Plate tectonics

Plate Tectonics

Continental Drift

4-1 Continental Drift – objectives…

Explain Wegener’s hypothesis of continental drift.

List evidence for Wegener’s hypothesis of continental drift.

Describe seafloor spreading.

Continental Drift

Began with observations made over 400 years ago.

The first reliable maps were studied and it was noticed that the continents fit together like a puzzle.

Hypothesis on Continental Drift.

The continents were once part of the same landmass – Pangea

Surrounding Pangea was a great ocean – Panthalassa

About 200 mya, Pangea began breaking apart.

This motion caused crumpling which led to today’s land features.

Alfred Wegener

A german scientist. Proposed his

hypothesis on continental drift in 1912.

Observations led to questions…

Were the continents once part of the same landmass?

If so, what caused this landmass to break apart?

What caused the continents to move to their present locations?

Evidence

Coastline similarities

Evidence

Fossils Mesosaurus Cynognatyhus Lystrosaurus Glossopteris

Geologic Evidence

Age and types of rocks

Mountains Applachian mountains

fit continuously with a band of mountains in Greenland.

Climatic Evidence

Layers of glaciers in southern Africa and South America

Coal deposits in the Eastern US, Europe, and Sibera

These climatic similarities are easy to explain if the continents were once joined.

Climatic Evidence - glaciation

Evidence of ancient glaciation indicated that parts of southern Africa, India, Australia, and South America were covered by a large ice sheet. Arrows indicate direction of ice movement.

There was little support for Wegner’s hypothesis….until.two decades after his death…

Seafloor Spreading

1947, mapping the Mid-Atlantic Ridge Discovered the ocean floor is much younger

than continental rocks. Oceanic rocks – 150 million years old Continental rocks – 4 billion years old

Harry Hess suggested a hypothesis Seafloor spreading

Rocks in/near the rift were younger as a result of the upwelling and solidification of magma.

Seafloor Spreading

Seafloor spreading provided the support Wegner needed for a mechanism for continental drift.

Still no proof…

Paleomagnetism

Magnetism resulting from the cooling of magma mirrors the existing magnetism of the earth.

Magnetic orientations were discovered that appeared normal then reversed…normal then reversed…

This supported seafloor spreading…and thus further anchored Wegner’s Hypothesis.

Paleomagnetism

Review of 4-1…

1. What observation first led to Wegner’s hypothesis of continental drift?

2. What types of evidence support Wegener’s hypothesis?

3. Describe the process of seafloor spreading.

4. Explain how scientists know that the earth’s magnetic poles have reversed themselves many times during earth’s history.

4-2 – Plate Tectonics - objectives

Summarize the theory of Plate Tectonics. Compare the characteristic geologic

activities that occur along the three types of plate boundaries.

Explain the possible role convection currents in plate movement.

Summarize the theory of suspect terranes.

A theory emerges…

1960’s… A combination of continental drift and

seafloor spreading… Describes continental movement but also

proposes a possible explanation of why and how continents move.

The Earth’s Layers

Two types of crust: Oceanic – ocean floor Continental – dry land

Both types of crust plus the rigid upper mantle make up the Lithosphere.

Himilayian Crust

The Earth’s Layers

Lithosphere – the thin outer layer Lithospheric plates

Asthenosphere – a layer of “plastic” rock Denser than the

lithosphere.

Earth’s Layers

Lithospheric Plate Boundaries

Lithospheric Plate Boundaries

Continent margins don’t necessarily follow plate boundaries.

30 plates have been identified at some speed of constant motion.

Types of Plate Boundaries

Divergent Boundaries

spreading centers where two lithospheric plates move away from each other

Divergent Boundaries

Convergent Boundaries

A converging boundary forms when two plates move toward each other

Two Types Collision Subduction

Convergent Boundary - Collision

Oceanic crust – Oceanic crust

Convergent Boundary - Collision

Oceanic crust – continental crust

Convergent Boundaries

Continental crust – continental crust

Transform Fault Boundaries

San Andreas fault, California

Causes of Plate Motion

Many scientists think that the movement of the lithospheric plates is due to convection currents.

Convection Currents