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The Structure of the Earth and Plate Tectonics

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Structure of the Earth

The Earth is made up of 3 main layers:

Core
Mantle
Crust

Inner core

Outer core

Mantle

Crust

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What is Plate Tectonics?

The interior of the Earth is divided into layers based on chemical and physical properties.

The Earth has an outer silica-rich, solid crust, a highly viscous mantle, and a core comprising a liquid outer core that is much less viscous than the mantle, and a solid inner core.

Working from the centre of the Earth out we have:

The inner core is a primarily solid sphere about 1220 km in radius situated at Earth's center.

Based on the abundance of chemical elements in the solar system, their physical properties, and other chemical constraints regarding the remainder of Earth's volume, the inner core is believed to be composed primarily of a nickel-iron alloy, with small amounts of some unknown elements.

The temperature is estimated at 5,000-6,000 degrees Celsius and the pressure to be about 330 to 360 GPa (which is over 3,000,000 times that of the atmosphere!)

The liquid outer core is 2300 km thick and like the inner core composed of a nickel-iron alloy (but with less iron than the solid inner core).

Iseismic and other geophysical evidence indicates that the outer core is so hot that the metals are in a liquid state.

The mantle is approximately 2,900 km thick and comprises 70% of Earth's volume. (the core makes up about 30% of Earth's volume, with the outer crust [where we live] <1%!!).

The mantle is divided into sections based upon changes in its elastic properties with depth.

In the mantle, temperatures range between 500-900 degrees Celsius at the upper boundary with the crust to over 4,000 degrees Celsius at the boundary with the core.

Due to the temperature difference between the Earth's surface and outer core, and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle (mantle convection cells). Hot material rises up as mantle plumes (like a lava lamp!), while cooler (and heavier) material sinks downward to be reheated and rise up again.

- We shall see that this process is very important for plate tectonic motion…

The outer most layer is the crust - this is the most familiar to us as it is where we live.

The distinction between crust and mantle is based on chemistry, rock types and seismic characteristics.

Presenter: Ask the students to guess what the most abundant element in the earth’s crust is…..they may be surprised to learn that it is actually Oxygen (46.6% Oxygen; 27.7% Silica; 8.1% Aluminum; 5.0% Iron; 3.6% Calcium; 2.8% Sodium; 2.6% Potassium; 2.1% Magnesium; plus trace elements)
Click to next slide for more on the Crust….

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If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.

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Plate Tectonics

The Earth’s crust is divided into 12 major plates which are moved in various directions.
This plate motion causes them to collide, pull apart, or scrape against each other.
Each type of interaction causes a characteristic set of Earth structures or “tectonic” features.
The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.

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What are tectonic plates made of?

Plates are made of rigid lithosphere.

The lithosphere is made up of the crust and the upper part of the mantle.

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Plate Movement

“Plates” of lithosphere are moved around by the underlying hot mantle convection cells

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Divergent

Convergent

Transform

Three types of plate boundary

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Spreading ridges

As plates move apart new material is erupted to fill the gap

Divergent Boundaries

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There are three styles of convergent plate boundaries

Continent-continent collision
Continent-oceanic crust collision
Ocean-ocean collision

Convergent Boundaries

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Forms mountains, e.g. European Alps, Himalayas

Continent-Continent Collision

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Himalayas

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Called SUBDUCTION

Continent-Oceanic Crust Collision

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Oceanic lithosphere subducts underneath the continental lithosphere
Oceanic lithosphere heats and dehydrates as it subsides
The melt rises forming volcanism
E.g. The Andes

Subduction

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When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.
The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench.
The world’s deepest parts of the ocean are found along trenches.

E.g. The Mariana Trench is 11 km deep!

Ocean-Ocean Plate Collision

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Where plates slide past each other

Transform Boundaries

Above: View of the San Andreas transform fault

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…what’s the connection?

Volcanoes and Plate Tectonics…

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Volcanism is mostly focused at plate margins

Pacific Ring of Fire

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- Subduction - Rifting - Hotspots

Volcanoes are formed by:

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Pacific Ring of Fire

Hotspot volcanoes

At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust.

This is called a Subduction Zone.

The oceanic crust descends into the mantle at a rate of centimetres per year. This oceanic crust is called the “Subducting Slab” (see diagram).

When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge (Presenter: explain all of this using the diagram).

The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes.

Subduction is a way of recycling the oceanic crust. Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled.

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Hot mantle plumes breaching the surface in the middle of a tectonic plate

What are Hotspot Volcanoes?

Photo: Tom Pfeiffer / www.volcanodiscovery.com

The Hawaiian island chain are examples of hotspot volcanoes.

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The tectonic plate moves over a fixed hotspot forming a chain of volcanoes.

The volcanoes get younger from one end to the other.

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…what’s the connection?

Earthquakes and Plate Tectonics…

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As with volcanoes, earthquakes are not randomly distributed over the globe

At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur.

Figure showing the distribution of earthquakes around the globe

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Where do earthquakes form?

Figure showing the tectonic setting of earthquakes

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Plate Tectonics Summary

The Earth is made up of 3 main layers (core, mantle, crust)
On the surface of the Earth are tectonic plates that slowly move around the globe
Plates are made of crust and upper mantle (lithosphere and the aesthenosphere)
There are 2 types of plates (oceanic and
continental)
There are 3 types of plate boundaries
Volcanoes and Earthquakes are closely linked to the margins of the tectonic plates