1 Geologic Structure of Earth - The Geologic Structure of Earth - The interior of interior of the Earth is layered. the Earth is layered. Concentric layers: crust, mantle, Concentric layers: crust, mantle, liquid outer core and solid inner liquid outer core and solid inner core. core. Evidence (indirect) for this structure Evidence (indirect) for this structure comes from studies of Earth’s comes from studies of Earth’s dimensions, density, rotation, dimensions, density, rotation, gravity, magnetic field, behavior of gravity, magnetic field, behavior of seismic waves and meteorites. seismic waves and meteorites. Chapter 3 – Earth Structure
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Geologic Structure of Earth - The interior ofGeologic Structure of Earth - The interior ofthe Earth is layered. the Earth is layered.
Evidence (indirect) for this structure comes Evidence (indirect) for this structure comes from studies of Earth’s dimensions, density, from studies of Earth’s dimensions, density, rotation, gravity, magnetic field, behavior of rotation, gravity, magnetic field, behavior of seismic waves and meteorites.seismic waves and meteorites.
Plate TectonicsPlate Tectonics
Chapter 3 – Earth Structure
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Density is a key concept for understanding the structure of Earth – differences in density lead to stratification (layers).
Density measures the mass per unit volume of a substance.
Density = _Mass_ Volume
Density is expressed as grams per cubic centimeter.
Water has a density of 1 g/cm3
Granite Rock is about 2.7 times more dense
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S waves – Secondary, ‘side-to-side’ or shear waves, S waves – Secondary, ‘side-to-side’ or shear waves, arrive second, arrive second, cannot pass through liquidcannot pass through liquid, pass , pass through solid, oscillate in the direction transverse to through solid, oscillate in the direction transverse to propagationpropagation
P waves – Primary, compressional, arrive first, pass P waves – Primary, compressional, arrive first, pass through solid, liquid and gas, oscillate in the through solid, liquid and gas, oscillate in the direction of propagationdirection of propagation
Waves associated with earthquakes:
Seismograph Minutes
P 0 10 30 40 50
Surface wavesMantle P S
Focus Core
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S
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Evidence that supports the idea that Earth has layers comes from the way seismic waves behave as they encounter different material inside Earth and as the material is either liquid or solid
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Earth’s layers – chemical composition and Earth’s layers – chemical composition and physical propertiesphysical propertiesCoreCore: ~ 3500 km thick, average density 13 g/cm: ~ 3500 km thick, average density 13 g/cm33, , 30% of30% of Earth’s mass and 16% of its volume Earth’s mass and 16% of its volume • Inner coreInner core: radius of 1200 km, primarily Fe & Ni : radius of 1200 km, primarily Fe & Ni @Temp of 4000-5500°C, solid, av. den. 16 g/cm3 @Temp of 4000-5500°C, solid, av. den. 16 g/cm3 • Outer coreOuter core: 2260 km thick, Temp of 3200°C, : 2260 km thick, Temp of 3200°C, liquid (partially melted), viscous, less denseliquid (partially melted), viscous, less denseMantlMantle: 70% Earth’s mass & 80% of its volume, 2866 e: 70% Earth’s mass & 80% of its volume, 2866 km km thick, @ Temp of 100-3200°C, Mg-Fe silicates, solid thick, @ Temp of 100-3200°C, Mg-Fe silicates, solid but but can can flowflow, average density 4.5 g/cm, average density 4.5 g/cm33 Note: inner core may be rotating faster than mantle – can be hotter than the Sun’s surface (more than 6, 500 Note: inner core may be rotating faster than mantle – can be hotter than the Sun’s surface (more than 6, 500 deg C!!)deg C!!)
