Subduction zones and mid-ocean ridges
Each plate has a variety of edges or boundaries. The boundaries of the plates are the most active parts of the surface and can be identified by concentrated tectonic activity, such as volcanoes and earthquakes. Where two plates meet, mountains form and sometimes one plate slides underneath the other. The areas where the plates are moving downwards are called subduction zones. Here the Earth is reclaiming rock from the crust and deep ocean trenches are formed due to the downward movement. On the opposite edge of the plate, new rock is being added to the Earth’s crust along mid-ocean ridges, akin to huge underwater volcanoes. This process moves continents slowly but steadily and is often called continental drift.
Distribution of plates, trenches and ridges
There are about eight major plates and various smaller ones on the Earth’s surface. Tectonic activity is infrequent in Britain today, showing that the British Isles are far from the boundary of the plate, and are part of the more stable inner-area (of the Eurasian plate). This has not always been the case.
Evidence for plate tectonics – It’s in the rocks
A continental jigsaw
Perhaps the most immediately obvious evidence for plate tectonics is the fit of the continents. Notice how the boundaries of the continents fit like a giant jigsaw, especially clear is the boundary between the east coast of South America and the west coast of Africa. There are other more subtle pieces to the jigsaw, such as regions of glacial deposits, which although separated now, also fit together when the continents are restored to their ancient positions. It may seem obvious now, but the concept was actually overlooked for a long period of time by geologists. In fact, the fit of the continents was noticed as early as 1620 and theories were proposed in the 1800’s akin to continental drift, but they were not taken seriously by scientists of the day!
The close fit between the coastlines of South America (orange) and Africa (green) is very clear.
The evidence for plate tectonics comes primarily from the oceanic crust, which forms the sea floor, rather than the familiar, more complicated and much less active continental earth crust. The continents are rather stable through time and while the oceanic crust is destroyed at subduction zones, the rocks of continents are rarely destroyed. The oldest rocks are to be found on the continents. At 2,700 million years old, the Lewisian rocks (metamorphic and igneous) of the Outer Hebrides, Scotland are the oldest rocks in Britain.