The planet Earth has a crust that undergoes stress.
The Earth's outermost surface is known as the crust, which is light and brittle. The crust is divided into 14 major continental plates around the planet. Different types of stress on these plates creates earthquakes and other geological conditions. A rock's response to stress varies according to the type of rock, the the surrounding temperatures, duration of the stress and the type of stress. There are known types of stress that affect the Earth's crust.
Causes of Crust Stress
Beneath the plates on the Earth's crust, the movement of superheated liquid rock called magma creates stress. On top of the Earth's crust, immensely heavy oceans compress both the plates and magma. The moon and planets result in wide variations in ocean tides. These stresses pull water outward to make a high tide, and pulled sideways to make a low tide. Sunspots, bursts of intense electro-magnetic energy, affects Earth's gravity, heat, wind and tides.
Compression Stress
Compression stress = squeezes rocks. This is the most common stress. Rocks squeezing together where plates are moving toward one another causes them to fold or break. These are called faults. Faults are cracks in the crust that cause earthquakes. According to the U.S. Geological Survey, the stress factor on a surface, such as a fault plane, results from "forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock."
Tensional Stress
Rocks pulled apart are under tensional stress. Tensional stress is the major type of stress that occurs at plate boundaries. This type of stress happens when rocks stretch and become thin in the middle.
Shear Stress
Shear stress occurs when forces are parallel but move in opposite directions. This is the most common stress found at plate boundaries. Rocks are pushed in different directions, causing them to slip apart or change shape. Shear stress manifests as a seismic body wave that shakes the ground.
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