Chapter 7 Plate Tectonics Underlies All Earth History.

Chapter 7 Chapter 7

Plate Tectonics Underlies Plate Tectonics Underlies All Earth HistoryAll Earth History

EarthquakesEarthquakes

Earthquake = vibration of the Earth produced by the rapid release of energy.

Seismic Waves Seismic Waves

• Focus = the place within the Earth where the rock breaks, producing an earthquake.

• Epicenter = the point on the ground surface directly above the focus.

• Energy moving outward from the focus of an earthquake travels in the form of seismic waves.

Types of Seismic Waves Types of Seismic Waves

1. Body waves

a. P-waves

b. S-waves

2. Surface waves


1. Body waves a. P-waves

Primary, pressure, push-pullFastest seismic wave (6 km/sec in crust; 8 km/sec in uppermost mantle)Travel through solids and liquids

b. S-waves2. Surface waves


1. Body waves a. P-wavesb. S-waves

Secondary, shaking, shear, side-to-sideSlower (3.5 km/sec in crust; 5 km/sec in upper mantle km/sec)Travel through solids only

2. Surface waves


1. Body waves a. P-wavesb. S-waves

2. Surface wavesL-waves or long wavesSlowestComplex motion – Up-and-down and side-to-sideCauses damage to structures during an earthquake

Seismogram showing Seismogram showing Seismic Wave ArrivalsSeismic Wave Arrivals

SeismographsSeismographs

• Earthquakes are recorded on an instrument called a seismograph.

• The record of the earthquake produced by the seismograph is called a seismogram.

Earth's Internal StructureEarth's Internal Structure

Determining the Earth's Determining the Earth's Internal Structure Internal Structure

Earth has a layered structure. Boundaries between the layers are called

discontinuities. – Mohorovicic discontinuity (Moho)

between crust and mantle(Named for discoverer, Yugoslavian seismologist Andrija Mohorovicic)

– Gutenberg discontinuity between mantle and core

Determining the Earth's Determining the Earth's Internal StructureInternal Structure

The layered structure is determined from studies of how seismic waves behave as they pass through the Earth.P- and S-wave travel times depend on properties of rock materials through which they pass.Differences in travel times correspond to differences in rock properties.


• Seismic wave velocity depends on the density and elasticity of rock.

• Seismic waves travel faster in denser rock.

• Speed of seismic waves increases with depth (pressure and density increase downward).


Curved wave paths indicate gradual increases in density and seismic wave velocity with depth. Refraction (bending of waves) occurs at discontinuities between layers.

S-wave Shadow ZoneS-wave Shadow Zone

Place where no S-waves are received by seismograph. Extends across the globe on side opposite from the epicenter.

S-waves cannot travel through the molten (liquid) outer core.Larger than the P-wave shadow zone.

P-wave Shadow ZoneP-wave Shadow Zone

Place where no P-waves are received by seismographs.

Makes a ring around the globe.Smaller than the S-wave shadow zone.

The Earth's Internal The Earth's Internal Layered Structure Layered Structure

• Crust

• Mantle

• Outer core

• Inner core

CrustCrust

• Continental Crust (granitic)

• Oceanic Crust (basaltic)

Continental Crust Continental Crust

• Granitic composition

• Averages about 35 km thick; 60 km in mountain ranges

• Less dense (about 2.7 g/cm3).

Oceanic CrustOceanic Crust• Basaltic composition• 5 - 12 km thick• More dense (about 3.0 g/cm3)• Has layered structure consisting of:

– Thin layer of unconsolidated sediment covers basaltic igneous rock (about 200 m thick)

– Pillow basalts - basalts that erupted under water (about 2 km thick)

– Gabbro - coarse grained equivalent of basalt; cooled slowly (about 6 km thick)

Lithosphere Lithosphere

Lithosphere = outermost 100 km of Earth. Consists of the crust plus the outermost part of the mantle. Divided into tectonic or lithospheric plates that cover surface of Earth

Asthenosphere Asthenosphere

• Asthenosphere = low velocity zone at 100-250 km depth in Earth (seismic wave velocity decreases).

• Rocks are at or near melting point.

• Magmas generated here.

• Solid that flows (rheid); plastic behavior.

• Convection in this layer moves tectonic plates.

Isostasy Isostasy

• Buoyancy and floating of the Earth's crust on the mantle.

• Denser oceanic crust floats lower, forming ocean basins.

• Less dense continental crust floats higher, forming continents.

