PLATE TECTONICSContinental Drift

& Seafloor Spreading

Continental Drift

• German scientist Alfred Wegener proposes the theory of Continental Drift in 1912

What does Continental Drift mean?

• It means exactly what is sounds like!

• Continental Drift = continents (land masses) drifting (moving)

• How many continents are there?• There are 7 continents.

•Wegener hypothesized that all the present continents once formed a single landmass, which he named Pangaea.

•Pangaea means “all lands”•Panthalassa was the huge

ocean surrounding Pangaea.

The break up of Pangaea

Evidence of Continental Drift

• 1. Similarities in the coastlines

Evidence of Continental Drift

•2. Fossils of the same plants and animals found in areas that had been adjoining part of Pangaea.

Mesosaurus• Small extinct land

reptile• Lived 270 million

years ago• Fossils found in

eastern South America and western Africa.

Glossopteris• Fossilized fern found

on 4 continents• Continents with very

different modern climates, led Wegener to believe these continents were once connected having a similar climate

Evidence of Continental Drift

• 3. Geologic Evidence• Age and types of rocks• Mountain age and structure• Climatic pattern• Debris form glaciers• Coal deposits

Did people accept Wegener’s theory?

• NO!• The evidence Wegener had to

support his theory was not enough to convince many people during his lifetime.

• Not until the 1960’s were his beliefs accepted by the scientific community.

• The truth was out there!

Seafloor Spreading• In the 1960’s, Harry Hess

developed his now famous and accepted theory of Seafloor Spreading.

• Hess proposed that hot, less dense material in the mantle is forced upward to the surface at a mid-ocean ridge.

• Example: Mid-Atlantic Ridge

Harold Hess

Evidence of Seafloor Spreading

• 1. Age evidence

• Studies by the Glomar Challenger in 1968 discovered that the youngest rocks were found at the mid-ocean ridges and rocks became increasingly older farther from the ridges on both sides.

Evidence of Seafloor Spreading

• 2. Paleomagnetism (Magnetic clues)• Important evidence that led to the

absolute belief in seafloor spreading• Scientist found that rocks on the ocean

floor showed many magnetic reversals.• Reverse back and forth in strips parallel

to the mid-ocean ridge.• Evidence needed to solidify Wegener’s

original hypothesis.

Theory of Plate Tectonics

• By the 1960’s Continental Drift was widely accepted and this led to the rise of another theory known as Plate Tectonics.

• The term tectonic comes from the Greek work tektonikos meaning “construction”

• Plate tectonics not only describes continental movement but also proposes a possible explanation of how and why the continents move.

•The theory of Plate Tectonics states Earth’s crust and upper mantle are broken into sections.

•These sections, called plates, move around on the mantle.

•About 30 lithospheric plates have been identified

•Some of these plates are moving towards each other, some are moving away from each other, and some are sliding past one another.

How fast do plates move per year?Plate movements vary, one

example:

Lithospheric Plate Boundaries

• The boundaries or edges of the plates have been divided into three types of plate boundaries. They are:

• 1. Divergent Plate Boundaries• 2. Convergent Plate Boundaries• 3. Transform Fault Boundaries

1. Divergent Plate Boundaries

• 2 plates moving away from each other

• Examples:• Mid-Atlantic Ridge• The Great Rift

Valley in eastern Africa

2. Convergent Plate Boundaries

•Plate colliding (pushing in to) with neighboring plates.

•There are 3 types of Convergent Plate Boundaries, They are as follows:

1. Ocean-Continent

• The area where the oceanic plate descends into the upper mantle is called a subduction zone.

• The oceanic crust subducts (moves under another) because it is less dense than the continental crust.

• Oceanic crust is generally basaltic in nature as continental crust generally granitic.

• Ocean trenches generally form along subduction zones.

• Many volcanic mountains also form along subduction zones.

• Examples: The Andes Mountains in South America where Nazca and South American Plates are colliding.

• Where does this happen in the U.S.?

2. Continent-Continent

• When two colliding plates have the same density neither plate is subducted, the plates experience uplifting instead.

• Example: • The Himalayan

Mountains• Formed when the

Indo-Australian Plate crashed into the Eurasian Plate.

• This is still going on today as the Himalayas are growing.

3. Ocean-Ocean

• Part of the subducted plate melts, and the resulting molten rock rises to the surface along the trench to form a chain of volcanic islands, called an island arc.

•Example:•The islands

of Japan are volcanic island arcs formed when two oceanic plates collided.

ReviewThe Three types of

Convergent Boundaries

• Ocean-Ocean (Subduction occurs)

• Ocean-Continent (Subduction occurs)

• Continent-Continent (Uplifting occurs)

3. Transform Fault Boundaries

• Forms when two plates are sliding past one another.

• Scrape and move in series of sudden spurts of activity separated by periods of little or no movement.

• Example: San Andreas Fault where the Pacific Plate is sliding past the North American Plate

Causes of Plate Tectonics

• Convection Currents• The entire cycle of heating, rising,

cooling, and sinking is called a convection current.

• The same process occurring in the mantle is thought to be the force behind plate tectonics.

• Convection occurs in a heated water filled beaker.

• The same process is thought to happen in the mantle as it is also experiencing heat gain rising from the mantle.

Effects of Plate Tectonics• 1. Tension forces• Rift Valleys form

where plates are diverging.

• Examples:• Great Rift Valley

of Africa• Earthquakes• East Pacific Rise• Tension also

forms normal faults.

Effects of Plate Tectonics

• 2. Compression Forces

• Mountain Building• Island Arcs and

Volcanoes form• Reverse Faults

also occur.• Earthquakes

Effects of Plate Tectonics

• Shearing forces create strike-slip faults

• Example:• San Andreas

Fault, California• Earthquakes