Plate Tectonics Continental Drift
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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
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