• As erosion removes part of the crust, it rises isostatically to a new level.

The Earth's The Earth's Internal Internal Layered Layered StructureStructure

Mantle Mantle

• Composed of oxygen and silicon, along with iron and magnesium (based on rock brought up by volcanoes, density calculations, and composition of stony meteorites). – Peridotite (Mg Fe silicates, olivine) – Kimberlite (contains diamonds) – Eclogite

• 2885 km thick• Average density = 4.5 g/cm3 • Not uniform. Several concentric layers with

differing properties.

Core Core

• Outer core– Molten Fe (85%) with some Ni. May contain

lighter elements such as Si, S, C, or O. – 2250 km thick – Liquid. S-waves do not pass through outer core.

• Inner core – Solid Fe (85%) with some Ni – 1220 km radius (slightly larger than the Moon)– Solid

Core and Magnetic FieldCore and Magnetic Field

• Convection in liquid outer core plus spin of solid inner core generates Earth's magnetic field.

• Magnetic field is also evidence for a dominantly iron core.

Crustal Structures Crustal Structures

FaultsFaults

• A fault is a crack in the Earth's crust along which movement has occurred.

• Types of faults: – Dip-slip faults - movement is vertical

• Normal faults • Reverse faults and thrust faults

– Strike-slip faults or lateral faults - movement is horizontal.

FaultsFaults

Normal FaultsNormal Faults

FoldsFolds

• During mountain building or compressional stress, rocks may deform plastically to produce folds.

• Types of folds– Anticline– Syncline– Monocline– Dome– Basin

FoldsFolds

A. Anticline

B. Syncline

C. Monocline

D. Dome

E. Basin

AnticlineAnticline

SynclineSyncline

Plate Tectonics Plate Tectonics

Plate Tectonics Plate Tectonics

Plate Tectonic theory was proposed in late 1960's and early 1970's. It is a unifying theory showing how a large number of diverse, seemingly-unrelated geologic facts are interrelated. A revolution in the Earth Sciences. An outgrowth of the old theory of "continental drift", supported by much data from many areas of geology.

The Data Behind Plate Tectonics The Data Behind Plate Tectonics

Geophysical data collected after World War II provided foundation for scientific breakthrough:

• Echo sounding for sea floor mapping discovered patterns of midocean ridges and deep sea trenches.

• Magnetometers charted the Earth's magnetic field over large areas of the sea floor.

• Global network of seismometers (established to monitor atomic explosions) provided information on worldwide earthquake patterns.

Evidence in Support of the Theory Evidence in Support of the Theory of Plate Tectonicsof Plate Tectonics

1. Shape of the coastlines - the jigsaw puzzle fit of Africa and South America.


2. Paleoclimatic evidence - Ancient climatic zones match up when continents are moved back to their past positions.

• Glacial tillites• Glacial striations• Coal deposits• Carbonate deposits• Evaporite deposits


3. Fossil evidence implies once-continuous land connections between now-separated areas

Image from U.S. Geological Survey


4. Distribution of present-day organisms indicates that they evolved in genetic isolation on separated continents (such as Australian marsupials).


5. Geologic similarities between South America, Africa, and India

– Same stratigraphic sequence (same sequence of layered rocks of same ages in each place)

– Mountain belts and geologic structures (trends of folded and faulted rocks line up)

– Precambrian basement rocks are similar in Gabon (Africa) and Brazil.


6. Geologic similarities between Appalachian Mountains and Caledonian Mountains in British Isles and Scandinavia.


7. Rift Valleys of East Africa indicate a continent breaking up.

Image from U.S. Geological Survey


8. Evidence for subsidence in oceans– Guyots - flat-topped sea mounts (erosion

when at or above sea level). – Chains of volcanic islands that are older

away from site of current volcanic activity -Hawaiian Islands and Emperor Sea Mounts (also subsiding as they go away from site of current volcanic activity).


9. Mid-ocean ridges are sites of sea floor spreading. They have the following characteristics:

– High heat flow. – Seismic wave velocity decreases at the

ridges, due to high temperatures. – A valley is present along the center of ridge. – Volcanoes are present along the ridge. – Earthquakes occur along the ridge.


10. Paleomagnetism and Polar Wandering Curves.

The Earth's magnetic field behaves as if there were a bar magnet in the center of the Earth

Paleomagnetism and Polar Paleomagnetism and Polar Wandering Curves Wandering Curves

• As lava cools on the surface of the Earth, tiny crystals of magnetite form.

