Plate Tectonics

Earth Science

Continental Drift

Alfred Wegener proposed the idea of continental drift in 1912. Looking at the continents, it is possible to visualize that they all fit together.

All the continents were together in one landmass called Pangaea.

There are several different clues that suggest this might be true.

Clues

Fossil Clues: Fossils from certain animals were found on several continents to suggest they were once together.

Climate Clues: Warm weather plants have been found in the arctic where they would normally not grow.

Rock Clues: Similar rock structures are found on different continents.

Continental Drift

What could make the continents drift apart?

Seafloor Spreading

Under the oceans, there are valleys and ridges throughout the world.

Mid-ocean ridges form underwater mountain ranges

Theory of seafloor spreading was introduced in the 1960s

Magma is pushed up through a mid-ocean ridge and forces the seafloor to spread

Seafloor Moves

Scientists found that the youngest rocks underwater were near the mid-ocean ridge and that there were no rocks older than 180 million years.

Magnetic Clues

Earths magnetic field has reversed itself several times throughout history.

Scientists measured the polarity of the rock in the ocean, and found that it did reverse itself.

Plate Tectonics

This theory states that the Earth’s crust is broken up into sections.

The sections are called plates and move around the mantle.

Plates are made up of the crust and part of the upper mantle, together called the lithosphere.

Composition

Lithosphere is rigid and about 100 km thick.

This layer floats on top of the asthenosphere below it.

Plate Boundaries There are 12

different tectonic plates

When plates interact they can…– Collide– Pull apart– Slide past each other

Boundary Types

Divergent: 2 plates moving apart from one another

Ex: Seafloor spreading

Mid-Atlantic Ridge Great Rift Valley

Convergent: 2 plates coming together

3 types In the collisions, one

plate will slide underneath another creating a subduction zone

Convergent Boundaries Volcanoes usually

occur in subduction zones.

Type 1: Ocean & Continental plates– Ocean plate will slide

beneath the continental plate.

– Creates a deep-sea trench & forms volcanic mountains

Type 2: Ocean-Ocean collision– Creates a deep sea

trench and an island arc of volcanoes

Type 3: 2 Continental Plate Collide– Usually no

subduction– Mountains form– Earthquakes

common

Transform Fault Occurs when 2 plates slide past each

other. Earthquakes often occur Ex: San Andreas Fault

Causes of Plate Tectonics

Convection currents – cycle of hot plasma rising and cooled plasma sinking causes the plates to move.– Hot plasma is less dense than cool plasma

Effects of Plate Tectonics

Earthquakes Volcanoes Form several different landforms

– Mountains– Faults– Rift Valleys

Landforms

Mountains, Arcs, & Volcanoes: formed from converging plates. – Ocean/Ocean = arcs

& volcanoes– Continental/Ocean =

volcanoes– 2 Continental =

mountains

Strike Slip Faults: when plates are sliding past each other.

Faults & Rift Valleys: form as divergence occurs