Geology Last Unit

Lessons between the last exam and the final exam

rock deformation

caused by plate tectonic forces, occurs along plate boundaries

outcrops

rocks exposed on Earth’s surface

rock or geologic structures

structural features that reflect a rock’s tectonic history in outcrops

folds, faults, joints

three types of geologic structure

compressional stress

compresses a rock, associated with convergent plate boundaries

tensional stress

pulls the rock apart, associated with divergent plate boundaries

shear stress

involves the movement of one part of a rock body relative to another, associated with transform plate boundaries

strain

when differential stress changes a rock’s shape, distortion

Elastic deformation

occurs when a rock temporarily responds to stress but returns to its original shape when the stress is removed

brittle deformation

occurs when a rock is deformed beyond its ability to respond elastically, usually resulting in breaks or bends

ductile deformation

occurs when an object changes shape without breaking (ex. clay)

temperature, confining pressure, type of rock, time

fours factors influence how a rock forms

folds

series of wave-like undulations, most result from compressional stress, results in lateral shortening and vertical thickening of the crust, hinge line, axial plane

anticlines

unfolded or arched sedimentary layers, oldest strata are in the center

synclines

downfolded or troughs of rock layers, youngest strata are in the center

domes

structures that occur when a broad upwarping of basement rock deforms the overlying sedimentary strata, circular or slightly elongated bulge, oldest rocks are in the center, can form due to intrusion of a laccolith

basins

down warped circular features, youngest rocks in the center, can form from subsidence of large sedimentary basins

monoclines

large steplike folds in otherwise horizontal sedimentary strata, uniquely coupled with faults, as blocks of basement rocks are displaced upward, the ductile sedimentary strata drape over them

faults

fractures along which motion has occurred, brittle deformation leads to fracturing of crust, fault zones, sudden movements produce earthquakes

fault zones

areas with displacements hundreds of kilometers in length

strike

compass direction of the line produced by the intersection of a fault with a horizontal plane

dip

angle of inclination of a fault from a horizontal plane

dip-slip fault

occur when movement is parallel to the inclination (dip)

hanging wall

rock surface above the fault

footwall

rock surface below the fault

normal fault

hanging wall moves down relative to footwall block, associated with tensional stress, lengthens crust

reverse fault

hanging wall moves up relative to footwall block, associated with compressional stress, crust shortens

horsts

uplifted blocks

grabens

down-dropped blocks

detachment faults

a low-angle nearly horizonal fault

thrust fault

angle less than 45 degrees, overlying plate moves almost horizontally, most pronounced along convergent plate boundaries

strike-slip faults

displacement that is horizontal and parallel to the strike of the fault, associated with transform plate boundaries

right lateral

as you face the fault, the opposite side of the fault moves to the right

left lateral

as you face the fault, the opposite side of the fault moves to the left

oblique-slip faults

exhibit both a strike-slip and dip-slip movement

fault scarps

long low cliffs produced by vertical displacement

slickensides

fault surfaces where the rocks become highly polished and striated (grooved) as crustal blocks slid past each other

joints

fractures in a rock where there has been no appreciable displacement, one of most common rock structures, most appear in parallel groups, produced when rocks in the outermost crust are deformed and experience brittle failure

aerial photography, satellite imagery, seismic reflection profiling, drill holes

four mapping techniques

strike and dip

sedimentary rock layers that are inclined or bent indicate that the layers were deformed following deposition

90

dip is always __ degrees to the strike

geologic map

representation of Earth’s surface, as viewed from above, that shows the locations and orientations of the rock units that outcrop at the surface

block diagram

three-dimensional view of a portion of Earth’s crust that allows you to visualize the rock layers at the surface and underground

earthquake

ground shaking caused by the sudden and rapid movement of one block of rock sliding past another, differential stress overcomes confining pressure

faults

fractures in the crust caused by rocks sliding past each other

focus or hypocenter

where rock slippage originates in the ground

epicenter

point on the ground surface directly above the focus

seismic waves

released stored up energy that radiates in all directions from the focus during an earthquake.

causes of earthquakes

volcanic eruptions, massive landslides, meteorites, stress build up from plate movement

elastic rebound

deformed rock snap back to their original position

foreshock

small earthquakes that sometimes precede a major earthquake by days or even years

aftershock

occur after a strong earthquake, typically lesser in magnitude, diminish in frequency and intensity in the months following, usually weaker, often trigger destruction of already weakened structures

megathrust fault

thrust fault in a subduction zone

creep fault

slow, gradual displacement

seismology

study of earthquake waves

2000 years

earliest study of earthquake waves date back almost ___ to the Chinese

seismographs or seismometers

record the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape, based on principle of inertia

vertical and horizontal

earthquakes cause ___ movement

seismograms

records obtained from seismographs

primary (P) waves

compression waves, can travel through all materials, push/pull waves, first to arrive at recording station, lowest amplitude

secondary (S) waves

shear waves, can only travel through solid material, shake at right angles to direction of travel, second to arrive at recording station

surface waves

travel in rock layers just below Earth’s surface, two different directions, lowest velocity, last to arrive at recording station, highest amplitude, causes greatest property damage

compressional, tensional, shear

types of stress

elastic, brittle, ductile

types of strain or deformation