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The Structure of the Earth The Structure of the Earth and Plate Tectonics

Structure of the Earth

• The Earth is made up of 3 main layers:– Core

Inner core

Outer core

Mantle

– Mantle– Crust

Crust

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The Crust• This is where we live!

• The Earth’s crust is made of:

Continental Crust Oceanic Crust

- thick (10-70km)- buoyant (less dense than oceanic crust) - mostly old rock

- thin (~7 km)- dense (sinks under continental crust)- young rock

How do we know what the Earth is made of?

G h i l i i i i • Geophysical surveys: seismic, gravity, magnetics, electrical, geodesy– Acquisition: land, air, sea and satellite– Geological surveys: fieldwork, boreholes, mines

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What is Plate Tectonics?

When 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 j

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” featurestectonic features.

• The word, tectonic, refers to the deformation (or changes) of the crust as a result of plate interaction.

World Plates

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What are tectonic plates made of?

• Plates are made of rigid lithosphere.

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

What lies beneath the tectonic plates?

Below the lithosphere is the plasticlike (neither solid nor liquid) nor liquid) asthenosphere.

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Plate Movement“Plates” of lithosphere are moved around byp y

the underlying hot mantle convection cells.

What happens at tectonic What happens at tectonic plate boundaries?

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Three types of plate boundary

1. Divergent

2. Convergent

3. Transform

Divergent Boundaries

• Spreading ridges (ex: Mid-Atlantic Ridge)– As plates move apart new material (basaltic

magma, or lava) is erupted to fill the gap

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Divergent Boundary

The Mid-Atlantic Ridge (longest underwater mountain range)

Age of Basaltic Oceanic Crust

Courtesy of www.ngdc.noaa.gov

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Polarity of Basaltic Oceanic CrustThe Earth’s The Earth s magnetic reversals provide more evidence for seafloor spreading.

The age and polarity

Courtesy of www.ngdc.noaa.gov

g p yof the basaltic ocean floor is mirrored on both sides of the spreading ridge.

• Iceland has a divergent l d

Iceland: An example of continental rifting

plate boundary running through its middle

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East African Rift: continental plate rifting

G l i t h Geologist have already given names to the resulting new plates: Nubianand SomalianPlatesPlates

Convergent Boundaries

• There are three types of convergent plate boundaries1. Continent-continent collision2. Continent-oceanic crust collision3 Oc n c n c llisi n3. Ocean-ocean collision

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Forms folded mountains like the European Alps, and the Himalayas

Continent-Continent CollisionHimalayas

• Usually, no subduction occurs at this type of convergent plate boundary.

Himalayas: Mt. Everest

“The roof of the world”

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Continent-Oceanic Crust CollisionEx. Juan de Fuca plate subducting under North Americanplate (Mt St Helens) Nazca plate subducting under the plate (Mt. St. Helens). Nazca plate subducting under the South American plate (Andes Mts.).

Subduction

• Oceanic lithosphere subducts underneath the continental lithosphere

• Oceanic lithosphere heats and dehydrates as it and dehydrates as it subsides

• The melt rises forming volcanism

• E.g. The Andes Mountains, Mt. Saint Helens

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Ocean-Ocean Plate CollisionThe Japanese The Japanese Islands were formed by subduction of the Pacific Plate under the E l Eurasian Plate.

Mt. Fuji, Japan

Wh t i l t llid

Ocean-Ocean Plate Collision• 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. This forms the deepest areas in the ocean floor pcalled trenches. – Ex. The Mariana Trench is 11 km deep!

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• Where plates slide past each otherTransform Boundaries

p p

Above: View of the San Andreas transform fault, California, USA.

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Volcanoes and Plate

…what’s the connection?

Tectonics…

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Pacific Ring of Fire

Volcanism is Volcanism is mostly focused at plate boundaries

- Subduction - Rifting - HotspotsVolcanoes are formed by:

g p

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Pacific Ring of Fire

Hotspot volcanoes

• Hot mantle plumes breaching the

What are Hotspot Volcanoes?Hot mantle plumes breaching the surface in the middle of a tectonic plate

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 older away from the hotspot.

Earthquakes and Plate

…what’s the connection?

qTectonics…

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

Figure showing the distribution f th k

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

of earthquakes around the globe

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 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)• There are 2 types of plate• There are 3 types of plate boundaries• Volcanoes and Earthquakes are closely

linked to the margins of the tectonic plates