Plate Tectonics

Earth’s Internal Structure

When a meteorite impacts a planet or moon, its energy of motion (called kinetic energy) is transformed into heat energy.As Earth grew larger and larger from continual impacts, its temperature increased.Radioactive decay of materials like uranium, thorium and potassium also added heat.Because Earth became partly fluid, less-dense molten materials (silicon, aluminum, sodium, and potassium) were freed to migrate toward the surface.Denser melted materials, such as molten iron, sank toward the center of the planet.

The Earth’s Interior

Planet Earth has three main parts: At the center is the densest part, the core

(metallic iron, nickel). Surrounding the core is the mantle. Surrounding the mantle lies the thinnest

and outermost layer, the crust.

Investigating the Earth’s Interior

How do we know anything about the composition of the crust, core and the mantle?

For example, the crust is not uniform.The oceanic crust on average is about 8 km thick.

The continental crust on average is about 45 km thick.

Answers from measuring the time required for earthquake waves to travel through Earth by different paths, we can determine the structure and composition of the materials through which they move.

The Layers of the Earth’s Interior (1)

The inner corePressures are so great that iron is solid, despite its high temperature.

The outer coreIron is molten and exists as a liquid.

The Mesosphere The mantle between the bottom of the asthenosphere to the core-mantle boundary.

The temperature at the core-mantle boundary is about 50000C.

The Layers of the Earth’s Interior (2)

The Asthenosphere: The region of the mantle where rocks become ductile, have little strength, and are easily deformed. It lies at a depth of 100 to 350 km below the surface.

The Lithosphere: The outer 100 km of the solid Earth, where rocks are harder and more rigid than those in the plastic asthenosphere.

Plate Tectonics

The Earth gets rid of heat and keeps a nearly constant internal temperature through convection in the mesosphere and asthenosphere.Plate tectonics is a group of processes by which large fragments (plates) of lithosphere move horizontally across the surface of the Earth. Through their movements and interactions, they generate:

Earthquakes. Volcanism.Mountain-building.

The next series of slides demonstrate a number of basic observations, in support of plate tectonics

Isostasy

The principle of isostasy governs the rise or subsidence of the crust until mass is buoyantly balanced.

Because of isostasy, all parts of the lithosphere are in a floating equilibrium.

As an in-class example, low-density Styrofoam chunks float high whereas higher-density wood blocks float low.

The continents average about 4.5 km elevation above the ocean floor.

They stand notably higher than the ocean basins because the thick continental crust is relatively light (average density 2.7 g/cm3).

The thin oceanic crust is relatively heavy (average density 3.0g/cm3).

The lithosphere floats on the asthenosphere.

Isostasy: Why Some Rocks Float Higher Than Others

Global Positioning System

In the 1960s, the U.S. Department of Defense established a network of satellites with orbits that could be used for reference in precisely determining location.

The Global Positioning System (GPS) detects small movements of the Earth’s surface.

Global Positioning System

It is accurate within a few millimeters.

Two measurement methods: A GPS campaign: researchers establish a network

of fixed reference points on Earth’s surface, often attached to bedrock. The position is re-measured every few months or years.

Continuous GPS measurement: the receivers are attached permanently to monuments, and position is estimated at fixed intervals of a few seconds or minutes.

Earthquake locations and focal depths: EQs do not occur in random locations

Fig. 2.26b

W. W. Norton

Basic Observation: Magnetic stripes

Fig. 2.26a

W. W. Norton

Magnetic Stripes

The Earth has a magnetic field that experiences polarity reversals in the geologic past.

The only way this polarity reversal and mapping on the oceanic crust can occur is by mid-ocean ridge spreading.

Movie 1 Magnetic Reversals

Summary of Basic Observations

From basic cosmology the Earth has a hot interior that has kept its heat through radioactive decayFrom earthquake seismic wave travel times, the earth has a basic interior structureFrom isostatic and buoyancy principles, continents float higher than oceansFrom GPS, the Earth has a series of rigid exterior portions that move coherently with little internal deformationFrom earthquake locations, the boundaries of these rigid portions deform in a brittle fashion (e.g., silly putty vs. wood block)

The only logical solution to all of the observations is plate tectonics with interior convectionconvection to transport the heat to the surface and thus drive plate motions.

