geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...

, the term "tension" refers to a

stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...

which stretches rocks in two opposite directions. The rocks become longer in a lateral direction and thinner in a vertical direction. One important result of tensile stress is jointing in rocks. However, tensile stress is rare because most subsurface stress is compressive, due to the weight of the overburden.

Jointing

Tensile stress forms joints in rocks. A

joint A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw ...

is a fracture that forms within a rock, whose movement to open the fracture is greater than the lateral movement that takes place. Joints are formed in the direction perpendicular to the least principal stress, meaning that they are formed perpendicular to the tensile stress. One way in particular that joints can be formed is due to fluid pressure, as well as at the crest of folds in rocks. This occurs at the peak of the fold or due to the

fluid pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

because a localized tensile stress forms, eventually leading to jointing. Another way in which joints form is due to the change in the weight of the

overburden In mining, overburden (also called waste or spoil) is the material that lies above an area that lends itself to economical exploitation, such as the rock, soil, and ecosystem that lies above a coal seam or ore body. Overburden is distinct from tai ...

. Since rocks lay under a great deal of overburden, they undergo high temperatures and high pressures. Over time, the rocks are

eroded Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust, and then sediment transport, tra ...

and the weight of the overburden is lifted, so the rocks cool and are under less pressure, which causes the rock to change shape, often forming breaks. As the compression is lifted from the rocks, they are able to react to the tension on them by forming these breaks, or joints.

Divergent boundaries

Geologic tension is also found in the tectonic regions of divergent boundaries. Here, a

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it upw ...

forms underneath

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...

and causes sea-floor spreading in the creation of new oceanic crust. Some of the force that pushes the two plates apart is due to

ridge push force Ridge push (also known as gravitational sliding) or sliding plate force is a proposed driving force for plate motion in plate tectonics that occurs at mid-ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosp ...

of the magma chamber. Weil, Arlo B. "Plate Driving Forces and Stress." Plate Driving Forces and Tectonic Stress. University of Michigan. Web. 22 Nov. 2010. Tension, however, accounts for most of the "opposite directions" pull on the plates. As the separating oceanic crust cools over time, it becomes more dense and sinks farther and farther away from the

ridge A ridge or a mountain ridge is a geographical feature consisting of a chain of mountains or hills that form a continuous elevated crest for an extended distance. The sides of the ridge slope away from the narrow top on either side. The line ...

axis. The cooling and sinking ocean crust causes a tensile stress that also helps drive the pulling apart of the plates at the ridge axis.

References

{{Structural geology Structural geology Geological processes