In this article, we will learn about continental drift.
Wagner’s continental drift hypothesis
The main objection to Wagner’s proposal was that it was unable to provide a mechanism for the continent actually moving and separating apart.
We’re going to talk about some of the different evidence that Wagner used when he was talking about continental drift. The first one he used is probably one of the most compelling and that we’re most familiar with and that would be the fit of South America and Africa together.
Some people use that to say that they don’t fit together. But when you look at the continental shelf, so not just the land that’s above the ocean but the oceanic shelf, these fit together really well.
Different types of volcanic hazards
Now one of the other types of evidence he used was fossil matches across the different types of seas. You can see that in some areas like South America you’re gonna get this fossil that goes from South America into Africa and you’ll even get some that will spin like a fossil of a fern that goes from Australia to Antarctica, India, Africa all the way to South America.
You can see that on the eastern coast of North America we have the same types of rocks. Same types of structures that you would see on Greenland and that you also see in Africa and some of the island areas.
So we have Earth’s rigid outer shell which is the lithosphere. It’s divided into numerous slabs that are called plates. So there are seven or eight major plates but within that, there are probably a dozen or so smaller ones.
Now we have the lithospheric plates. They’re moving relative to each other very slowly. The continuous rate is gonna average of about two inches a year.
So these grinding movements of the plates can generate earthquakes, create volcanoes, and form large masses of rock and mountains.
How fast and how slow plates are moving?
The absolute slowest one is going to move somewhere between one and four centimeters a year. That’s roughly the rate that your femur and heels will grow.
Now the fastest is about 16 centimeters a year and that’s the quickness that your hair actually grows out. In the middle of the North American plate and you can see that there are some larger plates but there are also some smaller ones there as well.
Today continents make up about one-third of the earth’s surface but contain the oldest rocks on the planet. Over 3.8 billion years old analysis of these rocks reveals even older zircon geochemical investigation of the zircons and smaller fragments within them shows that they formed at relatively low pressures and temperatures.
In molten material rich in water and silica at convergent plate boundaries such as volcanic island arcs this suggests that plate movement and subduction were active. And liquid water and continental crust were present four billion years ago.
Subduction of the primitive crust rocks led to selective melting with increasing heat at depth. Preferential melting of silicate minerals with the lowest melting points and relatively lower density formed magmas that roses into the crust and solidified forming granitic rock bodies near the surface.
These initial island arcs’ microcontinents and their granitic bodies grew further as they converged and joined together. It is likely that the first continental crust formed after a primitive crust had already developed and convection had started in the mantle.
Continental crust is informed when rocks in the mantle melt. Later solidity in the process becomes differentiated from the mantle. The process was probably particularly rapid above.
Sinking flows in the mantle and slower above rising flows where the continuous supply of mantle rocks slowed the rate of differentiation. Earth’s thin outer crust and upper mantle down to a depth of about 100 to 300 kilometers are divided into continent-sized plates that jostle against one another.
As the plates move oceans are created and later disappear and volcanoes and mountain chains are the mantles below. As they do so the mantle wells up and the crust bulges rupture along weak points called faults and eventually rift apart.
Pressure released allows the hot crust to melt forming magma that erupts as lava through ridges and valleys on either side as they slowly cool and shrink. The ridge flanks subside in their surface as smoothed out by the deposition of blankets of sediment.
A new crust is created by spreading ridges but the earth is not expanding. Divergence is one place results in convergence in another. On average the crust is less dense than the mantle and oceanic plates are denser than continental plates.
Because they contain a thinner crust as a result where oceanic and 3 melting and releasing magma which erupts at the surface volcanoes and earthquakes are violent expressions of the earth’s internal dynamic forces.
The vast majority occur at plate boundaries and are intimately connected to plate interaction. Diverging plates stretch and break generating shallow earthquakes and volcanic eruptions.
Most of which occur at spreading ridges in ocean depths and produce magma made up mainly of basalt. Converging plates however generate earthquakes as far as 700 kilometers.
Convergence brings different organisms together in competition while divergence separates species groups which then evolve in different conditions.
An example is the supercontinent of Gondwana formed around 500 million years ago evolving life-forms spread throughout this enlarged landmass leaving a record of themselves behind as fossils.
Thus fossils of the same species have been recovered in rocks from what are now widely separated continents. Only when isolating these creatures from one another did groups begin to evolve in different ways.
In the mantle is a layer of red-hot semi-molten magma and beneath that is an inner core with temperatures of about five thousand five hundred degrees centigrade. So you’d hope the crust protecting us from total meltdown.
It’s only between 25 and 100 kilometers thick. So the earth was like an apple. Then the crust is only about the thickness of its skin. Unlike the apple skin, the crust of all planets isn’t one solid surface.
Both earthquakes and tsunamis are caused by this process, the same destructive plate boundary has created composite volcanoes that affect japan and many other countries.
The massive pressure makes the earth’s crust buckle. The result folds mountains and that collision plate boundary is still forcing the Himalayas and Mount Everest higher and higher in the air every year.
In California conservative plate boundaries aren’t moving towards or away from each other but actually moving alongside each other at different speeds or in opposite directions.