Geology Chapter 10: Rock Deformation

The amount of force acting on a rock.

The change in shape or volume of a rock due to stress.

Forces push rocks together, shortening the rock.

Forces pull rocks apart, elongating the rock.

Forces push rocks parallel to one another, causing twisting.

Temporary shape change in a rock due to stress that is not permanent; rock returns to original shape.

Permanent shape change in a rock due to stress without breaking; flowing or bending.

Fracturing or breaking of a rock due to stress.

Rocks behave differently at different depths. Near the surface, rocks are cooler and experience less pressure, leading to brittle deformation (fracturing). At greater depths, higher pressure and temperature make rocks more pliable, resulting in ductile deformation (folding).

The composition of a rock influences its tendency to deform. Silicate-rich rocks (like granite and basalt) are strong and more likely to fracture (brittle). Non-silicate rich rocks (like shale) are weaker and more likely to fold (ductile).

The speed at which forces act on a rock determines its behavior. Rapid forces lead to brittle deformation (fracturing), while slow, sustained forces result in ductile deformation (folding).

An outcrop is a visible exposure of rock at the Earth's surface. It provides a window into the rock layers and structures beneath the ground.

To map rock structure, we need to identify outcrops, determine their rock types, measure the strike and dip of folds, faults, and layers, and plot this information on a geologic map.

The angle of inclination of the rock surface from the horizontal plane. Think of it as the path a ball would take rolling down the rock face. Dip is always perpendicular to the strike. It's reported as an angle and direction, for example: 30oE means the dip is 30 degrees in the eastward direction.

A map that shows the strike and dip of each rock structure, with different colors representing different rock formations. This map helps us understand the shape and orientation of structures beneath the ground.

The two sides of a folded rock layer. Think of them as the slopes of the fold.

A line drawn along the points of maximum curvature of each layer in a folded rock. It represents the line of greatest bending.

An imaginary surface that divides a fold as symmetrically as possible. It's like the plane of symmetry of the fold.

The angle of incline of the hinge line (axis) of a folded rock. It measures how much the axis dips into the Earth.

Steeply inclined, angular ridges formed when folded strata (rock layers) are resistant to weathering and erosion. They form outcrops.

The rock surface located above a fault.

The rock surface located below a fault.

A type of fault where the hanging wall moves upward relative to the footwall, compressing the crust.

A type of fault where the hanging wall moves downward relative to the footwall, stretching the crust.

A cliff-like feature on the surface that forms along the line of a fault.

Subduction zones are areas where tectonic plates collide, with one plate diving beneath the other. This process creates deep trenches, volcanic arcs, and earthquakes.

A plunging fold is a fold where the hinge line (the line of maximum curvature) dips below the horizontal.

The axial trace of a fold is the line of intersection between the fold's hinge line and the Earth's surface.

A monocline is a fold with only one limb, usually caused by vertical displacement along a fault.

Domes are circular upwarps where the oldest rocks are in the center, while basins are circular downwarps where the youngest rocks are in the center.

A region in the southwestern United States characterized by alternating mountain ranges and valleys formed by extensional forces pulling the crust apart.

A type of fault where the movement is horizontal and parallel to the strike (the line of intersection between the fault plane and the Earth's surface). It doesn't involve vertical displacement.

A fracture in rock where there is no noticeable displacement. Essentially, it's a crack with no movement across the crack.

Elongated, pillar-like structures formed in cooling igneous rock due to shrinkage fractures. These structures are perpendicular to the cooling direction.