Tectonics, Earthquakes, and volcanism

shields

the exposed regions of cratons; the oldest part of a continent

what are shields composed of?

crystalline rock; metamorphic and igneous

what is a craton?

the old, tectonically stable part of a continental crust; includes the continental shield and platform

continental platforms

the sedimentary rocks surrounding the continental shield; relatively flat or gently tilted sedimentary strata

what are basement rocks?

the metamorphic & igneous rocks underlying the sedimentary platform

how is continental crust formed?

starts from sea-floor spreading

what happens to oceanic crust?

its is eventually subducted and recycled

what generates magma?

the subducting plate

why does magma rise?

because it is hotter and less dense than the surrounding mantle material

how does magma add material to the crust?

the magma pools beneath or rises through

what are terranes?

migrating crustal pieces that are too thick, or too buoyant, to be subducted are fused to the edge of continents

what are terranes bounded by?

faults

what are faults?

fractures along which movement has taken place

deformation

a general term that refers to all changes in the original form and/or size of a rock body

what is stress?

the force applied to a given area

what is strain?

how rocks respond to the deformation

how is strain expressed?

folding (bending) or faulting (breaking)

what is another term for ductile deformation?

plastic deformation

when does ductile/plastic deformation occur

when temperatures and pressures are high

what kind of change does ductile/plastic deformation cause?

changes the size or shape of an object without fracturing; the deformation of modeling clay or caramel candy

what are the three types of stress?

tension, compression, and shear

what is compression stress?

stress resulting strain = shortening; shortens & thickens a rock body by folding (at depth) and faulting (near surface)

what is tension stress?

Tension stress resulting strain = stretching; elongates & thins a rock body; fractures surfaces rocks along faults; at great depths, rocks flow like putty (plastic

deformation)

what is shear stress?

stress resulting strain = twisting laterally; deforms along planes parallel to applied stress (lateral twisting of a rock body)

anticline

An arch-shaped upward fold; the rock layers slope downward and away from the central axis of the fold

syncline

A trough-shaped downward fold; the rock layers dip down and into the central axis

what are faults?

are fractures along which measurable displacement has occurred

what causes most earthquakes?

sudden movements along faults cause most earthquakes

when does faulting occur?

faulting occurs when rocks on either side of a fracture shift relative to the other side

what is the fault plane?

the fracture surface along which the two sides of a fault move

what are the three types of faults?

normal, reverse, and strike-slip (or transform)

what causes the three faults?

tensional stress, by compressional stress, and by lateral-shearing stress

strike

the compass direction (bearing) of the fault (or tilted rock layers intersecting a horizontal plane); expressed as an angle clockwise from North

Dip (inclination)

the angle that the fault (or rock unit) dips; the angle measured from horizontal (in upper image

how are faults classified

by their relative displacement which can be horizontal (lateral),

vertical (or oblique which involves both horizontal and vertical displacement)

dip-slip fault

a fault with vertical displacement

what is a strike-slip fault?

a fault in which the sides of the fault are displaced horizontally

displacement is…

horizontal and parallel to the strike of the fault

Right-lateral strike-slip fault

looking across the fault, the block on the opposite side of the fault is displaced to the right

Left-lateral strike-slip fault

looking across the fault, the block on the opposite side of the fault is displaced to the left

transform fault

a large strike-slip fault that cuts through the lithosphere; accommodates motion between lithospheric plates

hanging wall and foot wall

the rock above and below the fault plain

If the hanging‑wall moves down relative to the foot‑wall, the fault is called a…

normal fault; associated with crustal lengthening and thinning of the crust, caused by tensional (extensional) stress

If the hanging‑wall moves up relative to the foot‑wall, it is a…

reverse fault; associated with shortening and thickening of the crust (compressional stress)

thrust fault

a low angle reverse fault; dip angle is less than 45°

Orogenesis

mountain generating; involves large-scale deformation, crust thickening, and crustal uplift

what are the three types of convergent plate collisions that cause orogenesis?

oceanic plate–continental plate, oceanic plate–oceanic plate, continental plate–continental plate

what is an earthquake?

vibrations of the earth caused by the sudden release of energy as plates move past one another

magnitude

energy released from an earthquake

what is the focus?

the point of rupture, and the initial energy release; where the motion of seismic waves are initiated

what happens when distance from the focus is increased?

energy dissipates quickly

what is the epicentre?

the location on the Earth’s surface directly above the focus

seismometers/seismographs

instruments that record seismic waves

how much larger is a 5 to 7 on the richter scale?

32×32= 1024 so 1000

what is elastic rebound theory?

fault blocks are locked by friction, blocks deform (strain), building up and storing elastic energy as stress increases, when stress exceeds the frictional resistance that is holding the rocks together, slippage occurs at the weakest point (focus), both sides of the fault move and release the stored energy, displacement exerts stress farther along the fault where additional slippage occurs until most of the built-up stress is released, the deformed rocks rebound or “snap back” to their original undeformed shape

shield volcanoes

broad, slightly dome-shaped (so, low angle: 2-10º)

how are shield volcanoes produced?

by the accumulation of low-viscosity (runny) basaltic lava flows

averaging a few metres thick or by mild eruptions of large volumes of lava

Composite Volcanoes (Stratovolcanoes)

large, typically, symmetrical cone (hundreds of metres high and several kilometres wide at the base)

what are composite volcanoes composed of?

interbedded (generally intermediate/andesitic) lava flows and layers of pyroclastic debris

Cinder Cones (Scoria Cones)

built from scoria – ejected vesicular basaltic lava that hardened in flight (rock

with holes from gas bubbles)

what slope angle do cinder cones have?

steep slope angles

how long does a single eruption from a cinder cone last?

from a few days up to a few years