PlateTectonics

(Continental Drift)

Internal vs. External Geologic Processes

Internal processes occur in the earth and build up the planet’s surface while external processes occur on the earth’s surface and wear the surface down.

Internal processes are run by heat from the earth’s interior.

External processes (weathering and erosion) occur directly and indirectly from the sun’s energy.

Internal Processes

The internal processes of the earth are know as the theory of plate tectonics.

The theory of plate tectonics has changed over the past 100 years.

Plate tectonic started as the theory of continental drift in 1912 with a man name Alfred Wegner.

Theory of Continental Drift

By the 1950s, most scientists had accepted the idea as theory based on several pieces of evidence.

Evidence to support continental drift:

1. Continents fit together like puzzle pieces

2. Fossil evidence

3. Mountain (core rock) evidence

4. Mineral, rock, and glacial deposit evidence

In 1912, the German scientist Alfred Wegener proposed the idea of continental drift. Continental drift states that Earth’s continents were once joined in a single large landmass known as Pangaea that broke apart, and that these continents have “drifted” to their current location.

1. Puzzle PiecesOne early piece of evidence for continental drift was that the continents seem to fit together like puzzle pieces.

Remember that continents are not just the portion of land above the water, but extend into the oceans (continental shelves), so continents can look very different today due to deformation.

2. Fossil evidence Fossil evidence has been found on different continents.

These fossils appear to be of the same geological time period, but the present day arrangement of the continents does not explain their distribution.

If the continents were connected at one time, the plants and animals could have distributed themselves over these areas.

The fossils used most to support Continental Drift are:

Glossopteris, Mesosaurus, Lystrosaurus, and Cynognathus.

Glossopteris was a woody, seed-bearing shrub or tree, reaching 30 meters tall.

Seeds, much too large to be wind-borne, could not have blown across thousands of miles of open sea, nor is it likely they have floated across vast oceans.

Glossopteris fossils have been found in Antarctica while the plant preferred a warm, moist climate.

Mesosaurus was one of the first fresh water reptiles, and had many adaptations to a fully aquatic lifestyle. Mesosaurus was around 1 meter in length, with webbed feet, a streamlined body, and a long tail that may have supported a fin.

As Mesosaurus was a freshwater animal, and therefore could not have crossed the Atlantic Ocean, this distribution indicated that the two continents used to be joined together.

Lystrosaurus was a pig-sized mammal, about 3 feet long and weighing about 200 pounds.

They had very short snouts and no teeth except for the tusk-like upper canines.

They were also thought to have had horny beaks like those of turtles.

Cynognathus was a heavily built mammal, and measured around 1 meter in body length. It had a particularly large head (1 foot in length), with wide jaws and sharp teeth. Its hind limbs were placed directly beneath its body, like a mammal, but its front limbs sprawled outwards like a reptile.

3. Mountain Range Evidence

Mountain ranges and core Precambrian age rock of the same age period, composition, deformation, and strata layering, can be found on different continents.

4. Rock, Mineral, and Glacial Deposits Evidence

Besides fossils, there are also mineral, rock and glacial deposits that can not be explained at their present location.

Coal deposits found in arctic regions when the fern that originally formed the peat needed a tropical climate to grow.

Glacial deposits are found in tropical or desert areas when these deposits need cold climates to form.

Present day glacial deposits

Possible formation of glaciers

These are our present day climate zones. So the areas of Pangaea with specific deposits would have needed to line up with these current day climate zones.

For instance, any where that there are glacial deposits would have had to be near poles, while coral deposits would have to be near the equators.

The earth has changed a lot in the past 400 million years and will continue to change in the future. Click on the picture above to watch the transformation of the earth over this time and into the future.

Pangaea existed during the Permian period. What was the earth like before that time? The earth continues to move as the earth cools and the continents continue to shift. We don’t have a clear picture of the earth prior to Pangaea due to the limited number of fossils and deposits available from earlier time periods.

Click picture above for video.

Click on the picture above to watch the future of the earth. See if you can follow where Texas is during this transformation.