Continental Drift and Seafloor Spreading Notes
Continental Drift
Alfred Wegener
Alfred Wegener was a German scientist.
In 1915, he proposed the theory of continental drift.
The theory suggests that the continents were slowly drifting apart.
Wegener's Theory
Wegener proposed that all continents were once joined together in a supercontinent called Pangaea.
Pangaea existed approximately 200 million years ago.
About 200 million years ago, Pangaea began to break up.
The pieces drifted apart to form the present-day continents.
Evidence for Continental Drift
1. Fit of Continents
The coastlines of Africa and South America fit together like a jigsaw puzzle.
This suggests they were once joined before splitting apart.
2. Fossils
Fossils of the same plants and animals are found on widely separated continents.
These continents include Africa, South America, Antarctica, Australia, and Asia (India).
This suggests that the land was joined before it split apart.
Fossil Example: Mesosaurus
Mesosaurus fossils have only been found in South America and Africa.
Mesosaurus was a freshwater reptile.
It would have been difficult for it to migrate across the ocean.
Fossil Example: Lystrosaurus
Lystrosaurus fossils were found in Africa, India, and Antarctica.
Lystrosaurus was a land reptile.
It would have been even harder to imagine Lystrosaurus swimming across an ocean.
Fossil Example: Cynognathus
Cynognathus fossils are found in Africa and South America.
It’s unlikely Cynognathus could have made it across the ocean either.
Fossil Example: Glossopteris
Glossopteris fossils, a tropical tree, are found in South America, Africa, India, Australia, and Antarctica.
Fossil Evidence Summary
If the continents had been together as Wegener claimed, the animals that became fossils would have been living close together.
…and then been split apart as the continents moved.
Africa: Lystrosaurus fossils found.
India: Lystrosaurus fossils found.
South America: Mesosaurus and Cynognathus fossils found.
Australia: Glossopteris fossils found.
Antarctica: Lystrosaurus and Glossopteris fossils found.
3. Mountain Chains
Mountain chains match up on each side of the Atlantic Ocean.
The Appalachian Mountains seem to end in North America.
Mountains in Europe are the same age.
If the continents are put back to their pre-drift locations, the mountains form a continuous chain.
4. Climate
Evidence of glaciers is found on the equator.
Fossils of tropical forests are found in the arctic.
Climate Example
Evidence of glaciers is found along the equator (the warmest area of the planet).
Fossils of tropical rainforests are found in New Brunswick, northern Canada, and northern Europe.
Climate Explanation
It doesn’t make much sense unless the continents are moved back to their pre-drift location.
Now the glaciers are near the pole, and the palm trees are on the equator.
Reception of Wegener's Ideas
The scientific community rejected Wegener’s ideas.
He was forgotten for almost fifty years.
He couldn’t explain how the continents moved.
Seafloor Spreading
Even before Wegener, in 1872, people were laying telegraph cables on the ocean floor.
While surveying the ocean floor, undersea mountains (ridges) were discovered.
These ridges were confirmed in 1925 when sonar was invented.
In 1953, it was discovered how extensive these ridges were- right around the Earth.
Terms to Know
Seafloor spreading: The process of new crust forming at the ocean ridges and spreading outwards.
Subduction: The process by which areas of the earth’s crust are sinking down forming ocean trenches.
Ocean Trenches: Areas where subduction takes place.
Magnetic Striping: Patterns of magnetism trapped in rocks on either side of plate boundaries.
Harry Hess: Seafloor Spreading
In 1962, Harry Hess discovered cracks in the mountains, called “rifts”.
He proposed that new crust was forming and moving outward, away from the rifts.
The process is called seafloor spreading.
Evidence for Movement
Magnetic Striping
Age of Seafloor
Sediment Thickness
1. Magnetic Striping
Magnetite is a mineral found in many rocks.
A piece of magnetite acts as a compass.
When molten rock solidifies, the magnetite all lines up in the direction of the earth’s magnetic field.
Each piece of rock acts as a tiny magnet.
What We Know
The sea floor has bands of alternating north and south facing magnetite.
These bands are parallel to the mid-ocean ridges.
What We Propose
Is due to changes in the earth’s magnetic field over its history.
It is because earth’s magnetic field has changed every few million years.
The patterns on either side are symmetrical.
Meaning new seafloor is being added equally on each side of the ridge.
2. Age of Seafloor
When rocks on the sea floor were dated, they found that the rocks further away from the mid-ridge are older than those close.
The oldest seafloor rocks were 200 million years old, some continental rocks were thousands of millions of years old.
This means the sea floor is younger than the continents.
3. Sediment Thickness
Sediment is thicker the further away you get from the ridges.
This shows that sediment has been falling a longer time away from the ridges.
Sediment Thickness vs. Distance from Mid-Ocean Ridge
Thickest Sediment Continent Oldest Rock/Crust
Mid Ocean Ridge
Thinnest Sediment Youngest Rock/Crust
Continent Oldest Rock/Crust