What is it?
How do we know it is happening?
What are the different types of tectonic boundaries?
What are the effects of it?
Plate tectonics is the theory that plates of Earth's crust are sliding across the surface of Earth. This theory includes the effects of such movement.
But how do we know?
They look like they fit together...
Fossil and rock evidence indicates that they were once together...
The pattern of earthquakes and volcanoes seem to mark out chunks of Earth...
So, this man declared that the continents drift on top of the oceans:
His name was Alfred Wegener
And he was vilified for it.
More evidence was needed and to gather it they needed better tools.
Years passed and along comes WWII. In travels across the Atlantic, strong magnetic detectors were used by our ships to ensure that no German Subs were going under our defenses and they found the strangest pattern along the sea floor.
They had always known that molten rocks, when they solidify, will orient all their iron pointing toward the North Pole, but the data seemed to indicate that the North Pole wasn't always North. Take a look:
In this, black is facing North, white is facing South. This data spans the Atlantic Ocean.
What do you see in this?
Scientists wanted more data, so they dated those rocks and found the youngest were at the middle and the oldest at the edges.
Putting those together, they got this.
Since, they have even more evidence. Taking the iron alignment to the next level, the looked at the rocks in one place over time and found that the say that the North Pole moved!
Even more shocking, they found that rocks on different continents say the North Pole moved in different directions. How can that be? There cannot be two North Poles. But if the land were to have moved, and the Pole stay put, then we get this:
This is called "apparent polar wander path.
There has been additional evidence as well (like laser readings from New York to Paris indicating that those two cities are getting further apart), and very few scientists disagree with this theory.
The evidence shows that all the plates except the African plate are moving. The fastest, largest, and oldest plate is the Pacific plate which is moving at about 9 cm / year (that is the rate at which your fingernails grow!).
But is the Earth just growing? Or does it grow and then shrink? Where does the land that is created in the Atlantic go?
In the 1950s, a geologist studying earthquakes discovered the last piece of the puzzle...where the land disappears to.
He was researching the power and depth of earthquakes in the South Pacific and found this:
From where he was (the star), the earthquakes made a slope descending down into the Earth. This turned out to be the plate sliding under the other plate:
In other words, he discovered where the plates went.
As you can see, the land is sinking back into the Earth at these places and the force of that causes earthquakes and volcanoes. The earthquakes get weaker as the land slides past more magmafied rock further underground until some point where all earthquakes seem to stop. He concluded that this is the point where the rocks are too magmafied to resist the other rocks sliding (everything melts).
So, the three types of plate bounradies:
Looking closer at divergent boundaries:
These will have hot magma from the core (mafic and friendly) as well as weak, shallow earthquakes
Convergent plate boundaries:
These make powerful earthquakes and either tall mountains or dangerous (felsic) volcanoes.
Transform plate boundaries:
These cause powerful earthquakes!
These are the agreed upon plates: