Structure of the Earth

The structure of Earth

Traditional structure:

core

mantle

crust

Tectonic structure

inner core (solid)

outer core (liquid)

rigid mantle asthenosphere (plastic)

lithosphere (rigid)

Earth's vertical Profile

The lithosphere is composed of about 20 plates.

The plates slide on the asthenosphere.

The convected heat that drives plate movement includes both the asthenosphere and the mesosphere or perhaps only the asthenosphere.

The geomagnetic field is generated by differential movement between the outer core and the mesosphere.

Oceanic crust is basaltic, denser and thinner (5-10 km).

Continental crust is granitic, less dense and thicker (20-90 km).

Oceanic crust is recycled by volcanism to create continental crust.

Basalt is transformed to andesite, granite and other less dense rocks.

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The three types of plate boundary

Type	Landforms	Crust ...
Divergent	Rift valley and mid-ocean ridge	Created
Convergent	Trench, island arc, and magmatic arc	Subducted
Transform	Transform faults	Fragmented into microplates

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Types of Convergent Boundaries

The friction of the subducted slab on the overlying plate causes earthquakes.

The quakes are progressively deeper further from the trench as the slab pulled down into the lithosphere.

At some point it becomes hot enough to prevent friction; no more earthquakes.

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Continental-oceanic Convergence

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At the Japanese margin

The Pacific Plate is being subducted beneath the Eurasian Plate

Subduction is more rapid than the eastward movement of the Eurasian Plate

Sea of Japan is a back arc basin.

The continental crust under it is being stretched (tension).

Causes a depression to form.

Fore-arc basin is between trench and island arc.

Wedge of deep-sea sediment is scraped off subducting plate and pasted against over-lying one.

Seismic records show that earthquakes are deeper and deeper from trench under fore-arc basin and then under Japan itself.

They are occurring because of friction in the subduction zone between the subducted and overlying plate surfaces.

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Oceanic-oceanic Convergence

When an oceanic crust is subducted beneath oceanic crust, an island arc forms above the melting slab

When oceanic crust is subducted beneath a continent, a magmatic arc forms

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Continental-continental Convergence

After all the oceanic crust is consumed, eventually continents collide.

Subduction is greatly reduced and massive fold mountains are uplifted

Volcanic activity is minimal, but earthquakes are common.

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A contemporary example

The Indian Plate has been colliding with the Eurasian Plate for the past 40 million years and has uplifted and continues to uplift the Himalayas and the Tibetan Plateau.

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The Western

North American Mess

Transform boundaries are often quite chaotic.

Plates grind past one another and "splinters" are dislodged and may be dragged long distances along the plate boundaries

The west coast of North America is composed of slivers of old ocean crust, old island arc and old continental crust.

The Pacific Plate is rotating counter-clockwise past the North American Plate.

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