Mantle Convection

ConductionConduction is the process by which heat moves through solid rock, but rocks are poor conductors of heat, so Earth moves its internal heat by moving the rock itself.Earth’s heat can move in a second process called convection.

Convection can happen in gases, in liquids, or, given enough time, in ductile solids.A prerequisite condition for mantle convection is the thermal expansion of hot rock.

Mantle Convection

Rock expands as its temperature increases.

Its density thereby decreases slightly.

The hot rock is buoyant relative to cooler rock in its immediate neighborhood.

A 1 percent expansion requires an increase of A 1 percent expansion requires an increase of 300-400300-400ooC and leads to a 1 percent decrease in C and leads to a 1 percent decrease in density.density.

Viscosity is the propensity of rock to ductile flow.

Mantle Convection

Rock does not need to melt before it can flow.The presence of HThe presence of H22O encourages flow in solid O encourages flow in solid rock.rock.Convection currents bring hot rocks upward from Earth’s interior.The rock in the lithosphere is too cool for convection to continue.Heat moves through the lithosphere primarily by conduction.The lithosphere-asthenosphere boundary is 1300-1350oC, depending on depth.

Earth’s Convection: Driven From the Top

Below the lithosphere, rock masses in the deeper mantle rise and fall according to differences in temperature and buoyancy.

The densest lithosphere is most likely to sink back into the asthenosphere and the deeper mantle.

Ocean floor and the continents are slowly moving (up to 12 cm/yr).(up to 12 cm/yr).

Old, cold, more dense, thus less buoyant = sinks after 70 M years

New, hot, less dense, thus more buoyant = magma rises from

Plates have four kinds of boundaries or margins

Divergent margin (also called a spreading center): magma rises to form new oceanic crust between the two pieces of the original plate. As it ascends, the rock experiences a decrease in pressure and partially melts.Convergent margin/subduction zone: two plates move toward each other and one sinks beneath the other. For oceanic crust, the older lithosphere sinks beneath the edge of an adjacent plate.Convergent margin/collision zone: two colliding continental plates create a mountain range.Transform fault margin: two plates slide past each other, grinding and abrading their edges.

Do not confuse plate boundary types (left) with fault types (right). Plate boundaries occur in specific locations and can have all three types of faults (e.g., the Himalayan continent-continent collision has all three fault types). Faults, however, can occur in the interior of plates.

Movie 2 Basic Plate Boundaries

Earthquakes at midocean ridges occur only in the first 10 km beneath the seafloor and tend to be small.Normal faults form parallel lines along the rifted margin.Volcanic activity occurs at midocean ridges and continental rifts (along narrow parallel fissures).The midocean ridges rise 2 km or more above surrounding seafloor.

Characteristics of Spreading CentersSpreading Centers

Characteristics of Spreading CentersSpreading Centers

The principle of isostasy applies: lower-density rock rises to form a higher elevation.

The seafloor around midocean ridges for nearly all oceanic lithosphere is younger than 70 million years.

Movie 3 Motion of the ContinentsMovie 4 Sea Floor SpreadingMovie 5 Process of Rifting

Convergent MarginSubduction Zone

Over 70 million years, oceanic lithosphere can drift 1500 to 3000 km from the spreading center.

As the plate cools, it grows denser.

The principle of isostasy demands that the plate subsides as it grows denser.

The process by which lithosphere sinks into the asthenosphere is called subduction.

The margins along which plates are subducted are called subduction zones.

These are active continental margins.

The sinking slab warms, softens, and exchanges material with the surrounding mantle.The hot mantle rock immediately above the sinking slab starts to melt.

Magma rises to the surface to form volcanoes.Subduction zones are marked by an arc of volcanoes parallel to the edge of the plate.

Convergent MarginSubduction Zone

Movie 6 The Process of Subduction

Earthquakes in Subduction Zones

The largest and the deepest earthquakes occur in subduction zones.

