WELCOME• Volcanoes• Structure Of A Volcano• Volcanic Products.• Volcanic Eruptions• Features Of Volcanism• Earthquakes• Fault Zone And Earthquakes• Body Waves• Seismograph• Locating Earthquakes• Measuring The “Size” Of Earthquakes• World Earthquake Distribution• Effects Of Earthquakes

VOLCANOES

• The interior of the in the earth is quite hot, and at certain places, the interior crustal rocks may exist molten state. This molten material or liquid rock when heavily charged with gases and volatile substances existing below the earth’s surface, is called MAGMA.

• The process of movement of magma within the earth’s curst and eruptions is called volcanism or volcanic activity.

• The eruptions through long fissures are normally quite, are called fissure type eruption.

STRUCTURE OF A VOLCANO• In addition to emission of gases and molten lavas , vast quantities of

fragmental materials (pyro clasts) are also produced during volcanic eruptions. This material accumulates around the vent.

• Conical-hill like masses formed in this way are called volcanic cones. Volcanoes often have side vents as well . The smaller cones formed around these side vents are called parasitic cones.

• The erupted products spread in such a way that a typical circular depression or pit is formed at the top of the hill, which is known as crater.

VOLCANIC PRODUCTS.• Solid products• Enormous quantities of solid materials are thrown out by volcanoes during

an eruption. They consist of fragments of rocks or pieces of already cooled lava.

• The ejection of the solid materials are usually accompanied by violent explo sions. The solid materials, during the initial stages of volcanism, mostly contain the fragments of the crustal rocks through which the pipe of the volcano passes; but at later stages they consist mostly the fragments of solidified lava, resulted from the partial solidification in the molten reservoir beneath the surface as well as the solidified lava of earlier eruptions.

The rock fragments ejected during volcanic-eruptions are called pyroclasts or tephra. Generally, larger fragments fall at the edge of the crater and slide down its inner and outer slopes, while smaller ones are thrown into the surrounding plains or pile up at the foot of the cone.

According to their size and shape the pyroclastic materials are classified as follows:

(i) Volcanic blocks

These are the largest masses of rock blown out. These are either the masses of the solidified lava of earlier eruptions or those of the pre-existing rocks. They are usually angular and the diameter of the fragments is always above 32 milimeter. Thus they are the huge solid fragments ejected during a volcanic activity.

(ii) Volcanic bombs

These are rounded or spindle-shaped masses of hardened lava, which may develop when clots of lava are blown into the air and get solidified before reaching the ground.

Their ends are twisted, indicating rapid rotation in the air while the material was plastic. Because of their somewhat rounded appearance, they are known as volcanic bombs. The diameter of these fragments are always above 32 millimeter.

Bread-crust bombs are those volcanic bombs which present a cracked surface, may be due to the approxi mately solid state of the material from which they have been formed, which gives the appearance of the crust of a bread.

(iii) Cinders or lapilli

The size of the fragments is between 4 mm to 32 mm, and are shaped very much like bombs. The term 'lapilli' is used when the fragments are not conspicuously vesicular; and in case of vesicular fragments they are known as cinders. Still smaller fragments are called volcanic-sand.

(iv) Ash

These particles range in size from 0.25 mm to 4mm and as such, are the fine particles of lava.

(v) Fine-ash or volcanic dust

These are the minute pyroclas- tic materials, and their diameter is always less than 0.25 mm. In many instances volcanic dust was carried by wind to enormous distances and scattered over a vast territory forming volcanic dust layers.

Pyroclastic materials accumulating on the slopes and adjoin ing areas of a volcano with gradual compaction and cementation gives rise to rocks called Volcanic-tuffs.These tuffs when consist of angular fragmental materials, they are known as Volcanic- breccia; when volcanic bombs are predominant in the tuffs, they are referred to as Volcanic-agglomerates.The diameter of these fragments are always larger than 20 mm. The welded tuffs are commonly known as Ignimbrites. In certain instances, a great cloud of superheated vapours and incandescent rock material and volcanic ash are violently emitted during the eruption. These are called Nuees ardentes and are sometimes referred to as glowing avalanches.(b) Liquid productsLavas are the major and the most important liquid product of a volcano. As we know, the magma that has flowed out on to the surface is called lava. All lavas contain gases, but because of the high pressure that prevails in the interior of the earth the content of gases and vapours in the magma is more.According to the composition and the gas content, the temperature of lavas during eruptions usually ranges between 900°C to 1200°C. Like magma, lava is also divided in to three types viz. acidic, medium and basic, depending on the silica contentAcid lavas contain a high proportion of silica, have a high melting point and are usually very viscous and therefore their mobility is low. They cool very slowly and contain many gases in a dissolved state.They congeal at relatively short distances from the crater. Rhyolites, composed of orthoclase feldspar and quartz are the examples of acid lavas.

