Chapter 6 Chapter 6 Earth’s Crust & Earth’s Crust & Plate Tectonics Plate Tectonics

The Earth’s Crust and Plate Tectonics

The Earth’s Crust This is where we live!

The Earth’s crust is made of:

Continental Crust

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

Oceanic Crust

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

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…

World Plates

Plate Tectonics

• The Earth’s crust is divided into many 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.

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.

Pangaea Alfred Wegener proposed the Continental Drift Hypothesis in 1912, that stated Earth’s continents were once joined in a single landmass and gradually moved or drifted apart.

Theory of Plate Tectonics Fossils, Climate, & Geology

• http://library.thinkquest.org/17701/high/pangaea/• http://www.pbslearningmedia.org/resource/ess05.sci.ess.earthsys.plateintro/plat

e-tectonics-an-introduction/

Evidence from the Sea Floor

http://library.thinkquest.org/17457/platetectonics/4.php

Three types of plate boundary

Divergent

Convergent

Transform

Divergent Boundaries

Also called spreading ridgesAs plates move apart new material is erupted to fill the gap

Iceland has a divergent plate boundary running through its middle

Divergent Boundaries

Where plates slide past each other

Transform Boundaries

Convergent Boundaries

• 2 plates that are pushing together • There are three types of convergent

plate boundaries– Continent-continent collision– Continent-oceanic crust collision– Ocean-ocean collision

Continent-Continent Collision

Forms mountains, e.g. European Alps, Himalayas

Himalayas

Oceanic-Continental Subduction

• Oceanic lithosphere subducts underneath the continental lithosphere

• Oceanic lithosphere heats and dehydrates as it subsides

• The melt rises forming volcanism

• Deep ocean trenches & Coastal mountains

Oceanic-Oceanic Plate Subduction

• When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.

• Island arcs form • The subducting plate is bent

downward to form a very deep depression in the ocean floor called a deep ocean trench.

• The worlds deepest parts of the ocean are found along trenches. – E.g. The Mariana Trench is 11

km deep!

Convection Currents

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

Magnetic Reversal • After molten lava emerges from a volcano, it solidifies to a

rock. In most cases it is a black rock known as basalt, which is faintly magnetic (iron)

• Its magnetization is permanently fixed like tiny compass needles pointing north & south.

• Instruments can measure the magnetization of basalt.• Surprisingly, this procedure suggested that times existed

when the magnetization had the opposite direction from today's. All sorts of explanation were proposed, but in the end the only one which passed all tests was that in the distant past, indeed, the magnetic polarity of the Earth was sometimes reversed.