What is Fault (geology)?
[[File:The Blue Anchor Fault - geograph.org.uk - 2455274.jpg|250px|thumb| Normal fault on the coast of [[Somerset]], England. On left, younger blue-grey rock is late [[Triassic]] to early [[Jurassic]]. On right, the red rock is older [[Triassic]] [[mudstone]]. Strata of the red rock deformed when it moved upwards. The line of the fault runs along the beach, shown by the rock/sand divide.]] [[File:Nor rev.png|thumb|right|Cross-sectional illustration of normal and reverse dip-slip faults]] [[File:strike slip fault.png|thumb|right|Schematic illustration of the two strike-slip fault types]] [[File:Fault types.svg|thumb|Three types of faults:<br /> A. <!--[[strike-slip fault|-->Strike-slip<br /> B. <!--[[Normal fault|-->Normal<br /> C. <!--[[Reverse fault|-->Reverse ]] [[File:Oblique slip fault.svg|thumb|250px|Oblique-slip fault]] [[File:Fault-Horst-Graben.svg|right|thumb|300px|[[United States Geological Survey|USGS]] terminology of normal (basic) faults]] A '''fault''' is a fracture, or break, in the Earth's crust ([[lithosphere]]). Some faults are active. Here, sections of [[Rock (geology)|rock]] move past each other. This sometimes makes [[earthquake]]s. Faulting occurs when [[shear stress]] on a rock overcomes the [[Force (physics)|forces]] which hold it together. The fracture itself is called a ''fault plane''. When it is exposed at the Earth's surface, it may form a cliff or steep slope called a ''[[fault scarp]]''. The angle between the fault plane and an imaginary [[horizontal]] plane is called the ''dip angle'' of the fault. Faults may dip shallowly or steeply. Faults are categorized into three types: #A '''normal fault''' is one where the fault dips toward the downthrown block. Normal faults occur in rifted terranes, such as [[Mid-Ocean Ridge]]s, the [[Great Rift Valley|African Rift]], and the [[Basin and Range]] Province of western North America. It is shown on geologic maps as a black line with either a block pattern on the downthrown side, or the letters U/D showing the upthrown and downthrown sides.