The best evidence for plate tectonics is made possible by modern technology. Now, we can use GPS to monitor movement. For example, to learn how to San Andreas fault is moving and how fast it might be moving, researchers put GPS stations along the fault. Then they can track the movement every day and over a year they can map exactly how fast it is moving, and in which directions. Scientists do this along tectonic plates all over the world so we can track plate movement (which often correlates to earthquake monitoring because earthquakes often occur along the edges of tectonic plates).

There is so much evidence that I could not possibly explain it all. Arguing against the theory of plate tectonics has become like arguing against the theory of gravity. Plate tectonics causes earthquakes, and earthquakes happen as a result of plates sliding past one-another, which is why earthquakes all cluster around edges of plates. Plate tectonics causes the formation of new ocean crust at mid-ocean ridges, and we can measure the age of ocean crust via magnetic minerals inside of the crust, and we can see that the ridges create new crust because that's where the youngest crust is. Plate tectonics causes continents to move; we know this because continents fit together, because the ranges of fossil organisms link up when the continents are fit together, and because the earthquakes on continents create mountain ranges around the edges of plates. Last, but hardly least, using laser-sensitive instruments and spacecraft, we can actually watch the continents move.

Volcanic eruptions, earthquakes and mountain ranges are signs that the Earth beneath us is moving. For example, the Atlantic ridge has lava coming out all the time because the American plates are moving away from the African and European plates. If you look at a map, you will see that Africa and South America look like they fit together; this is because they were originally next to each other and drifted apart.

Volcanic eruptions in mountain ranges (like the Andes) occur because one tectonic plate is going under another tectonic plate (subduction zone), which pushes lava to the surface. The lava is under a lot of pressure, which is why the eruptions are so impressive. Oceanic plates usually go under continental plates, because they are denser.

You will also find mountain ranges near subduction zones (e.g. the Andes, the Rockies…) because the oceanic plate under pushes the continental plate upwards.

Earthquakes occur when two tectonic plates slide against each other. The plates are usually stationary, but when enough pressure builds up, they slide violently at areas called fault lines, causing earthquakes. Our major fault line in California is the San Andreas fault line.

See this picture: plate tectonics