ESC 101: 7.1 Stress in the Earth's Crust

• Enormous slabs of lithosphere move unevenly, resulting in earthquakes.
• This section considers what happens to rocks when exposed to stress, mountain building and earthquakes.

Causes and Types of Stress

• Stress is the force applied to an object
• In geology, stress measures force per unit area on a rock.
• There are four types of stress that act on materials:
• Confining stress occurs when deeply buried rocks are pushed down by overlying material causing rocks to not deform.
• Compression squeezes rocks, causing folds or fractures and is common at convergent plate boundaries.
• Tension pulls rocks apart, causing them to lengthen or break and is common at divergent plate boundaries.
• Shear stress occurs when forces are parallel but move in opposite directions and is common at transform plate boundaries.
• Strain/Deformation occurs when stress causes a material to change shape, commonly happening in geologically active areas.
• A rock's response to stress depends on the rock type, the temperature, pressure, duration, and type of stress.
• Rocks have three possible responses to increasing stress:
• Elastic deformation allows the rock to return to its original shape after stress removal.
• Plastic deformation means the rock doesn't return to its original shape after stress removal.
• Fracture is the breaking of rock.

Geologic Structures

• Sedimentary rocks help decipher a region's geologic history because they follow rules:
• Sedimentary rocks form with the oldest layers at the bottom and the youngest at the top.
• Sediments are deposited horizontally, like sedimentary rock layers and volcanic rocks (ash falls).
• Sedimentary rock layers that aren't horizontal are deformed, which affect geologic structures like folds, joints, and faults.

Folds

• Folds form when rocks deform plastically under compressive stresses.
• Monocline: A simple bend in rock layers, making them no longer horizontal.
• Anticline: A fold that arches upward, with rocks dipping away from the center, and the oldest rocks in the center.
• Dome: When rocks arch upward in a circular structure.
• Syncline: A fold that bends downward, with the youngest rocks in the center and the oldest on the outside.
• Basin: When rocks bend downward in a circular structure.

Faults

• A rock under enough stress will fracture.
• Joint: A fracture with no movement on either side.
• Fault: A fracture where the blocks of rock move on one or both sides.
• Slip is the movement distance of rocks along a fault.
• The dip (angle) of a fault defines its type:
• Dip-slip fault: The fault's dip is inclined relative to the horizontal.
• Normal fault: The hanging wall drops down relative to the footwall.
• Reverse fault: The footwall drops down relative to the hanging wall.
• Thrust fault: A reverse fault with a nearly horizontal fault plane angle.
• Strike-slip fault: A fault where the dip of the fault plane is vertical, resulting from shear stresses.
• Faults are either right-lateral or left-lateral.

Stress and Mountain Building

• Converging continental plates smash upwards, creating mountain ranges.
• Stress from uplift causes folds, reverse faults, and thrust faults, allowing the crust to rise.
• Subduction of oceanic lithosphere at convergent plate boundaries also builds mountain ranges.
• Tensional stresses pulling the crust apart cause blocks to slide up and down along normal faults, creating alternating mountains and valleys (basin-and-range).
• Horsts: Uplifted blocks that form ranges
• Grabens: Dropped blocks that form basins