Plate Tectonics

What is Plate Tectonics?

Earth’s surface is made of rigid slabs of rock called plates that move with respect to each

other.

Alfred WegenerNearly 100 years ago, a German scientist named

Alfred Wegener proposed a radical idea. He proposed to his colleagues that the continents

were not fixed to the earth’s surface, but rather they were in constant motion which he called

Continental Drift.

Wegener’s Proposal cont.

In addition, Wegener also proposed that millions of years ago, all the continents were

part of a supercontinent which he named Pangaea

Pangaea

Evidence

As you may guess, no one believed him. He need proof. So he began his investigation to

prove his theory. He researched three different areas: Climate, Rocks, and Fossils.

Climate

Wegener suggested, at the time of Pangaea, parts of South America, Africa, India, and

Australia were located closer to Antarctica and would have been much colder and possibly

covered by glaciers.

Rocks

Wegener analyzed sediment deposits and glacial grooves cut into the rock from each

continent. He found similarities in sediment deposits and grooves carved by glaciers that

helped support his theory.

Evidence of Glaciation

Rock EvidenceThe Caledonian and Appalachian Mountains appear to

be a single mountain chain composed of the same rock type, age, and structure.

Fossil Evidence

GlossopterisAn ancient plant that lived in a warm wet climate. The fossils have been found in

South America, Africa, India, and Australia.Today, these continents are separated by vast

oceans and it would impossible for seeds to travel that far.

Glossopteris

Mesosaur

Distribution of Glossopteris and Mesosuar

More Fossil Evidence

Separation of the Supercontinent

120 million years ago Pangaea split into two giant land masses, Gondwanaland and

Laurasia

Gondwanaland

Gondwanaland remained in the southern hemisphere and consists of Africa, South

America, Arabia, India, Australia, and Antarctica.

Laurasia

Laurasia moved to the north and was composed of North America and Europe and

Asia

The Breakup of Pangaea

How the Continents Move

Lithosphere

The cold, rigid outermost layer of the Earth is known as the lithosphere. The lithosphere is thicker below the continents and thinnest at the mid-ocean ridges. The lithosphere forms

the floating plates. The largest plate is the Pacific plate and the smallest is the Juan de

Fuca plate.

Asthenosphere

The layer below the lithosphere is the asthenosphere. This layer is so hot that it behaves like a plastic material that flows.

Plate Boundaries

Plate boundaries are located anywhere that two or more plates interact. There are three types

of boundaries: Divergent, Convergent, and Transform.

Divergent Plate Boundaries

A divergent plate boundary forms where two plates separate.

Rift Valley

A rift valley is a divergent plate boundary that occurs in the middle of a continent.

Rift Valleys

Mid-Ocean RidgesMid-ocean ridges are mountain ranges in the

middle of the oceans.

Seafloor Spreading

Seafloor spreading is the process by which new oceanic crust forms along a mid-ocean ridge. As new crust is formed, older oceanic crust moves

away for the ridge.

Seafloor Spreading

Convergent Plate Boundaries

Convergent Plate boundaries form where two plates collide. The denser plate sinks below the

more buoyant plate in a process called subduction. The are three main types of

convergent boundaries: Ocean-to-Continent,Ocean-to-Ocean, and Continent-to-Continent

Ocean-to-Continent

Ocean-to-Continent

Mount Rainier in Washington State

Ocean-to-Ocean

The Marianas Trench

The Marianas Trench is the deepest location on the planet at 36,200 feet, (6.9 miles), deep. At

that depth the pressure on a body is8 tons or 16,000 lbs!

Mariana Trench

SCARY CREATURES IN MARIANAS TRENCH

Continent-to-Continent

Continent-to-Continent

The Himalaya mountains are created by the collision of the Eurasian and Indian Plates

Forces Causing Plate Movement

Convection Currents

Convection current is the circulation of material caused by differences in temperature and

density. Convection currents in the mantle are created by the decay of radioactive elements

such as uranium, thorium, and potassium heating the surrounding rock.

Convection Currents cont.

The heated rock becomes less dense, rises to the crust, cools and sinks back down to be

heated again. These convection currents cause three forces to act on the crust: Basal Drag,

Ridge Push, and Slab Pull

Basal Drag

Convection currents in the asthenosphere circulate and drag the lithosphere similar to a

conveyor belt.

Ridge Push

Mid-oceanic ridges have a greater elevation than the surrounding seafloor. Because the

mid- ocean ridges are higher, gravity pulls the surrounding rock down and away from the

ridge. The rising mantle material at the mid-ocean ridge creates the potential for the plates to move away from the ridge with a force called

ridge push.

Ridge Push cont.

Ridge push moves the lithosphere in opposite directions away from the mid-ocean ridge.

Slab Pull

When tectonic plates collide, the denser plate will sink into the mantle along a subduction

zone. This plate is called a slab. Because this slab is old and cold, it sinks, pulling on the rest

of the plate with a force called slab pull.

In the end, Hess’s work with sonar mapping proved Wegener's basic idea right and clarified

the mechanism that broke the once-joined continents into the seven we know today and so

began the modern Theory of Plate Tectonics