The location of most earthquakes define the top surface of a slab as it slides into the mantle (the surface to as deep as 670 km).

Quakes deeper than 100 km are more likely associated with faults caused by stresses within the slab.

Transform Fault Margin

Along a transform fault margin, two plates grind past each other in horizontal motion.

These margins involve strike-slip faults in the shallow lithosphere and often a broader shear zone deeper in the lithosphere.

Most transform faults occur underwater between oceanic plates.

Between the two ridges, a transform fault results in strike-slip earthquakes, but no earthquakes occur beyond the ridges.

Movie 7 Transform Faults

Two of Earth’s most notorious and dangerous transform faults are on land.

The North Anatolian Fault in Turkey.

The San Andreas Fault in California.

Convergent Margin/Collision Zone

Collision zones that mark the closure of a former ocean form spectacular mountain ranges.

The Alps.

The Himalayas

The Appalachians.

Convergent Margin/Collision Zone

Continental crust is not recycled into the mantle.

Continental crust is lighter (less dense) and thicker than oceanic crust.

When two fragments of continental lithosphere converge, the surface rocks crumple together to form a collision zone.

Movie 8: India-Asia Collision

Answer These Questions

The next slide contains a topographic and bathymetric map of the earth. Are the Andes Mountains similar or different from the Himalaya? Can you explain why the Alps exist? Why are there no mountains in the interior of Canada and Brazil? Why is there a chain of mountains in the Atlantic and Pacific Oceans?What is indicated on the two slides of South America? What do the arrows indicate? Did you understand the subduction process in the movie? Why did the city of Concepcion move more than 3 meters? Is this typical? Would you expect Columbus to move that much? Would you expect St. Louis to move that much?

February 27, 2010 Chile Earthquake

• Read this news story: http://researchnews.osu.edu/archive/chilemoves.htm

• Magnitude 8.8• Using GPS stations, Mike Bevis

and colleagues measured co-seismic (or nearly co-seismic) movement of the crust

• Concepcion moved more than 3m westward.

• Other nearby cities moved much less, <30cm

• Apparently little to no movement in the interior of the continent

• Why did Concepcion and other cities move west?• Watch movie to find out why

February 27, 2010 Chile Earthquake

Movie 9: 2010 Chile EQ and Tsunami

March 11, 2011 Japan Earthquake• What is the plate tectonic setting of Japan?• How fast is the Pacific plate moving, with

respect to the North American plate?• How much motion occurred along the fault?• Why did the tsunami occur?• How far did the land surface move,

horizontally and vertically?• Why did the land move?

http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/#summary

http://www.gsi.go.jp/cais/topic110315-index-e.html

Tohoku, Japan Earthquake: Tectonic Summary

The magnitude 9.0 Tohoku earthquake on March 11, 2011, which occurred near the northeast coast of Honshu, Japan, resulted from thrust faulting on or near the subduction zone plate boundary between the Pacific and North America plates. At the latitude of this earthquake, the Pacific plate moves approximately westwards with respect to the North America plate at a rate of 83 mm/yr, and begins its westward descent beneath Japan at the Japan Trench.

The location, depth, and focal mechanism of the March 11 earthquake are consistent with the event having occurred on the subduction zone plate boundary.

Modeling of the rupture of this earthquake (red shading, approx.) indicate that the fault moved upwards of 30-40 m, and slipped over an area approximately 300 km long (along-strike) by 150 km wide (in the down-dip direction). The rupture zone is roughly centered on the earthquake epicenter along-strike, while peak slips were up-dip of the hypocenter, towards the Japan Trench axis. The March 11 earthquake was preceded by a series of large foreshocks over the previous two days, beginning on March 9th with a M 7.2 event approximately 40 km from the epicenter of the March 11 earthquake, and continuing with another three earthquakes greater than M 6 on the same day.

Tohoku, Japan Earthquake: GPS DisplacementsGeospatial Information Authority of Japan

Horizontal Displacement

Tohoku, Japan Earthquake: GPS DisplacementsGeospatial Information Authority of Japan

Vertical Displacement