• When the lava cools to a certain temperature, known as the Curie point, the crystals become magnetized and aligned with Earth's magnetic field.

• The orientation of the magnetite crystals records the orientation of the Earth's magnetic field at that time.


• As tiny magnetite grains are deposited as sediment, they become aligned with Earth's magnetic field.

• The grains become locked into place when the sediment becomes cemented.


The orientation of Earth's magnetic field is described by inclination and declination.

InclinationInclination

Inclination = the angle of the magnetic field with respect to the horizontal (or the dip of the magnetic field). – Inclination = 90o at North magnetic pole– Inclination = 0o at the equator

Inclination can be used to determine the latitude at which a lava body cooled, or at which sedimentary grains were deposited.

DeclinationDeclination

Declination = the angle between where a compass needle points (magnetic north) and the true geographic north pole (axis of the Earth).

Apparent Polar WanderingApparent Polar Wandering

• Paleomagnetic data confirm that the continents have moved continuously.

• When ancient magnetic pole positions are plotted on maps, we can see that they were in different places, relative to a continent, at different times in the past.

• This is called apparent polar wandering. The poles have not moved. The continents have moved.

Apparent Polar WanderingApparent Polar Wandering

• Different polar wandering paths are seen in rocks of different continents.

• Put continents back together (like they were in the past) and the polar wandering curves match up.

The lithosphere is divided into plates The lithosphere is divided into plates (about 7 large plates and 20 smaller ones).(about 7 large plates and 20 smaller ones).

Lithosphere and AsthenosphereLithosphere and Asthenosphere

• Lithosphere = rigid, brittle crust and uppermost mantle.

• Asthenosphere = partially molten part of upper mantle, below lithosphere.

• Rigid lithospheric plates "float" on flowing asthenosphere.

• Convection in asthenosphere moves tectonic plates.

Two types of crust are present in the upper part of the lithosphere:

1. Oceanic crust - thin, dense, basaltic

2. Continental crust - thick, low density, granitic

Types of plate boundaries Types of plate boundaries

• Divergent - The plates move apart from one another. New crust is generated between the diverging plates.

• Convergent - The plates move toward one another and collide. Crust is destroyed as one plate is pushed beneath another.

• Transform - The plates slide horizontally past each other. Crust is neither produced nor destroyed.

Divergent Plate Boundaries Divergent Plate Boundaries

• Plates move apart from one another

• Tensional stress

• Rifting occurs

• Normal faults

• Igneous intrusions, commonly basalt, forming new crust

Seafloor Spreading at Seafloor Spreading at Divergent Plate BoundaryDivergent Plate Boundary

Convergent Plate Boundaries Convergent Plate Boundaries

• Plates move toward one another

• Compressional stress

• Continental collision

• Subduction

Convergent Plate BoundariesConvergent Plate Boundaries

A. Continental collision

B. Subduction

Continental Collision Continental Collision

• Continental collisions form mountain belts with:– Folded sedimentary rocks– Faulting– Metamorphism – Igneous intrusions

• Slabs of continental crust may override one another

• Suture zone = zone of convergence between two continental plates

Subduction Subduction

• An oceanic plate is pushed beneath another plate, forming a deep-sea trench.

• Rocks and sediments of downward-moving plate are subducted into the mantle and heated.

• Partial melting occurs. Molten rock rises to form:– Volcanic island arcs– Intrusive igneous rocks

Ocean-to-Ocean Subduction Ocean-to-Ocean Subduction

An oceanic plate is subducted beneath another oceanic plate, forming a deep-sea trench, with an associated basaltic volcanic island arc.

Image courtesy of U.S. Geological Survey

Ocean-to-Continent Subduction Ocean-to-Continent Subduction An oceanic plate is subducted beneath a continental plate, forming a trench adjacent to a continent, and volcanic mountains along the edge of the continent.

Image courtesy of US Geological Survey

Ocean-to-Continent Ocean-to-Continent Subduction Zone Includes:Subduction Zone Includes:

1. Accretionary prism or accretionary wedge - Highly contorted and metamorphosed sediments that are scraped off the descending plate and accreted onto the continental margin.

2. Mélange - A complexly folded jumble of deformed and transported rocks.

Ocean-to-Continent Ocean-to-Continent Subduction Zone Includes:Subduction Zone Includes:

3. Ophiolite suite - Piece of descending oceanic plate that was scraped off and incorporated into the accretionary wedge. Contains:– Deep-sea sediments– Submarine basalts (pillow lavas) – Metamorphosed mantle rocks (serpentinized

peridotite) 4. Blueschists – metamorphic minerals

(glaucophane and lawsonite) indicating high pressures but low temperatures.