The lavas of intermediate or medium composition have the silica content between 55 to 60%. Andesite lavas are the best examples of the lavas of intermediate nature and they mostly characterize extrusions around the margins of the Pacific.

The basic lavas contain low percentage of silica, which is usually 50% or less. These lavas melt at lower temperature, and have a high density as well as liquid consistency. They cool quickly and contain little gas.

These lavas are highly mobile and spread over large distances, forming flows or sheets. Basalts are the best examples of the basic lava.

Since the lava behave differently depending on their chemi cal composition they give rise to different configurations when consolidated, as described below:

(i) Lava tunnels

Sometimes the outer surface of the lava flows; cools and solidifies first forming a crust while the lava is still in a liquid state inside. This enclosed liquid may drain out through some weak spots of the solidified flow forming a tunnel called a lava-tunnel.

(ii) Block lava

It is also known as aa-lava. In this case, the gases escape explosively from the partly crystallized flows thus break the congealing crust in to an assemblage of rough and uneven blocks.

The escape of gases increase the viscosity of the lava and helps in rapid cooling, giving rise to a solidified lava flow with spiny, rubbly surface. It is therefore the Hawaiian name, aa (pro nounced ah-ah meaning rough or spiny) is applied to this type of lavas.

(iii) Ropy-lava

Lavas with low-viscosity remain mobile for a longer period. These lavas usually contain much en trapped gas and cool very slowly.

The lava spreads out in thin sheets and congeals with a smooth surface which wrinkles or twisted into ropy form like that of a stream of flowing pitch. It is also called Pahoehoe-structure.

(iv) Pillow lava

Lava erupted under water-logged sediments in sea-water, beneath ice-sheets, or in to rain soaked air, characteristically emerges as a pile of rounded bulbous blobs or pillows. Basic lava of spilitic type often presents pillow structure.

(v) Vesicular or Scoriaceous structureWhen lavas heavily charged with gases and other volatiles are erupted on the surface, the gaseous constituents escape from the lava, due to the decrease of pressure, giving rise to a large number of empty cavities of variable dimensions on the surface of the lava-flows.Due to the presence of vesicles or cavities, the resulting structure is known as vesicular- structure. These cavities when filled up subsequently with secondary minerals, the structure is called amyg- daloidal structure and the infillings as amygdales.A highly vesicular rock, which contains more gas space than rock, is known as 'Scoria'. In more viscous lavas, when the gases cannot escape easily and the lava quickly congeals, it forms Pumice or 'Rock -froth', which contains so much void space that it can float in water.(vi) JointingAs a consequence of contraction due to cooling joints are developed in the lava flows, which may be ma nifested in the form of sheet, platy or columnar structures, (c) Gaseous Products Volcanic activity is invariably associated with emanation of steam and various gases from the volcanoes.Water vapor constitutes about 60 to 90% of the total content of the volcanic gases. Second in abundance to steam among volcanic gases is carbon-di-oxide.Amongst other gases which have been detected in considerable quantities, hydrochloric acid, sulphuretted hydrogen, Sulphur-dioxide, hydrogen, nitrogen, boric-acid va pours, phosphorous, arsenic vapour, argon, hydrofluoric acid etc. are the most importantThe vents emitting sulphurous vapours are called Solfataras' when carbon-dioxides are emitted they are called 'Mofettes' and in the case of emission of boric-acid vapours, they are known as Saffioni.

Volcanic Eruptions

The most common type of volcanic eruption occurs when magma (the term for lava when it is below the Earth's surface) is released from a volcanic vent. Eruptions can be effusive, where lava flows like a thick, sticky liquid, or explosive, where fragmented lava explodes out of a vent. In explosive eruptions, the fragmented rock may be accompanied by ash and gases; in effusive eruptions, degassing is common but ash is usually not.

Volcanologists classify eruptions into several different types. Some are named for particular volcanoes where the type of eruption is common; others concern the resulting shape of the eruptive products or the place where the eruptions occur. Here are some of the most common types of eruptions:

VOLCANIC ERUPTIONS

Hawaiian Eruption 

In a Hawaiian eruption, fluid basaltic lava is thrown into the air in jets from a vent or line of vents (a fissure) at the summit or on the flank of a volcano. The jets can last for hours or even days, a phenomenon known as fire fountaining. The spatter created by bits of hot lava falling out of the fountain can melt together and form lava flows, or build hills called spatter cones. Lava flows may also come from vents at the same time as fountaining occurs, or during periods where fountaining has paused. Because these flows are very fluid, they can travel miles from their source before they cool and harden. 

