Plate Tectonics
What are the plates that sit under our feet?
Plate tectonics
Did you know the Earth’s crust is made up of a bunch of “tectonic” plates which are constantly shifting? Don’t worry, each plate moves super slowly -- think a few millimeters to a few centimeters per year, depending on the place.
How many tectonic plates are there? No one knows for sure, but most estimates fall in the range of 15-25, with a few big ones taking up the most space. For example, you may hear terms like “the Eurasian plate”, which takes up most of Eurasia, though pieces of other, smaller plates underlie parts of Asia.
Tectonic plates can be made up of “oceanic crust” or “continental crust”. Some plates are made up of a bit of both. As you can probably guess, oceanic crust sits low underneath most of the oceans while continental crust sits higher and parts of it reach the surface, allowing us to use land as what we call “continents”. This is because oceanic crust is denser (aka heavier) than continental crust. As a result of this, when two plates, one of oceanic crust and one of continental crust are moving towards each other, the oceanic plate sinks beneath the continental crust.
When one continental plate crashes into another continental plate, they go up, causing mountain ranges. Oftentimes the crusts stay connected, leaving the differentiation between crusts and their underlying plates hard to determine. Generally, a piece of crust is considered its own plate if it still has its own range of motion. For example, the Australia-India plate is still moving northward relative to the Eurasia plate, raising the heights of the Himalayas at a rapid pace of 5 millimeters per year, so we consider them as separate plates.
When two plates spread apart, usually where two plates connect in the ocean, this is called a “ridge” or a “ridge line”. As the plates pull apart, lava flows out from the mantle (layer of the Earth beneath the crust). We can think of this as the yolk from an egg flowing out when its shell is cracked. The lava cools and turns to rock, leaving young rock at the ridge lines and older rock further away. We can see the process of lava flowing out of the ridge line to make new, young rocks actively happening at the Mid-Atlantic ridge as one piece of evidence for plate tectonics. We can add to this evidence by checking ages of rocks near the surface as we move away from ridge lines and see the rocks get older and older.
Evidence for plate tectonics
There are a handful of pieces of evidence we can look towards for plate tectonics. In the last section, we touched on mountain ranges and ages of ocean floors. Some other evidence includes paleomagnetism, earthquakes, shapes of continents, and fossils.
Why are fossils important here? Well frequently fossils of as species of very similar ages are found on the edges of two continents, but not across anywhere which currently connects the two continents. For example, fossils of the same species of the same ages can be found on the Eastern coasts of North and South America, and on the Western Coast of Africa. In other words, either the coasts would have had to have been connected during the time which the species was alive, or the coasts were once connected and have now separated. To be clear, dinosaurs did NOT have oceanfaring vessels, meaning the evidence points towards a much more likely conclusion that the continents used to be stuck together..
Further more, if we look closesly at the edges of some continents with similar fossil records, we also see the possibility of continent shapes fitting together.
This is what gives us Earthquakes, mountains, and volcanoes.