Transform Plate Boundaries Transform Plate Boundaries

• Plates slide past one another

• Shear stress

• Transform faults cut across and offset the mid-ocean ridges

• A natural consequence of horizontal spreading of seafloor on a curved globe

• Example: San Andreas Fault

Types of Transform FaultsTypes of Transform Faults

Because seafloor spreads outward from mid-ocean ridge, relative movement between offset ridge crests is opposite of that in ordinary strike-slip faults. Note arrows showing direction of movement.

Plate BoundariesPlate Boundaries

Red = Midoceanic ridges Blue = Deep-sea trenches

Black = Transform faults

Wilson Cycles Wilson Cycles

Plate tectonic model for opening and closing of an ocean basin over time.

1. Opening of new ocean basin at divergent plate boundary

2. Seafloor spreading continues and subduction begins

3. Final stage of continental collision

Wilson CyclesWilson Cycles

1. Opening of a new ocean basin at a divergent plate boundary.

Sedimentary deposits include:a. Quartz sandstones

b. Shallow-water platform carbonates

c. Deeper water shales with chert


2. Expansion of ocean basin as seafloor spreading continues and subduction begins. Sedimentary deposits include:

a. Graywacke

b. Turbidites

c. Volcanic rocks

Also mélange, thrust faults, and ophiolite sequences near the subduction zone.


3. Final stage of continental collision.

Sedimentary deposits include: – Conglomerates – Red sandstones– Shales

Deposited in alluvial fans, rivers, and deltas as older seafloor sediments are uplifted to form mountains, and eroded.

What Forces Drive Plate Tectonics?What Forces Drive Plate Tectonics?

The tectonic plates are moving, but with varying rates and directions.

What hypotheses have been proposed to explain the plate motion? – Convection Cells in the Mantle– Ridge-Push and Slab-Pull Model – Thermal Plumes

Convection Cells in the MantleConvection Cells in the Mantle

• Large-scale thermal convection cells in the mantle may move tectonic plates.

• Convection cells transfer heat in a circular pattern. Hot material rises; cool material sinks.

• Mantle heat probably results from radioactive decay.

Image courtesy of U.S. Geological Survey.

Convection Cells in the MantleConvection Cells in the Mantle

Rising part of convection cell = rifting (mid-ocean ridge)

Descending part of convection cell = subduction (deep sea trench)

Image courtesy of U.S. Geological Survey.

Ridge-Push and Slab-Pull ModelRidge-Push and Slab-Pull Model

Crust is heated and expands over a mid-ocean ridge spreading center. Crust tends to slide off the thermal bulge, pushing the rest of the oceanic plate ahead of it.

This is called ridge-push.

Ridge-Push and Slab-Pull ModelRidge-Push and Slab-Pull Model

Near subduction zones, oceanic crust is cold and dense, and tends to sink into the mantle, pulling the rest of the oceanic plate behind it.

This is referred to as slab-pull.

Thermal PlumesThermal Plumes

• Thermal plumes are concentrated areas of heat rising from near the core-mantle boundary. Hot spots are present on the Earth's surface above a thermal plume.

• The lithosphere expands and domes upward, above a thermal plume. The uplifted area splits into three radiating fractures, forming a triple junction. Rifting occurs, and the three plates move outward away from the hot spot.


A triple junction over a thermal plume. Afar Triangle.


• Thermal plumes do not all produce triple junctions.

• Hot spots are present across the globe. If the lava from the thermal plume makes its way to the surface, volcanic activity may result.

• As a tectonic plate moves over a hot spot (at a rate as high as 10 cm per year), a chain of volcanoes is formed.

Map of Major Hotspots Map of Major Hotspots

Verifying Plate Tectonic Theory Verifying Plate Tectonic Theory

Paleomagnetic Evidence Paleomagnetic Evidence

• Magnetic reversals have occurred relatively frequently through geologic time.

• Recently magnetized rocks show alignment of magnetic field consistent with Earth's current magnetic field.

• Magnetization in older rocks has different orientations (as determined by magnetometer towed by a ship).

Paleomagnetic Evidence Paleomagnetic Evidence

Magnetic stripes on the sea floor are symmetrical about the mid-ocean ridges (Vine and Matthews, 1963).

