The Birth of a Theory: Continental Drift

Throughout history, most people believed that the continents had always been in the same positions that they are today.

In 1915, a German scientist, named Alfred Wegener, proposed the idea of “Continental Drift”. Alfred Wegener

1. The earth is always changing, land masses (continents) are constantly moving. What is the theory of continental drift? The hypothesis :

What is the theory of continental drift? 2. Originally all of the continents were joined together in one super-continent called Pangaea and slowly moved to their current positions.

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What evidence did Wegener use to support his theory of continental drift?

1 - Earth’s landmasses—especially South America and Africa—fit together like a jigsaw puzzle.

Fossil Evidence 2 - The fossils from the exact same land-living animals and plants are found on continents now separated by the Atlantic Ocean.

Matching Rock Layers 3 - The rocks of South America match the rocks of Africa in age and layers.

Matching Mountains 4 – The Appalachian Mountains match the mountains in Ireland and Scotland.

Fossils 5 – Fossils of tropical swamp plants were found in the eastern United Sates, Europe, and Siberia.

Glacier Evidence 6 - Glaciers scars are found on continents such as Asia, India, and Australia which are too warm for glaciers today.

Panthalassa

Evidence of “continental drift”—. Physical fit of continents Fossil evidence Rock layer sequences Glacial evidence Matching mountain ranges Key concepts Alfred Wegener

Was Wegener’s theory believed? Scientists did not believe Wegener’s ideas because he couldn’t explain HOW the continents moved!!! NO

Harry Hess Harry Hess was a navy sea captain and scientist that proposed the idea of seafloor spreading.

A bit of history… Sonar was developed during World War II to locate German submarines.

After the war, sonar was used to map the ocean floor.

Many new features were discovered… 1 – a huge volcanic mountain range in the middle of the Atlantic Ocean (M.A.R.) M.A.R.

2 – deep trenches in the Pacific Ocean

Hypothesis: The ocean floor is getting bigger at the ridges and smaller at the trenches. Evidence : The newest rocks are on the ocean floor along the Atlantic Ridge. oldest youngest oldest The oldest rocks are on the continents. The continents are much older than the ocean floor!

Volcanoes located along ocean ridges erupt, creating new ocean floor.

Iceland’s Volcanoes

World Earthquake Data

The earth’s lithosphere is broken into huge sections called plates that are in constant motion.

Plate Tectonics Plate Tectonics – The idea that the earth’s surface is broken into huge pieces called plates. These plates are constantly moving and bumping into each other, carrying the continents and ocean floor to new locations.

How fast are the plates moving?

You may wonder… How are the continents moving?

Convection Cells! Magma circulates under the ground, pushing around the plates. (Convection Cells)

What are the plates made of? Ocean plates are made of basalt. Continental plates are made of granite.

Plates —continental crust, oceanic crust Features —faults, trenches, mid-ocean ridges, folded mountains, hot spots, volcanoes Related actions —earthquakes, volcanic activity, seafloor spreading, mountain building, convection in mantle. Key concepts

Divergent Plate Boundaries Two land or ocean plates move apart in opposite directions. Magma flows to the surface between them creating new crust. ocean floor M.A.R. granite basalt

Mid-Atlantic Ridge = Divergent Boundary

Iceland – a continent directly over the Mid-Atlantic Ridge

Subduction Zones An ocean plate and a continental plate hit head-on. The ocean plate subducts under the continent forming a trench. The subducting plate melts. Magma rises to the surface creating a string of volcanic mountains parallel to the shoreline. trench volcanic mountains basalt granite

What are the plates made of? Ocean plates are made of basalt. Continental plates are made of granite.

Andes Mountains Subduction zones form chains of volcanic mountains along the shoreline.

Pacific Ocean

Cascade Mountains

The Cascade Mountains

Collision Zones Two continents hit head-on, crinkling up the land into a high mountain chain. granite mountains

India’s Collision with Asia Himalayas

The Himalayas Are Born…

Island Arcs Two ocean plates hit head-on. One ocean plate is forced to subduct under the other forming an ocean trench. The subducting plate melts. Magma rises to the surface forming a string of volcanic islands parallel to the trench. basalt

The Aleutian Islands

Island Arc Divergent Boundary Subduction Zone

How does tectonic activity affect the earth’s crust? Builds mountains Creates deep ocean trenches Causes earthquakes Create volcanoes

Ocean Trenches

Volcanoes

Most volcanoes occur at plate boundaries…

Recent patterns of earthquake and volcanic activities; maps showing the direction of movement of major plates and associated earthquake and volcanic activity Compressional boundaries: folded mountains, thrust faults, trenches, lines of volcanoes (e.g. Pacific “ring of fire”) Tensional boundaries: mid-ocean ridges, rift valleys Shearing boundaries: lateral movement producing faults (e.g. San Andreas Fault). Real-world contexts:

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Lithosphere