Hawaiian eruptions get their names from the Kilauea volcano on the Big Island of Hawaii, which is famous for producing spectacular fire fountains. Two excellent examples of these are the 1969-1974 Mauna Ulu eruption on the volcano's flank, and the 1959 eruption of the Kilauea Iki Crater at the summit of Kilauea. In both of these eruptions, lava fountains reached heights of well over a thousand feet. 

Strombolian Eruption

Strombolian eruptions are distinct bursts of fluid lava (usually basalt or basaltic andesite) from the mouth of a magma-filled summit conduit. The explosions usually occur every few minutes at regular or irregular intervals. The explosions of lava, which can reach heights of hundreds of meters, are caused by the bursting of large bubbles of gas, which travel upward in the magma-filled conduit until they reach the open air. 

This kind of eruption can create a variety of forms of eruptive products: spatter, or hardened globs of glassy lava; scoria, which are hardened chunks of bubbly lava; lava bombs, or chunks of lava a few cm to a few m in size; ash; and small lava flows (which form when hot spatter melts together and flows downslope). Products of an explosive eruption are often collectively called tephra. 

Strombolian eruptions are often associated with small lava lakes, which can build up in the conduits of volcanoes. They are one of the least violent of the explosive eruptions, although they can still be very dangerous if bombs or lava flows reach inhabited areas. Strombolian eruptions are named for the volcano that makes up the Italian island of Stromboli, which has several erupting summit vents. These eruptions are particularly spectacular at night, when the lava glows brightly. 

Vulcanian Eruption

A Vulcanian eruption is a short, violent, relatively small explosion of viscous magma (usually andesite, dacite, or rhyolite). This type of eruption results from the fragmentation and explosion of a plug of lava in a volcanic conduit, or from the rupture of a lava dome (viscous lava that piles up over a vent). Vulcanian eruptions create powerful explosions in which material can travel faster than 350 meters per second (800 mph) and rise several kilometers into the air. They produce tephra, ash clouds, and pyroclastic density currents (clouds of hot ash, gas and rock that flow almost like fluids). 

Vulcanian eruptions may be repetitive and go on for days, months, or years, or they may precede even larger explosive eruptions. They are named for the Italian island of Vulcano, where a small volcano that experienced this type of explosive eruption was thought to be the vent above the forge of the Roman smith god Vulcan. 

Plinian Eruption

The largest and most violent of all the types of volcanic eruptions are Plinian eruptions. They are caused by the fragmentation of gassy magma, and are usually associated with very viscous magmas (dacite and rhyolite). They release enormous amounts of energy and create eruption columns of gas and ash that can rise up to 50 km (35 miles) high at speeds of hundreds of meters per second. Ash from an eruption column can drift or be blown hundreds or thousands of miles away from the volcano. The eruption columns are usually shaped like a mushroom (similar to a nuclear explosion) or an Italian pine tree; Pliny the Younger, a Roman historian, made the comparison while viewing the 79 AD eruption of Mount Vesuvius, and Plinian eruptions are named for him. 

Plinian eruptions are extremely destructive, and can even obliterate the entire top of a mountain, as occurred at Mount St. Helens in 1980. They can produce falls of ash, scoria and lava bombs miles from the volcano, and pyroclastic density currents that raze forests, strip soil from bedrock and obliterate anything in their paths. These eruptions are often climactic, and a volcano with a magma chamber emptied by a large Plinian eruption may subsequently enter a period of inactivity.

Geysers, fumaroles (also called solfataras), and hot springs are generally found in regions of young volcanic activity. Surface water percolates downward through the rocks below the Earth's surface to high-temperature regions surrounding a magma reservoir, either active or recently solidified but still hot. There the water is heated, becomes less dense, and rises back to the surface along fissures and cracks. Sometimes these features are called "dying volcanoes" because they seem to represent the last stage of volcanic activity as the magma, at depth, cools and hardens.

Erupting geysers provide spectacular displays of underground energy suddenly unleashed, but their mechanisms are not completely understood. Large amounts of hot water are presumed to fill underground cavities. The water, upon further heating, is violently ejected when a portion of it suddenly flashes into steam. This cycle can be repeated with remarkable regularity, as for example, at Old Faithful Geyser in Yellowstone National Park, which erupts on an average of about once every 65 minutes.

FEATURES OF VOLCANISM

Fumaroles, which emit mixtures of steam and other gases, are fed by conduits that pass through the water table before reaching the surface of the ground. Hydrogen sulfide (H2S), one of the typical gases issuing from fumaroles, readily oxidizes to sulfuric acid and native sulfur. This accounts for the intense chemical activity and brightly colored rocks in many thermal areas.

