The evidence for Plate Tectonics is very conclusive. It is very well supported theory, and while scientific debate continues about small parts or local effects, the overall concept is accepted as good as fact.
So what are the pieces of evidence?
Shape of continents
From almost the creation of the first true maps of the Earth, people started seeing how continents would be able to fit together. In particular, people noticed that South America fits almost exactly into Africa.
You should be aware that while world maps were around early in the 1600's and better defined by the end of the 1700's, those maps were not 'public' but were treated as state secrets. And so it was not till much later that the broader 'science' community had access to good quality and accurate world maps.
It is now known that most of the major continental masses can be fit together in a jigsaw process. In fact we now know that the continents were indeed once all joined together as one land mass - the supercontinent of Pangaea.
If you look at most world maps you will ponder how this is evidence, as the continents really don't appear to 'fit' together very well. What you need to understand is that the vast majority of maps are drawn using a map projection that has the north and south poles (which are points) are a line the width of the map at the top and bottom. This means that the map distorts the true shape of the Earth's land and oceans as you love closer to the poles. Some other map projections distort less, but have the map shape looking like segments of an orange. See here for a range of map projections. Check out the way Greenland changes size in each!
What is more remarkable, is the next piece of evidence when added to the shape of continents.
Location of mountains and fossils
What is remarkable, when you reassemble all the continents together, the ancient mountain ranges (many now eroded to their cores) and the location of rock types and fossils all match up.
Earthquakes and Volcanoes
As you can imagine, if plates that are made up of huge slabs of rock are grinding past or under each other, then those would be the places where the vast majority of earthquakes would take place. If we map the location of all the earthquakes recorded over time, almost all of them occur along what we recognize as plate boundaries.
Likewise, the exception of special volcanoes called hot spots (see below), all of the active volcanoes on Earth are found on some of the boundaries between plates.
In this diagram, we see the Aleutian area off North America. The black dots and circles represent recorded earthquakes. You can see they form a zone. Look above and below the zone, and there are almost none. So the earthquakes define the boundary. Not only that, they show that earthquakes are shallow in depth on the southern side of the zone getting deeper and deeper as you go northwards. This is because the Pacific Plate to the south is being forced under the North American Plate in the north. As it is forced under it is getting deeper and deeper until it finally melts into the underlying mantle.
The red triangles are active volcanoes. They also follow the earthquake pattern but are found on the far side of the zone, along with the deep earthquakes.
Hot Spot Volcanoes
Deep in the Earth, possibly at the boundary between the outer crust and the mantle, some form of disturbance funnels vast heat up towards the Earth's surface in places we call Hot Spots. These Hot Spots are localized intense zones that can melt the overlying crust and form a volcano.
Because the Hot Spot is completely independent of what is happening with the crust, the resulting volcanoes are not related to the position of the plates. So many hot spot volcanoes occur nowhere near a boundary.
But what is more significant is that when the plate moves away from the hotspot, the hot spot volcano no longer has the heat and becomes extinct. The new area of plate above the hot spot then melts and a new volcano is formed. So we end up with a chain of volcanoes, the active volcano over the hotspot and the remains of volcanoes getting older and older as you move away from the hot spot.
The Hawaiian Islands are an example of this hot spot phenomenon. The most active volcano, Kilauea, is above the hotspot, and the volcanoes moving north-west get older and older. The largest of these still stick out of the ocean, but many more remnants of volcanoes that are now further away from the hot spot and most are under the ocean surface.
Because we know the age of the rocks on each of these old volcanoes, and we know how far away they are from the hot spot, we can calculate how fast the crustal plate is moving over the hotspot. In Hawaii's case, the Pacific Plate on which the island sit is moving towards the north-west at around 10 cm a year.
Other hotspots exist around the globe which can give us the speeds of other plates.
For more of our blogs on Plate Tectonics see these posts: