Intrusions cut through layers of rock, and faults cause displacement.
Faulting commonly occurs in rock due to the natural stresses and strains of the earth. Intrusions are often associated with faulting and result from magma movement along weaknesses in rock. The different types of faults and intrusions can be distinguished from each other by characteristics such as shape, orientation, or sense of movement. Most faulted or intruded rocks occur on a large scale.
Instructions
1. Find the type of fault that may be occurring in the rock. Normal faults occur when the hanging wall on one side of the fault drops down relative to the foot wall on the other side (See Resources). Reverse faults occur when the hanging wall goes up relative to the foot wall. Transform faults occur when rocks slide past each other, usually with little to no vertical displacement.
2. Look for displacement in the rock. Displacement is sometimes identifiable in outcrops. Strata usually lie in horizontal layers, according to Steno's laws of stratigraphy. If a normal or reverse fault occurs within a stratigraphic column, displacement of the layers will be visible.
3. Look for a repeating section in the rock. Within a stratigraphic column, each layer represents a different point in time. Therefore, if you are walking on top of a certain type of rock and then the rock type changes to one you know to be older, this indicates a reverse fault is the culprit. This can also be seen in drill cores through the faults themselves.
4. Look for dikes in the rock. A dike is a discordant igneous intrusion. These are injected from magma chambers along weaknesses in the rock and sometimes fracture the rock as they intrude. Identifying a dike in an outcrop is often quite simple, as it will have a different composition as the rock through which it has intruded. Dikes can be identified as having intruded a rock if they cut through the rock.
5. Look for sills in the rock. Sills are igneous intrusions that lie parallel to bedding, so they are often horizontal. They can be identified in the same fashion as dikes -- they will have a different composition and will cut through rock or be spread in between layers of pre-existing rock where there had been planes of weakness.
6. Look for laccoliths in the rock. Laccoliths are mushroom-shaped igneous intrusions. They can be identified in outcrops by their differing composition and their shape, and will often bend the rocks above them so the rocks on top of either side of the laccolith dip in opposite directions.
7. Look for batholiths in the rock. Batholiths are large igneous bodies that are the crystallized remains of former magma chambers. They can be identified by their very large size and their composition, often crystalline rock such as granite. Smaller versions of batholiths are called stocks.
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