Chapter 18

Plate Tectonics

History of Earth’s interior

• Earth formed ~4.6 billion years ago in the solar nebula

• Early Earth had molten surface from bombarding materials

• Surface cooled and crystallized to igneous rocks• Heat accumulated from radioactive decay led to

a second melting of the interior. This time

melting occurred in pockets, not throughout interior.

Seismic Wave

• A seismic wave is a vibration that moves through any part of the earth. They can be used by geologists to learn about the earth’s interior.

• Seismic waves radiate outward from an earthquake, spreading in all directions through the solid earth’s interior like sound waves from an explosion.

Earth’s Internal Structure

• The 3 main zones of the earth’s interior, determined through seismic studies are:

1. The crust, is the outer layer of rock that forms a thin shell around the earth.

2. The mantle is below the crust and is a much thicker shell than the crust.

3. The core is the center part. There is an inner core and a liquid outer core.

The structure of the earth’s interior.

The Crust• The crust is a thin skin that covers the

entire earth. It is below oceans and also makes up continents, although the two are different because they are made of different types of rock:

1. The oceanic crust is much thinner and denser, it is much younger and it is composed of basalt.

2. The continental crust is much thicker and less dense, it is much older and it is composed of granite and other rocks.

Continental crust is less dense, granite-type rock, while the oceanic crust is more dense, basaltic type rock. Both types of crust behave as if they were floating on the mantle, which is more dense than either type of crust.

The Crust

• The boundary between the crust and the mantle is called Mohorovicic discontinuity, Moho for short.

The Mantle• The mantle is a thick layer between the core and

the crust. It takes up about 80% of the total volume of the earth and accounts for about 2/3 of the earth’s total mass.

• The mantle is composed of silicates, particularly the ferromagnesian silicate olivine.

• The evidence for the composition of the mantle comes from seismic studies, studies of the composition of meteorites, and studies of materials that have been ejected to the earth’s surface by volcanoes.

The Core• The core has been found to have 2 parts: 1. A liquid outer core 2. A solid inner core• The core makes up about 15% of the earth’s total

volume and about 1/3 of its mass.• The core has been found to be mostly iron. This is

believed because the magnetic field which is produced indicates that it is composed of an electrical conductor, a metal such as iron.

• Meteorites that fall to earth are either stony (made of rock which are silicate materials), or iron meteorites made up of iron or a nickel-iron alloy.

• Since the earth has a silicate-rich crust and mantle, by analogy the earth’s core must consist of iron or a nickel and iron alloy.

Earth’s Interior Structure

Iron meteorites – composed of iron or nickel-iron alloy

Stony meteorites – composed of silicates

What happens when both melted on early planet?

Iron and nickel sank to core

Earth’s Interior Structure

Silicates remained near surface

Differentiation – separation of materials while in molten stage

Asthenosphere

• Another way of looking at the earth’s structure is by considering the following:

• There is also a part of the mantle called the asthenosphere, after the Greek for “weak shell”.

• It is weak because it is plastic like, mobile, and yields to stresses. It is a hot, elastic, semi liquid layer that extends around the entire earth.

• In some regions it is completely liquid, containing pockets of magma.

The Lithosphere• The rocks above and below the asthenosphere are rigid,

solid and brittle.• The solid layer above the asthenosphere is called the

lithosphere, after the Greek for “stone shell”.• The lithosphere is also known as the strong layer in

contrast to the asthenosphere, which is the weak layer.• The lithosphere includes the entire crust, the Moho, and

the upper part of the mantle.• The lithosphere is one of the important sources of

magma that reaches the earth’s surface.• It is also an important part of the mechanism involved in

the movement of the crust. It is made up of comparatively rigid plates that are moving, floating in the upper mantle like giant ice sheets floating in the ocean.

The earth’s interior, showing the weak, plastic layer called the asthenosphere. The rigid, solid layer above the asthenosphere is called the lithosphere. The lithosphere is broken into plates that move on the upper mantle like giant ice sheets floating on water. This arrangement is the foundation for plate tectonics, which explains many Changes that occur on the earth’s surfacesuch as earthquakes, volcanoes, and mountain-building.

Theory of Plate Tectonics• Continental shapes suggest that the continents were at

one time together, breaking apart and moving to their present position sometime in the past. For example, the Americas and Europe and Africa.

• A geologist named Wegener in the early 1900’s believed that at one time there was a single land mass called Pangaea, from the Greek meaning “all lands”. He pointed out that similar fossils found in landmasses on both sides of the Atlantic Ocean today must be from animals and plants that lived in Pangaea, which later broke up into smaller continents.

• Wegener’s concept became known as continental drift, the idea that individual continents could shift positions on earth’s surface.

• In the 1950’s this idea resurfaced.

Theory of plate tectonicsLand masses appeared to belong to one giant land mass, Pangaea.

Plate Tectonics-Evidence from the Ocean

• Ocean basins are the bottom of the ocean floor. They are covered by 3-4 miles of water.

• There are valleys, hills, mountains, and mountain ranges. Oceanic ridges are long, high, and continuous chains of mountains that seem to run clear around the earth.

• The Mid-Atlantic ridge is an oceanic ridge that is located at the center of the Atlantic Ocean basin. It divides the Atlantic Ocean into two nearly equal parts. Where it is high enough to reach sea level it makes oceanic islands such as Iceland.

The Mid-Atlantic Ridge divides the Atlantic Ocean into two nearly equal parts. Where theRidge reaches above sea level, it makes oceanic islands, such as Iceland.

Plate Tectonics-Evidence from the Ocean

• The following has been observed in relation to the Mid-Atlantic Ridge:

1. Submarine earthquakes which occur in a narrow band under the crest of the Mid-Atlantic ridge.

2. A rift (a long, continuous, and deep valley) which runs along the crest of the Mid-Atlantic Ridge for its length.

3. Large amounts of heat found to be escaping from the rift.

Plate Tectonics-Evidence from the Ocean

• It was concluded that the rift may be a crack in the earth’s crust, a fracture through which basaltic lava flowed to build up the ridge.

• This led to the hypothesis of seafloor spreading, that hot, molten rock moved up from the interior of the earth to emerge along the ridge, flowing out in both directions to create new rocks along the ridge.

• The creation of new rock would tend to spread the seafloor in both directions.

Plate Tectonics-Evidence from the Ocean• Fossil samples taken by drilling into the ocean’s

floor has indicated that the layers of sediments near the ridge were thin and became progressively thicker toward the continents.

• This is what indicated that the seafloor was spreading, since the older layers would have more time to accumulate greater depths of sediments. The oldest layers of sediments proved to be the ones near the continents, about 150 million years old.

• This indicates that the Atlantic Ocean did not exist until 150 million years ago. At that time a fissure formed between Africa and South America.

The pattern of seafloor ages on both sides of the Mid-Atlantic Ridge reflects seafloor spreading activity. Younger rocks are found closer to the ridge.

Plate Tectonics• According to plate tectonics, the lithosphere is

broken into a number of fairly rigid plates that move on the asthenosphere.

• Some plates contain continents and part of an ocean basin, while other plates contain only ocean basins.

• The plates move and the movement is helping to explain why mountains form where they do, the occurrence of earthquakes and volcanoes, and in general, the entire changing surface of the earth.

• Earthquakes, volcanoes, and most rapid changes in the earth’s crust occur at the edge of a plate, which is called a plate boundary.

The major plates of the lithosphere that move on the asthenosphere.

Plate Boundaries• There are 3 types of plate boundaries that

describe how one plate moves relative to another: divergent, convergent, and transform.

• Divergent boundaries occur between two plates moving away from each other. Magma forms as the plates separate. New crust is then added to the edges of the separating plates. The divergent boundary is a new crust zone.

A diverging boundary at a mid-oceanic ridge. Hot asthenosphere wells upward beneath the ridge crest. Magma forms and squirts into fissures. Solid materialthat does not melt remains as mantle in the lower part of the lithosphere. As thelithosphere moves away from the spreading axis, it cools, becomes denser, andsinks to a lower level.

Divergent Boundaries

• The Mid-Atlantic Ridge is a divergent boundary between the South American and African Plates, extending north between the North American and Eurasian plates.

• The results of divergent plate movement can be seen in Iceland, where the Mid-Atlantic Ridge runs as it separates the North American and Eurasian Plates. In the northeastern part of Iceland, ground cracks are widening, often accompanied by volcanic activity. Between 1975 and 1984 displacements caused a total separation of about 23 feet.

At some point along the mid-ocean ridge, it comes above sea-level.

The major plates of the lithosphere that move on the asthenosphere.

Divergent Boundaries

• The measured rate of spreading along the Mid-Atlantic Ridge ranges from 1-6 cm per year.

• Another major ocean may be in the making in East Africa, where a divergent boundary has already moved Saudi Arabia away from the African continent, forming the Red Sea.

This section moving away from mainland Africa and could be a large island.

Lithospheric plates and boundaries

Divergent boundaries – two plates moving away from each other.

Convergent Boundaries• Convergent Boundaries occur between two

plates moving towards each other. • The creation of new crust at a divergent

boundary means that old crust must be destroyed somewhere else or else the earth would have an ever expanding diameter. Old crust is destroyed by returning to the asthenosphere at convergent boundaries.

• The collision produces an elongated belt of down-bending called a subduction zone.

Convergent Boundaries• The lithosphere of one plate, which

contains the crust, is subducted beneath the second plate and partially melts, then becoming part of the mantle.

• The oceanic lithosphere is being recycled, which explains why ancient seafloor rocks do not exist.

• Earthquakes, volcanoes, and mountain ranges result from convergent boundaries.

Lithospheric plates and boundaries

Convergent boundaries – two plates moving toward each other.

Lithospheric plates and boundaries

Since our Earth is not growing in size, what is created at divergent boundaries goes away somewhere else.

