No body is perfect; forces on both the inside and outside of them push and pull them around. This leads to cracks and wrinkles on surfaces which represent the passage of time. On planetary bodies, this is referred to as tectonics.
Most bodies have tectonic features spread out over their surface. Earth is different. The surface is divided up into interlocking blocks which move around over time, called tectonic plates. These plates are constantly moving: colliding, separating or moving past each other, this movement causes the majority (but not all) of earthquakes and volcanoes on the Earth. The vast majority of tectonic features are related to either current or historical plate boundaries.
The Earth’s tectonic plates, courtesy of the U.S. Geological Survey
The driving force behind plate tectonics is dominated by the gradual overturning of the mantle as it moves heat from the core to the crust in a process called convection. The plates are also pulled by old cold crust sinking down and pulling the plate with it.
These forces linked to the moving of plates transfer stresses through the crust and lead to the rocks deforming. On a large scale, this leads to several features on the Earth’s surface which can be seen from space. In future posts, we’ll look at some of the features on other planets but first, let’s have a look at a few features formed by plate tectonics on the Earth:
A fault offsets banded rock south of the Tien Shan mountains as seen by Landsat 8 (USGS/NASA)
Faults are breaks in rocks along which movement occurs and are associated with earthquakes. They release strain built up in the crust and are found on all plate margins, including those that slide past each other like the San Andras fault. These are often seen as breaks and offsets in geological patterns.
In areas where the crust is under tension it can be pulled apart, the rocks break into a series of faults, which form a staircase-like feature dropping down to form a valley. As the crust gets thinner as it is pulled apart it enables volcanism to start and volcanoes form along the valley (such as in the East African Rift valley).
This rifting can continue and the crust can break into two separate plates. with a volcanic ridge in the middle which generates new crust as the two sides move apart. These features form the seas and the oceans.
East African rift valley, formed by multiple faults (seen as lines going diagonally across the image) which lead to low lying areas as the plate thins (NASA/JPL)
Mt Everest and other mountains as seen from STS-066 (NASA, JSC)
In areas where plates collide with each other, the outcomes vary depending on the type of crust colliding. If it involves a thinner denser crust called oceanic crust (which is the type generated by mid-ocean ridges) subduction occurs – an oceanic plate will be forced down and sink into the mantle. This activity leads to a trough and then a line of volcanoes behind them. Where two pieces of thicker, less dense continental crust collide the parts crash into each other they crumple rather thank sink and form mountain ranges such as the Himalayas.
Earth is unique with its highly active plate tectonic system defining its surface, other planets have been and are tectonically active but in different ways with features spread out and controlled by different tectonic systems.