Anticline and Syncline

Anticline and Syncline, in geology, terms used to describe folded rock layers where the relative ages of the layers are known. Specifically, anticlines are folds in which the oldest rocks are in the core, or center of the fold, while synclines have the youngest rocks in their cores. In most cases, anticlines are arch-shaped while synclines are trough-shaped. In unusual cases where older rock layers lie on top of younger layers, anticlines are trough-shaped and synclines are arch-shaped. If the relative ages of the rock layers are unknown, then the terms antiform and synform describe arch and trough shapes respectively. The size of folds, measured from the crest of one arch to the crest of the next arch, can range from less than a meter (about 3 ft) to greater than 10 km (about 6 mi).

Anticlines and synclines form when rock layers are compressed, just as a carpet folds when its sides are pushed together along a floor. When rocks are compressed, they can either bend or fracture. When rocks bend they create folds, and when they fracture they can produce faults. Consequently, many folds are associated with faults.

Anticlines and synclines are useful to geologists for a variety of reasons. They give information regarding geologic history (see stratigraphy). By determining the age of a fold, geologists can determine when the earth’s crust in a given area was compressed. Furthermore, the orientation of the fold provides information regarding the direction of forces within the crust.

Geologists who explore for oil and gas seek out anticlines because the arches form natural traps for these hydrocarbons. Oil and gas are less dense than water and tend to migrate upward through permeable rock (rock that allows movement of fluids). If the rock is folded into an anticline and capped by an overlying impermeable rock, then oil and gas will migrate up the slope of the fold to the crest and accumulate there.

Hydrologists looking for an underground water supply, or aquifer, seek out synclines because synclines can form natural traps for water. Water tends to migrate downward through permeable rock until it reaches impermeable rock or reaches a level, called the water table, where groundwater fills all of the available spaces below it. If permeable rock is folded into a syncline and impermeable rock lies at the bottom of the syncline, then water will accumulate at the bottom of the syncline. Sometimes the level of the impermeable rock at the bottom of a syncline lies above the level of the water table. When this occurs, water accumulated at the base of the syncline will be perched above the water table. Such accumulations of water are called perched aquifers. Perched aquifers are valuable because their relatively shallow depths make the cost of drilling wells far less expensive than drilling wells down to the water table.