Hot springs occur in many thermal areas where the surface of the Earth intersects the water table. The temperature and rate of discharge of hot springs depend on factors such as the rate at which water circulates through the system of underground channelways, the amount of heat supplied at depth, and the extent of dilution of the heated water by cool ground water near the surface.

EARTHQUAKES•An earthquake is a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath Earth’s surface

– Tectonic forces within the Earth produce stresses on rocks that eventually exceed their elastic limits, resulting in brittle failure

•Energy is released during earthquakes in the form of seismic waves

– Released from a position along a break between two rock masses (fault)

•Elastic rebound theory - earthquakes are a sudden release of strain progressively stored in rocks that bend until they finally break and move along a fault

Fault zone and earthquakes

Body Waves - Travel through the earth- 2 types, have different motion:Primary (P) waves •Particle motion is parallel to wave direction•Travel fastest (arrive first)•Travel through solid or fluid

Secondary (S) waves •Particle motion is perpendicular to wave direction•Travel slightly slower•Only travel through solid

Surface Waves

•Slowest type of seismic waves produced by earthquakes•Love waves - side-to-side motion of the ground surface

–Can’t travel through fluids•Rayleigh waves - ground moves in an elliptical path opposite the direction of wave motion

–Extremely destructive to buildings

Measuring Earthquakes

•Seismometers - used to measure seismic waves•Seismographs - recording devices used to produce a permanent record of the motion detected by seismometers•Seismograms - permanent paper (or digital) records of the earthquake vibrations

–Used to measure the earthquake strengths

Seismograph•Measures horizontal motion (P waves)

Locating Earthquakes

•P- and S-waves leave earthquake focus at the same time•P-wave gets farther and farther ahead of the S-wave with distance and time from the earthquake•Travel-time curve - used to determine distance to focus

– based on time between first P- and S-wave arrivals

Locating earthquakes

•Plotting distances from 3 stations on a map, as circles with radii equaling the distance from the quake, locates earthquake epicenter•Depth of focus beneath earth’s surface can also be determined

–Shallow focus 0-70 km deep–Intermediate focus 70-350 km deep–Deep focus 350-670 km deep

Measuring the “Size” of Earthquakes

•Earthquake “size” measured two ways - intensity and magnitude•Intensity - a measure of the effects an earthquake produces (on both structures and people)

–Modified Mercalli scale

•Size of earthquakes measured in two ways - intensity and magnitude•Magnitude is a measure of the amount of energy released by an earthquake

– Richter scale•Moment magnitude - more objective measure of energy released by a major earthquake

– Uses rock strength, surface area of fault rupture, and amount of movement

– Smaller earthquakes are more common than larger ones

Effects of Earthquakes•Earthquakes produce several types of effects, all of which can cause loss of property and human life

–Ground motion is the familiar trembling and shaking of the land during an earthquake

•Can topple buildings and bridges–Fire is a problem just after earthquakes because of broken gas and water mains and fallen electrical wires–Landslides can be triggered by ground shaking, particularly in larger quakes–Liquefaction occurs when water-saturated soil or sediment sloshes like a liquid during a quake

World earthquake distribution•Most earthquakes occur in narrow geographic belts which mark tectonic plate boundaries•Most important concentrations in circum-pacific and mediterranean-himalayan belts•Shallow-focus earthquakes common along the crests of mid-oceanic ridges•Nearly all intermediate- and deep-focus earthquakes occur in benioff zones

–Inclined seismic activity associated with descending oceanic plate at subduction zones)

Earthquakes and plate tectonics

•Earthquakes are caused by plate inter-actions along tectonic plate boundaries•Plate boundaries are identified and defined by earthquakes•Earthquakes occur at each of the three types of plate boundaries: divergent, transform, and convergent

–At divergent boundaries, tensional forces produce shallow-focus quakes on normal faults–At transform boundaries, shear forces produce shallow-focus quakes along strike-slip faults–At convergent boundaries, compressional forces produce shallow- to deep-focus quakes along reverse faults

Earthquake prediction and seismic risk

•Accurate and consistent short-term earthquake prediction not yet possible, three methods assist in determining probability that an earthquake will occur:

–Measurement of changes in rock properties, such as magnetism, electrical resistivity, seismic velocity, and porosity, which may serve as precursors to earthquakes–Studies of the slip rate along fault zones–Paleoseismology studies that determine where and when earthquakes have occurred and their size

•Average intervals between large earthquakes and the time since the last one occurred can also be used to assess the risk (over A given period of time) that A large quake will occur