Earth’s surface is always changing. Some changes, caused by erosion, are slow. Some changes, caused by earthquakes, are fast. The Earth’s crust is like a puzzle. It is made up of huge pieces called tectonic plates. The tectonic plates float on top of the mantle. The mantle is a soft, hot rock layer located between the crust and the core. Heat is transferred from the core to the crust via convection currents (look at the arrows). Heat in the core causes magma to rise in columns in the mantle. The hot magma columns move the plates and then when the magma cools, it sinks back towards the core.
The currents of hot magma pull and push the plates, causing the plates to smash together, slide apart, or grind past each other. This results in earthquakes, volcanoes, and shapes many of Earth’s landforms.
The theory of Plate Tectonics was defined in the 1970s. It follows Alfred Wegener’s 1912 theory of continental drift. He believed the continents were once one landmass. The landmass split apart and moved to their current locations. His evidence was the shape of the continents which suggests they were once joined. Fossils are found in similar locations across multiple continents where the organisms would be found if the continents were once joined. This was a first step towards the plate tectonics theory.
Tectonic plates are either Oceanic Plates or Continental Plates. Oceanic plates are primarily ocean basalt. They are dense and mostly made of basalt rock. Continental plates rise above sea level. They are less dense and made mostly of granite. Plates can move towards each other, away from each other, or slide past each other. Plates only move 3-5 cm per year, which makes their normal movement impossible to observe.
A convergent plate boundary occurs when two plates push together. When two continental plates collide high mountains, such as the Himalayas or Andes, can be formed. When two plates of different densities collide, such as a continental plate and an oceanic plate, subduction occurs. The denser plate (oceanic) sinks beneath the less dense plate. Volcanoes are frequently found at convergent boundaries.