Earth’s outer layer is the Crust: cool, rigid, thin surfaceEarth’s outer layer is the Crust: cool, rigid, thin surface layer – rocks on crust side are chemically different layer – rocks on crust side are chemically different thanthan rocks on mantle side – separation is called rocks on mantle side – separation is called MohoroviMohoroviččiićć discontinuitydiscontinuity
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Earth’s Crust: cold, brittle
thin layer, 0.4% of Earth’s mass and 1% of its volume
Continental Crust: •Primarily granitic type rock (Na, K, Al, SiO2)•40 km thick on average•Relatively light, 2.7 g/cm3
Oceanic Crust•Primarily basaltic (Fe, Mg, Ca, low SiO2)•7 km thick•Relatively dense, 2.9 g/cm3
cool, solid crust and upper (rigid) mantle “float” and move over hotter, deformable lower mantle
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Lithosphere & Asthenosphere:: More detailed description of Earth’s layered structure according to mechanical behavior of rocks, which ranges from very rigid to deformable
1. lithosphere: rigid surface shell that includes upper mantle and crust (here is where ‘plate tectonics’ work), cool layer
2. asthenosphere: layer below lithosphere, part of the mantle, weak and deformable (ductile, deforms as plates move), partial melting of material happens here, hotter layer
(100 – 200 km)
(200 – 400 km)
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Layer Chemical Properties
Continental Crust Composed primarily of graniteDensity = 2.7 g/cm3
Oceanic Crust Composed primarily of basaltDensity = 2.9 g/cm3
Mantle Composed of silicon, oxygen, iron, and magnesiumDensity = 4.5 g/cm3
Core Composed primarily of ironDensity = 13 g/cm3
Summary Table 1 – Physical Properties
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Summary Table 2 – Composition
Layer Physical Properties
Lithosphere Cool, rigid, outer layer
Asthenosphere Hot, partially melted layer which flows slowly
Mantle Denser and more slowly flowing than the asthenosphere
Outer Core Dense, viscous liquid layer, extremely hot
Inner Core Solid, very dense and extremely hot
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Isostasy
A term used to refer to the state of gravitational equilibrium between the lithosphere and the asthenosphere, which makes the plates (seem like) “float” at an elevation that depends on their thickness and density – areas of Earth’s crust get to this equilibrium after rising and subsiding until their masses are in balance.
Less dense continental blocks “float” on the denser mantle
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Buoyancy: a 10 kg object can float if it lands on a liquid (water) body large enough that the object can displace a volume of liquid that weighs 10 kg and there is still more liquid left
Buoyancy: depends on the mass and density of the object and of the liquid in which object floats
Icebergs: 10% of volume above water, 90% of volume below surface
displaced water
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Isostatic equilibrium: continental mountains float high above sea level because the lithosphere sinks slowly into the deformable asthenosphere until it has displaced a volume of asthenosphere equal to the mass of the mountain’s mass.
Very slow process – if it goes too fast for some reason then the rock will crack (fracture) and a fault occurs, and cause earthquakes
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Isostasy
A term used to refer to the state of gravitational equilibrium between the lithosphere and the asthenosphere, which makes the plates (seem like) “float” at an elevation that depends on their thickness and density – areas of Earth’s crust get to this equilibrium after rising and subsiding until their masses are in balance.
Less dense continental blocks “float” on the denser mantle
can pass through Earth. Analysis of how these waves are changed, and the time required for their passage, has told researchers much about conditions inside Earth.
Earth is composed of concentric spherical layers, with the least dense layer on the outside and the most dense as the core. The lithosphere, the outermost solid shell that includes the crust, floats on the hot, deformable asthenosphere. The mantle is the largest of the layers.
Large regions of Earth’s continents are held above sea level by isostatic equilibrium, a process analogous to a ship floating in water.
Plate motion is driven by slow convection (heat-generated) currents flowing in the mantle. Most of the heat that drives the plates is generated by the decay of radioactive elements within Earth.
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Chapter 3 – Plate Tectonics
Plate Tectonics – a unified model with ideas Plate Tectonics – a unified model with ideas from continental drift and sea floor spreading from continental drift and sea floor spreading
Lithosphere broken into platesLithosphere broken into plates
Plates move Plates move
Boundaries between plates are sites of Boundaries between plates are sites of geologic activitygeologic activity
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Movement of the Continents – Continental DriftMovement of the Continents – Continental Drift
• Continents had once been together Continents had once been together advanced by advanced by Alfred Wegener during the 1920’sAlfred Wegener during the 1920’s• Ultimately rejected – Until Ultimately rejected – Until new technology provided evidence to support his ideas.
Seismographs revealed a pattern of volcanoes and earthquakes.Radiometric dating of rocks revealed a surprisingly young oceanic crust.Echo sounders revealed the shape of the Mid-Atlantic Ridge
Plate Tectonics
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The Fit between the Edges of Continents Suggested That They Might Have Drifted
The fit (first noticed by Leonardo da Vinci) of all the continents around the Atlantic at a water depth of about 137 meters (450 feet), as calculated in the 1960s. This well-known graphic was a very effective kick-off to the ‘tectonic revolution’.
Age of Earth was not easily determined, nor accepted as ‘that old’!
