Introduction

Geologic structures such as faults and folds are the architecture of the earth’s crust. Geologic structures influence the shape of the landscape, determine the degree of landslide hazard, bring old rocks to the surface, bury young rocks, trap petroleum and natural gas, shift during earthquakes, and channel fluids that create economic deposits of metals such as gold and silver.

Folds, faults, and other geologic structures accommodate large forces such as the stress of tectonic plates jostling against each other, and smaller forces such as the stress of gravity pulling on a steep mountainside. An understanding of the structures that shape the earth’s crust can help you see when and where the crust was subjected to pushing or pulling, terrane accretion or crustal rifting.

Physical Behavior of Rocks: Stress and Geologic Structures

Before exploring geologic structures, we need to look at how rocks respond to the forces that create the structures. Stress refers to the forces that cause rocks to deform. There are three basic types of stress that deform rocks:

In response to stress, rocks will undergo some form of bending or breaking, or both. The bending or breaking of rock is called deformation or strain.

If rocks tend to break, they are said to be brittle. If a rock breaks, it is said to undergo brittle behavior. If rocks tend to bend without breaking, they are said to be ductile. If a rock bends but is able to return to its original position when the stress is released, it is said to undergo elastic behavior. If a rock bends and stays bent after stress is released, it is said to undergo plastic behavior.

A combination of elastic and brittle behavior causes earthquakes. Rocks get bent in an elastic fashion until they reach their limit, then they break in brittle fashion. The rocks on either side of a break act like rubber bands and snap back into their original shape. The snap is an earthquake and the break along which the rocks slide back to their original shape is a fault.

Earthquakes and faults occur in the shallow crust, where rocks are relatively cold and therefore brittle. In the deep crust and deeper, in the earth’s mantle, rocks are very hot and subject to high pressure caused by the weight of the overlying rock. This heat and pressure causes deep crustal and mantle rocks to be ductile. In fact, rocks deep in the continental crust and upper mantle can be so hot and soft that they behave almost like a slow-moving liquid, even though they are actually solid. They “flow,” or bend, or stretch, in a plastic manner, at a geological pace.

Now let us look at the specific types of geologic structures – the breaks and bends that deform rock in response to stress.

Folds

Ductile rocks behave plastically and commonly become folded in response to stress. Folding can happen in the shallow crust if the stress is slow and steady and gives the rock enough time to gradually bend. If the stress is applied too quickly, rocks in the shallow crust will behave as brittle solids and break. However, deeper in the crust, where the rocks are more ductile, folding happens more readily.

Refer to this table of folds and how they are symbolized on a geologic map.

Anticlines and Synclines

The most basic types of folds are anticlines and synclines.

Anticlines are “up” folds; synclines are “down” folds. In box diagrams like these, the top of the box is the horizontal surface of the earth, the map view. The other two visible sides of the box are cross-sections, vertical slices through the crust. The colored layers represent layered geologic formations that were originally horizontal, such as sedimentary beds or lava flows. Use the block diagrams to visualize the three-dimensional shapes of the geologic structures. Keep in mind that erosion has stripped away the upper parts of these structures so that the map views reveal horizontal slices through the structures.

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