Tectonic plates are large plates of rock that make up the foundation of the Earth's crust and the shape of the continents. The tectonic plates comprise the bottom of the crust and the top of the Earth's mantle. There are ten major plates on Earth and many more minor ones. They float on a plastic-like part of the Earth's mantle called the asthenosphere. The plates are most famously known for being the source of earthquakes.
The tectonic plates are about 100 km (60 miles) in thickness, with continental plates tending to be thicker than oceanic ones. The composure of the two types of plate is also quite different. Oceanic plates consist of thicker basaltic rocks, compressed by the pressure of kilometers of water. Contintental plates have a lower average density, containing granitic rocks with a heavy composition of aluminum and silica.
The mantle underneath the tectonic plates is constantly recirculating, causing the plates to float around slowly in a process called tectonic drift. This process was described well by the theory of plate tectonics, which solved several scientific dilemmas about the distribution of species when it was introduced. When plates push up against each other, they create mountain ranges and volcanoes. Mt. Everest was created in this way.
Because the plates are so large, each wraps over a considerable portion of the Earth's surface, making them curved. This is a different shape than the flatness the word "plate" suggests.
Over time, plate tectonics has caused the world's continents to be reshaped. Every continent on Earth was once part of an ancient supercontinent known as Pangaea, and Antarctica was once located in a temperate climate. Marine fossils can be found on the peaks of the world's tallest mountains. The tectonic plates continue to move slowly, but it is unlikely that their movement will cause the world's face to change more rapidly than the growing technological influence of mankind will. One day the plates' great momentum and pressure might even be used as a source of geological energy.