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Evidence for Seafloor Spreading
• Earthquake epicenters• Heat flow• Radiometric dating of rocks of ocean
and continental crust• Magnetism
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Figure 2.13
Age of sea floor vs. distance from ridge crest
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Age and thickness of sea floor sediment
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Earthquake EpicentersEarthquake Epicenters
Shallow epicenters – crustal movement(less than 100 km)
Mid-deep epicenterssubduction(greater than 100 km)
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Lithosphere
A plate is the cooled surface layer of a convection current in upper mantle.
Heat source
heated water rises, cools at the surface and falls around the container’s edge
heat transfer: conduction (contact)
convection (motion of an agent, currents)
tectonic plate is the cool surface, the result of a convection current rising from the (hot) upper mantle (spreading center) – as it cools it becomes denser so gravity ‘pulls’ it down (subduction zone)
Mantle Convection
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Model of Mantle Convection
Improved Mapping, WWII
spreading centers – where new sea floor and oceanic lithosphere form
subduction zones – where old oceanic lithosphere descends
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Divergent plate boundary marked by mid-ocean ridge (spreading center)
Asthenosphere
Transform fault
Oceanic lithosphereSubduction fueling volcanoes
AsiaAfrica
Mantle upwelling
Descending plate pulled down by gravitySuperplume
Philippine TrenchOuter core Mariana Trench
MantleMid-Atlantic
Ridge Inner core
Hot
Rapid convection
at hot spotsPossible
convection cells
ColdSouth America
Peru–Chile Trench
Hawaii
East Pacific Rise
Convergent plate boundary marked by trench
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Plate Tectonics:
Synthesis of Continental Drift and Seafloor Synthesis of Continental Drift and Seafloor Spreading --> Theory of Plate TectonicsSpreading --> Theory of Plate TectonicsMain points of theory (Wilson, 1965):Main points of theory (Wilson, 1965):
Earth’s outer layer is divided into lithospheric plateEarth’s plates float on the asthenospherePlate movement is powered by convection currents in the asthenosphere seafloor spreading, and the downward pull of a descending plate’s leading edge.
Hess and Dietz in 1960 proposed a model to Hess and Dietz in 1960 proposed a model to explain features of ocean floor and of explain features of ocean floor and of continental motion powered by heat continental motion powered by heat mantle mantle convectionconvection
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Chapter 3 – Plate Tectonics
Plate Tectonics – a unified model with ideas Plate Tectonics – a unified model with ideas from continental drift and sea floor spreading from continental drift and sea floor spreading
Lithosphere broken into platesLithosphere broken into plates
Plates move Plates move
Boundaries between plates are sites of Boundaries between plates are sites of geologic activitygeologic activity
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Seven major plates – Pacific, African, Eurasian, North American, Seven major plates – Pacific, African, Eurasian, North American, Antarctic, South American, Australian Antarctic, South American, Australian
Minor plates – Nazca, Indian, Arabian, Philippine, Caribbean, Minor plates – Nazca, Indian, Arabian, Philippine, Caribbean, Cocos, Scotia, Juan de FucaCocos, Scotia, Juan de Fuca
Plates Rigid Slabs of Rock
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Plate boundaries in action: (1) plates move apart, (2) plates move toward each other, (3) plates move past each other
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Extension of divergent boundaries causes splitting and rifting.
Compression at convergent boundaries produces buckling and shortening
Translation at transform boundaries causes shear.
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As plates float on the deformable As plates float on the deformable aesthenosphere, they interact aesthenosphere, they interact among each other. The result of among each other. The result of these interactions is the existence these interactions is the existence of 3 types of boundaries:of 3 types of boundaries:
• Divergent: plates move away from each other, examples:
* Divergent oceanic crust: the Mid-Atlantic Ridge * Divergent continental crust: the Rift Valley of East Africa (b) Convergent: plates move toward
each other. Three possible combinations: continent-ocean, ocean-ocean, continent-continent
(c) Transform: neither (a) nor (b), plates slide past one another – transform faults. * Example: San Andreas fault
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Fracture Zones-Transform faults
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Oceans are created along divergent boundaries
Recall that seafloor spreading was an idea proposed in 1960 to explain the features of the ocean floor. It explained the development of the seafloor at the Mid-Atlantic Ridge. Convection currents in the mantle were proposed as the force that caused the ocean to grow and the continents to move.
The breakdown of Pangea showing spreading centers and mid-ocean ridges
2 kinds of plate divergences
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Modern divergenceEast African Rift System
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East African Rift System
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Island Arcs Form, Continents Collide, and Island Arcs Form, Continents Collide, and Crust Recycles at Convergent Plate Crust Recycles at Convergent Plate BoundariesBoundariesConvergent Plate Boundaries - Regions where plates are pushing together can be further classified as:
• Oceanic crust toward continental crust - the west coast of South America.
• Oceanic crust toward oceanic crust - occurring in the northern Pacific.
• Continental crust toward continental crust – one example is the Himalayas.
3 kinds of plate convergences
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• Continent – OceanWest Coast of S. America
• Ocean – OceanCaribbean Island
Continent – Continent
Convergent Plate Boundaries
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• Continent – Ocean
• Mount St. Helens
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Island Arcs Form, Continents Collide, and Island Arcs Form, Continents Collide, and Crust Recycles at Convergent Plate Crust Recycles at Convergent Plate BoundariesBoundaries
The formation of an island arc along a trench as two oceanic plates converge. The volcanic islands form as masses of magma reach the seafloor. The Japanese islands were formed in this way.
Motion of the platesMotion of the plates:: Mechanisms – not fully understoodMechanisms – not fully understood Rates: average 5 cm/yearRates: average 5 cm/year Mid-Atlantic Ridge = 2.5 – 3.0 Mid-Atlantic Ridge = 2.5 – 3.0 cm/yrcm/yr East-Pacific Rise = 8.0 – 13.0 East-Pacific Rise = 8.0 – 13.0 cm/yrcm/yr
• Rocks record the direction of magnetic field (Magnetite)
• Magnetic field direction changes through geologic time – polar reversals recorded in rocks
* 560 °C = rock solidifies (Curie Point)* Captures magnetic signature
• Particles of Magnetite align with the direction of Earth’s magnetic field at the time of rock formation. Magnetites occur naturally in basaltic magma and act as compass needles
Many discoveries contribute to the theory of plate tectonics but the most compelling evidence comes from The Earth’s Magnetic Field
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The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes.
(a) When scientists conducted a magnetic survey of a spreading center, the Mid-Atlantic Ridge, they found bands of weaker and stronger magnetic fields frozen in the rocks. (b) The molten rocks forming at the spreading center take on the polarity of the planet when they are cooling and then move slowly in both directions from the center. When Earth’s magnetic field reverses, the polarity of new-formed rocks changes, creating symmetrical bands of opposite polarity
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Plate Movement above Mantle Plumes Plate Movement above Mantle Plumes and Hot Spots Provides Evidence of and Hot Spots Provides Evidence of Plate TectonicsPlate Tectonics
Formation of a volcanic island chain as an oceanic plate moves over a stationary mantle plume and hot spot. In this example, showing the formation of the Hawai’ian Islands, Loihi is such a newly forming island.
(See also Figure 3.33 on page 89 of textbook)
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Chapter 3: Key ConceptsChapter 3: Key Concepts
Plate tectonics theory suggests that Earth’s surface is not a static arrangement of continents and ocean, but a dynamic mosaic of jostling segments called lithospheric plates. The plates have collided, moved apart, and slipped past one another since Earth’s crust first solidified.
The confirmation of plate tectonics rests on diverse scientific studies from many disciplines. Among the most convincing is the study of paleomagnetism, the orientation of Earth’s magnetic field frozen into rock as it solidifies.
Most of the large-scale features seen at Earth’s surface may be explained by the interactions of plate tectonics. Plate tectonics also explains why our ancient planet has surprisingly young seafloors, the oldest of which is only as old as the dinosaurs – that is, about 1/23 of the age of Earth.
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Chapter 3: SummaryChapter 3: Summary
Keep in mind that the important points in this chapter Keep in mind that the important points in this chapter are:are:
2.2. P and S waves – used to study Earth’s layered structure P and S waves – used to study Earth’s layered structure 3.3. Lithosphere and Asthenosphere – defined according to Lithosphere and Asthenosphere – defined according to
mechanical behavior of rocksmechanical behavior of rocks4.4. Isostasy – pressure balance between overlying crust and Isostasy – pressure balance between overlying crust and
astheosphere deformationastheosphere deformation5.5. Continental drift – plates/continents moving about surface; Continental drift – plates/continents moving about surface;
deduced from definitive evidence: ridges, rise, trench system, deduced from definitive evidence: ridges, rise, trench system, sea-floor spreading, spreading centers, subduction zonessea-floor spreading, spreading centers, subduction zones
