabatic lapserate. More generally, the actual rate at which the temperature drops with altitude is calledthe environmental lapse rate. In the troposphere, the average environmental lapse rate isabout -6.5 C/km in increased height.The environmental lapse rate is not usually equal to the adiabatic lapse rate. If the upper airis warmer than predicted by the adiabatic lapse rate (dS/dz> 0), then when a parcel of airrises and expands, it will arrive at the new height at a lower temperature than its

surroundings. In this case, the air parcel is denser than its surroundings, so it sinks back to itsoriginal height, and the air is stable against being lifted. If, on the contrary, the upper air iscooler than predicted by the adiabatic lapse rate, then when the air parcel rises to its newheight it will have a higher temperature and a lower density than its surroundings, and willcontinue to accelerate upward.Temperatures decrease at middle latitudes from an average of 15C at sea level to about -55C at the top of the tropopause. At the poles, the troposphere is thinner and the


19/20



temperature only decreases to -45C, while at the equator the temperature at the top of thetroposphere can reach -75C.

The tropopause is the boundary region between the troposphere and the stratosphere.Measuring the temperature change with height through the troposphere and the stratosphereidentifies the location of the tropopause. In the troposphere, temperature decreases with

altitude. In the stratosphere, however, the temperature remains constant for a while andthen increases with altitude. The region of the atmosphere where the lapse rate changesfrom positive (in the troposphere) to negative (in the stratosphere), is defined as thetropopause. Thus, the tropopause is an inversion layer, and there is little mixing between thetwo layers of the atmosphere.

There are three "levels" often used by meteorologists to describe the height and verticalrange of an atmospheric event or phenomenon. Each level is roughly defined, and so therecan only be approximate definitions of each.

The lower level of the atmosphere from the surface up to 1,800 to 2,400 meters abovesea level.

The mid level lies between the lower and upper levels, corresponding to aroundroughly 1,800 to 7,600 meters.

This is the highest of the three levels. The term applies to the portion of theatmosphere that is above the lower troposphere, generally 850 hPa and above.

The flow of the atmosphere generally moves in a west to east direction. This however canoften become interrupted, creating a more north to south or south to north flow. Thesescenarios are often described in meteorology as zonal or meridional. These terms, however,tend to be used in reference to localised areas of atmosphere (at a synoptic scale)). A fullerexplanation of the flow of atmosphere around the Earth as a whole can be found in the three-cell model.

a. A zonal flow regime is themeteorological term meaning that thegeneral flow pattern is west to eastalong the Earth's latitude lines, withweak shortwaves embedded in the flow.The use of the word "zone" refers to theflow being along the Earth's latitudinal"zones". This pattern can buckle andthus become a meridional flow.Meridional flow

b. When the zonal flow buckles,the atmosphere can flow in a morelongitudinal (or meridional) direction,and thus the term "meridional flow"arises. Meridional flow patterns featurestrong, amplified troughs and ridges,with more north-south flow in thegeneral pattern than west-to-east flow.

c. The three cells model attempts to describe the actual flow of the Earth's atmosphereas a whole. It divides the Earth into the tropical (Hadley cell), mid latitude (Ferrelcell), and polar (polar cell) regions, dealing with energy flow and global circulation. Its


20/20



fundamental principle is that of balance - the energy that the Earth absorbs from thesun each year is equal to that which it loses back into space, but this however is not abalance precisely maintained in each latitude due to the varying strength of the sun ineach "cell" resulting from the tilt of the Earth's axis in relation to its orbit. Itdemonstrates that a pattern emerges to mirror that of the ocean - the tropics do notcontinue to get warmer because the atmosphere transports warm air pole-ward and

cold air equator-ward, the purpose of which appears to be that of heat and moisturedistribution around the planet.

Forcing is a term used by meteorologists to describe the situation where a change or an eventin one part of the atmosphere causes a strengthening change in another part of theatmosphere. It is usually used to describe connections between upper, middle or lower levels(such as upper-level divergence causing lower level convergence in cyclone formation), butcan sometimes also be used to describe such connections over distance rather than heightalone. In some respects, tele-connections could be considered a type of forcing.

Date post:	04-Apr-2018
Category:	Documents
Upload:	kipngenokering
View:	225 times
Download:	0 times

EGS 2101 Set II.docx

Documents