Density differences force one plate to go below another. Oceanic crust goes under continental crust

Lithospheric plates and boundaries

Subduction zones where old crust is sent back to asthenosphere for recycling.

Trenches created at subduction zones.

3 types of convergent boundaries: Continental-Oceanic, Oceanic-Oceanic, Continental-Continental

Continental-oceanic Gives rise to the Andes Mountains and deep-seated earthquakes.

Lithospheric plates and boundaries

Andes mountains

Peru-Chile trench

Continent-ocean plate convergence. This type ofPlate boundary accounts for shallow and deep-seated earthquakes, an oceanic trench, volcanic activity,And mountains along the coast.

Oceanic-oceanic. Gives rise to curving chains of volcanic islands called island arcs that occur over the belt of deep-seated earthquakes. Aleutian Islands Japanese, Marianas, and Indonesians.

Lithospheric plates and boundaries

Aleutian islands, Japanese Islands

Ocean-ocean plateconvergence. This type ofplate convergence accounts for shallowand deep-focused earthquakes,an oceanic trench, and a volcanic arcAbove the subducted plate.

Continent-continent

Lithospheric plates and boundaries

Indian-Australian plate Eurasian plate

Continent-continent plate convergence. Rocks are deformed,and some lithosphere thickening occurs, but neither plate is subducted to any great extent. Gave rise to the Tibetan Plateau and theHimalayan Mountains.

Transform Boundaries

• Transform boundaries occur between two plates sliding by each other.

• Crust is neither created nor destroyed at transform boundaries as one plate slides horizontally past another along a long, vertical fault.

• The Pacific Plate is moving slowly to the northwest, sliding past the North American Plate.

• The San Andreas Fault is one boundary along the California coastline. Vibrations from plate movement along this boundary are the famous California earthquakes.

Lithospheric plates and boundaries

Transform boundaries – two plates next to each other.

The AlpsThe Himalayas

Th

e A

nd

es

The New Zealand “Alps”

JapanAleutian Islands

Present-Day Understanding• Today what is generally accepted about the

plate tectonic theory is the understanding that the solid materials of the earth are engaged in a continual cycle of change.

• Oceanic crust is subducted, melted, then partly returned to the crust in volcanic igneous rocks in island arcs and along continental plate boundaries.

• Other parts of the subducted crust become mixed with the upper mantle, returning as new crust at diverging boundaries.

• The materials of the crust and the mantle are thus cycled back and forth in a mixing that may include the deep mantle and the core as well.

Exercises

• Applying the concepts.

p 458 # 1, 2, 4, 5, 7, 8, 9, 10

New Book: p. 504-506

# 1, 2, 4, 7, 8, 9, 10, 11, 13, 14, 16, 17, 18, 19, 20, 21, 27, 28, 30, 31, 32, 33, 36, 38, 41, 42, 43, 45, 46, 47, 48, 49, 50, 52

Review• The Earth was molten at one time.• Seismic waves and seismic studies.• The 3 parts of the earth: crust (thinner, on top), mantle

(thicker, middle), core (solid inner and liquid outer, innermost layer).

• Oceanic crust is thinner, younger, denser, made up of basalt. Continental crust is thicker, less dense, older, and composed of granite.

• The Moho is the boundary between the crust and the mantle.

• The mantle is made up largely of the ferromagnetic silicate called olivine.

• The core is made up largely of iron (evidence from meteorites, and the magnetic field of the Earth.

• The asthenosphere is a part of the mantle which is a weak, plastic layer.

• The lithosphere includes the entire crust, the Moho, and the upper part of the mantle. It is made up of rock. It is the “strong layer”.

• The lithosphere is comparatively rigid plates that are moving, floating in the upper mantle like giant ice sheets floating in the ocean.

• The continental drift.• Earthquakes, volcanoes, and most rapid changes in the

earth’s crust occur at the edge of a plate, which is called a plate boundary.

• Oceanic ridges are long, high, and continuous chains of mountains that seem to run clear around the earth. The Mid-Atlantic ridge is an oceanic ridge that is located at the center of the Atlantic Ocean basin.

• There are 3 types of plate boundaries: divergent, convergent and transform.

• Divergent boundaries-Plates moving away from each other-new crust is formed from magma. Ocean crust near continents is older. e.g. the Midatlantic Ridge.

• Convergent boundaries-Plates moving towards each other. Result in earthquakes, volcanoes, mountains.

• The collision produces an elongated belt of down-bending called a subduction zone.

• The oceanic lithosphere is being recycled, which explains why ancient seafloor rocks do not exist.

• Continent-Ocean convergence leads to earthquakes and mountains along the coast, like the Andes in South America.

• Ocean-ocean convergence leads to volcanic islands like Japan and the Aleutian islands. These are called island arcs.

• Continent-continent convergence leads to tall mountain ranges like the Himalayas. There is not much subduction.

• Transform boundaries-2 plates sliding past each other-No land is recycled but earthquakes result, e.g. San Andreas fault.