Paleomagnetic EvidencePaleomagnetic Evidence

Normal (+) and reversed (-) magnetization of the seafloor about the mid-ocean ridge. Note the symmetry on either side of the ridge.

Magnetic Reversal Time Scale Magnetic Reversal Time Scale

Reversals in sea floor basalts match the reversal time scale determined from rocks exposed on land. Continental basalts were dated radiometrically and correlated with the oceanic basalts.Using this method, magnetic reversals on the sea floor were dated.

Calculating Rates of Seafloor Calculating Rates of Seafloor Spreading Spreading

• Width of magnetic stripes on sea floor is related to time.

• Wide stripes = long time

• Narrow stripes = short time

• Knowing the age of individual magnetic stripes, it is possible to calculate rates of seafloor spreading and former positions of continents.

Rates of Seafloor Spreading Rates of Seafloor Spreading

The velocity of plate movement varies around the world.

• Plates with large continents tend to move more slowly (up to 2 cm per year).

• Oceanic plates move more rapidly (averaging 6-9 cm per year).

Youth of Ocean Basins Youth of Ocean Basins and Sea Floor and Sea Floor

• Only a thin layer of sediment covers the sea floor basalt.

• Sea floor rocks date to less than 200 million years (most less than 150 million years).

• No seafloor rocks are older than 200 million years.

Measurement of Plate Tectonics Measurement of Plate Tectonics from Space from Space

• Lasers

• Man-made satellites in orbit around Earth - Global Positioning System

• By measuring distances between specific points on adjacent tectonic plates over time, rates of plate movement can be determined.

Seismic Evidence for Seismic Evidence for Plate Tectonics Plate Tectonics

• Inclined zones of earthquake foci dip at about a 45o angle, near a deep-sea trench. Benioff Zones, (or Wadati-Benioff Zones).

• The zone of earthquake foci marks the movement of the subducting plate as it slides into the mantle.

• The Benioff Zone provides evidence for subduction where one plate is sliding beneath

another, causing earthquakes.

Gravity Evidence Gravity Evidence

• A gravity anomaly is the difference between the calculated theoretical value of gravity and the actual measured gravity at a location.

• Strong negative gravity anomalies occur where there is a large amount of low-density rock beneath the surface.

• Strong negative gravity anomalies associated with deep sea trenches indicate the location of less dense oceanic crust rocks being subducted into the denser mantle.

Gravity EvidenceGravity Evidence

Negative gravity anomaly associated with a deep sea trench. Sediments and lower density rocks are subducted into an area that would otherwise be filled with denser rocks. As a result, the force of gravity over the subduction

zone is weaker than normal.

Thermal Plumes, Thermal Plumes, Hot Spots and HawaiiHot Spots and Hawaii

• Volcanoes develop over hot spots or thermal plumes.

• As the plate moves across the hot spot, a chain of volcanoes forms.

• The youngest volcano is over the hot spot.• The volcanoes become older away from the

site of volcanic activity.• Chains of volcanic islands and underwater sea

mounts extend for thousands of km in the Pacific Ocean.


A new volcano, Lo'ihi, is forming above the hot spot, SE of the island of Hawaii. The Hawaiian islands are youngest near the hot spot, and become older to the NW.


This chain of volcanoes extends NW past Midway Island, and then northward as the Emperor Seamount Chain. The volcanic trail of the Hawaiian hot spot is 6000 km long. A sharp bend in the chain indicates a change in the direction of plate motion about 43 million years ago.

Exotic Terranes Exotic Terranes

• Small pieces of continental crust surrounded by oceanic crust are called microcontinents.

• Examples: Greenland, Madagascar, the Seychelles Bank in the Indian Ocean, Crete, New Zealand, New Guinea.

Exotic Terranes Exotic Terranes

Microcontinents are moved by seafloor spreading, and may eventually arrive at a subduction zone.

They are too low in density and too buoyant to be subducted into the mantle, so they collide with (and become incorporated into the margin of) a larger continent as an exotic terrane.

Exotic TerranesExotic Terranes

Exotic terranes are present along the margins of every continent.

They are fault-bounded areas with different structure, age, fossils, and rock type, compared with the surrounding rocks.

Exotic TerranesExotic Terranes

• Green terranes probably originated as parts of other continents.

• Pink terranes may be displaced parts of North America.

• The terranes are composed of Paleozoic or older rocks accreted during the Mesozoic and Cenozoic Eras.

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Chapter 7 Plate Tectonics Underlies All Earth History.

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