Continental Drift and Plate Tectonics

Layered Earth Cutaway

Take-Away Points1. How we know plate tectonics happens

2. Most earthquakes and volcanoes occur along plate boundaries

3. New ocean crust is created along mid-ocean ridges

4. Old ocean crust is recycled at subduction zones

5. Subduction zones are where mountain-building (orogeny) occurs

6. Small pieces of crust (terranes) are important in building up continents

7. Hot spots are long lived stationary magma sources

Development of Continental Drift

Lots of people had noticed that the coastlines of Africa and South America are similar

Frank Taylor (1910)

Alfred Wegener (1912) Die Entstehung Der Kontinente Und Ozeane

1. How we know plate tectonics happens

The Permian Ice Age Problem

1. How we know plate tectonics happens

Similar rock formations are found scattered all over the world on different continents. The best explanation is that they were once all joined.

Wegener’s Theory

1. How we know plate tectonics happens

Wegner also noted that later formations of coal would have had to have originally been warm, wet, coastal areas as shown in the top image, now having moved to cold and/or arid and/or mountainous regions like Kentucky, Russia and China.

Frank Taylor

In some respects, Taylor’s ideas were more modern than Wegener’s

Taylor always thought Wegener had stolen credit from him

Used evidence of narrow straits, mid-Atlantic ridge and some mountain ranges to prove continental drift.

Frank Taylor

Recognized role of Mid-Atlantic Ridge

Never reconstructed the continents like Wegener did

1. How we know plate tectonics happens

The Geomagnetic Reversal Time

Scale

Magnetic resonance in rocks also provides more evidence to prove the Continental Drift Theory.

This shows how many studies have shown that the Earth’s magnetic field has reversed itself 3 times in the last 4 million years. The direction of tiny magnetic crystals in rock samples can show where they originally formed.

Discovery of Sea-Floor Spreading

1. How we know plate tectonics happens

mid-Atlantic ridge has been carefully studied and tracked to measure the actual movements on each side.

Where Does Ocean Crust Go? Hugo Benioff, 1954

1. How we know plate tectonics happensHugo Benioff described subduction to explain many oceanic features

Benioff’s Interpretation

1. How we know plate tectonics happensHeavy oceanic crust gets pushed under lighter continental crust.

Benioff’s Interpretation

Updated

1. How we know plate tectonics happens

Subduction creates trenches behind and pushes up mountains ahead.

Worldwide Standardized Seismic Network

Plotting where volcanoes and earthquakes occur shows the outline of the continental plates.

What Drives It: Convection

As cooler crust gets pushed into the mantle, warmer magma rises to the surface, forming convection currents in the liquid mantle.

How Plates Move

1. Hot magma rising heats and stretches the crust above it, swelling it up and thinning it until it rips apart. 2. The rotational motion of the convection adds to the motion of the crust away from the rift. 3. The weight of the continental crust pulls more crust under it.

Three forces move the oceanic plates:

1.

2.

3.

Fit of Contine

nts Across

the Atlantic

Iceland was not part of Pangaea because it had not been formed yet.

Anatomy of a Mid-Ocean Ridge

Magma from the mantle pushes the ocean crust up.

A Subduction Zone

As ocean crust is subducted, the rock above it can fracture, allowing magma to rise to the surface fueling volcanic activity.

Why Mountains are High

Even if there is no eruption, hot magma can bend and push rock up.

Plate Boundaries and Earthquakes

Irregularities on the plates create earthquakes as they grind past each other.

Hot Spot

sAreas where especially hot magma rises close to the crust, melts through it and erupts to the surface. When the molten rock (now called lava) solidifies, new mountains and/or islands are formed.

Hot Spots: Hawaii

As the Pacific plate moves northwest, new islands are formed to the southeast of the older ones, forming the chain.

Copy these in your Journal

1. How we know plate tectonics happen

2. Most earthquakes and volcanoes occur along plate boundaries

3. New ocean crust is created along mid-ocean ridges

4. Old ocean crust is recycled at subduction zones

5. Subduction zones are where mountain-building occurs

6. Hot spots are long lived stationary magma sources

*When done, get out ‘Tectonic Plate PPT Notes’ handout and ‘Layered Earth’ diagram you colored.*