Rocks have often been subjected to numerous events. Faulting, folding, igneous activity, and erosion are just some of the processes that may have helped form the images we see today. In order for us to unravel the complex geologic history of an area, that is, to interpret the order of geologic events that have occurred, we have to understand and apply several fundamental geologic principles. Although these principles may seem obvious, it is important to note that their recognition, beginning in the seventeenth century, played a crucial role in the development of geology as a science.
Principle of Superposition - New sediments are deposited on top of previously deposited sediments. In an undisturbed sequence of sedimentary layers, the oldest layer is found at the bottom, with the layers getting progressively younger moving upward.
Principle of Original Horizontality - Sediments settle out from water under the influence of gravity and are deposited in approximately horizontal layers, roughly parallel to the Earth's surface. A rock layer that is significantly inclined from the horizontal must have been tilted after deposition and lithification.
Principle of Lateral Continuity - Sediments are typically deposited over vast areas, and extend laterally in all directions, until pinching out or terminating at the edge of the depositional basin. The continuity may be disrupted by later events, such as erosion.
Principle of Cross-cutting Relationships - An igneous intrusion or fault that cuts across a rock unit must be younger than the rocks being intruded or displaced.
Principle of Components - The components of a sedimentary rock must be older than the time when the rock was deposited. Likewise, inclusions in igneous rocks are older than the rocks containing them.
Principle of Faunal Succession - Life-forms on Earth have evolved over time. Species appear, then often go extinct. Fossil assemblages follow one another through time in an orderly and determinable fashion. Because of this, fossils can be used to determine the relative ages of rock units.
A geologic contact is the surface where rock bodies of two different types or ages meet. There are several ways in which different rocks may be brought into contact. Therefore, there are several distinct types of contacts: normal depositional contacts, unconformities, faults, and intrusive contacts.
Normal depositional contacts: When sediments are continuously deposited, the resulting layers of sediment are said to have been deposited conformably. Conformable deposition will result in a sequence of beds that are parallel to sub-parallel (original horizontality). Normal depositional contacts are contacts that form between layers within a conformable rock sequence.
Unconformities: An unconformity is a surface of non-deposition or erosion separating younger and older rock layers. Because no rocks are being preserved during the period of non-deposition or erosion, unconformities represent a break in the record of geologic time. There are three major types of unconformities you need to know about.
Nonconformity - a nonconformity is an unconformity separating older igneous or metamorphic rock from younger overlying sedimentary rock.
Angular unconformity - An angular unconformity separates tilted or folded rock layers from overlying units that have a different attitude.
Disconformity - A disconformity is an unconformity that separates parallel to sub-parallel sedimentary layers.
Faults: A fault is a fracture surface in rock along which movement has occurred. Faults are classified by the sense of movement along the fault surface. There are three major categories of faults: normal, reverse, and strike-slip.
Normal faults are fractures caused by tensional forces pulling the rock apart. The result is a fault in which the hanging wall (upper block of rock) has moved downward relative to the footwall.
Reverse faults are fractures caused by compressional forces squeezing the rock until it breaks. In this case, the hanging wall will move upward relative to the footwall.
Strike-slip faults, associated with shearing, are surfaces where rock bodies slide past one another horizontally, with no vertical movement. Strike-slip faults may be classified as either left- or right-lateral. To determine which, look at the relative direction of movement across the fault. If objects on the far side appear to have been displaced to the right, it is a right-lateral fault. If the relative movement is to the left, it is a left-lateral fault.
Intrusive Contacts: As magma, molten rock, travels through the Earth's crust, it intrudes the older rock that is already in place. The geologic contact between the igneous rock and the pre-existing country rock is known as an intrusive contact. The intruding magma is, of course, very hot. Because of this, the country rock near the intrusion often undergoes contact metamorphism. The igneous rock that forms as the magma cools may contain inclusions of country rock which broke free as the magma intruded.
