How the Earth works lecture 8: Deformation in the Earth's crust Flashcards
List the three tectonic forces/boundaries
Divergent boundary – two plates move away from each other
Convergent boundary – two plates move towards each other
Transform boundary – two plates slide past each other
Define stress
Stress is the force applied across a unit area.
Stress in the Earth’s crust is a result of large-scale plate tectonic forces which results in deformation of the Earth’s crust.
Define strain
Strain is the change in shape caused by deformation.
E.g. stretching, shortening, shear
Strained rocks can change shape, be tilted, folded, faulted, altered and metamorphosed.
Deformation results in change in the rocks.
Define compression and when it takes place
Compression takes place when an object is squeezed.
Deformation shortens and thickens the material.
Horizontal compression drives plate-tectonic collision and orogenesis (mountain building via plate collision).
What are the twp types of deformation
Two major deformation styles: brittle and ductile.
The type of deformation depends on temperature and pressure conditions.
Define the Brittledeformation
Brittle deformation occurred in the ~upper 15 km of the crust.
BRITTLE deformation happens where chemical bonds are snapped in order for the fractures to form.
Occurs in the shallower crust; rocks break by fracturing.
Can result in a series of brittle fault rocks including breccias and slickensides
Define ductile deformation
Ductile deformation occurs in the mid and lower crust
Occurs at higher temperature and pressure conditions, which causes rock to deform by flowing and folding.
What are some controlling factors which determine what kind of deformation occurs
Temperature – hot rocks tend to deform in a ductile manner
However even relatively ‘cold’ rocks in the upper crust can deform in a ductile fashion if stresses are applied slowly over a long period of time.
Pressure – rocks under high pressure will be less likely to behave in a brittle fashion than rocks under low pressure
Composition – salt can flow in a ductile manner at temperatures of under 100°C but granite would need to be heated up to over 450-500°C to deform the same (ductile)
Strain rate - depends on rate at which stress is applied
Define a fault
Faults are planar fractures that show offset called displacement
Define a drip slip fault
Dip slip faults – block moves up or down.
Diagonal fracture between blocks
Define a reverse fault
In a reverse fault, the hanging wall moves up the fault slope. A thrust fault is a special, low-angle type of reverse fault.
Reverse faults are most common in regions experiencing horizontal compression.
Define a thrust fault
A thrust fault is a special type of reverse fault with a dip below 30 degrees.
Define an antiform
Antiform: limbs dip out and away from the hinge
Define a Synform
Synform- limbs dip inward towards the hinge
Define a joint
Joints are planar rock fractures without any offset that develop from tensile stress in brittle rock.
Systematic joints occur in parallel sets.
Joints often control the weathering of the rock in which they occur.
Define a Vein
Veins- mineralized extensional planar features are called veins- these record records of paleo-fluid flow
Define strike slip fault
Strike-slip faults have fault motion parallel to the strike of the fault.
These faults are classified by the relative sense of motion of the block on the far side of the fault from the observer.
Left lateral—opposite block moves to observer’s left.
Right lateral—opposite block moves to observer’s right.
Explain an Earthquake
Earth shaking is caused by a rapid release of energy, most of which is due to tectonic forces.
Before an earthquake, rock bends elastically, like a stick arch in your hands.
Eventually, the rock breaks. The break generates vibrations
Explain stick-slip behavior
Faults move in jumps. Once movement starts, it quickly stops due to friction. Over time, strain builds up again, leading to repeat failure.
This behavior is termed stick-slip behavior.
Stick—friction prevents motion
Slip—friction briefly overwhelmed by movement
Define the hypocentre/focus of an earthquake
The hypocenter (or, focus) is the location where fault slip occurs. It is usually on a fault surface.
Define the epicentre of an earthquake
The epicenter is the land surface directly above the hypocenter. Maps often portray the location of epicenters.
What are the three seismic waves associated with earthquakes?
P waves
S waves (Secondary waves)
L waves (Love waves)
R waves (Rayleigh waves)
Define seismic P waves
P-waves are the first to arrive. They produce a rapid, bucking, up-and-down motion.
P-waves travel by compressing and elongating the material in the direction of travel.
P-waves are the fastest seismic waves, and they travel through solids, liquids, and gases.
Define seismic S waves
S-waves/Secondary waves arrive next (second). They produce a pronounced back-and-forth motion.
This motion is much stronger than that from P-waves. S-waves cause extensive damage
S-waves travel by moving material back and forth, perpendicular to the wave-travel direction. Material looks like it is wiggling like a snake or worm from the side (NOT FROM OVERHEAD)
S-waves are slower than P-waves, and they travel only through solids, never liquids or gases.
Define seismic L waves
Define Surface waves
Surface waves are seismic waves that only travel through solids, specifically along the surface.
Surface waves come after the Body waves (P & S waves).
The two surface wave types are Love waves and Rayleigh waves
Describe Love waves
L-waves (Love waves) are S-waves that intersect the land surface. They move the ground back and forth like a writhing snake when looked at from overhead.
Describe R waves (Rayleigh waves)
R-waves (Rayleigh waves) are P-waves that intersect the land surface. They cause the ground to ripple up and down like water.
Look like ocean waves. With elliptical pattern.
R-waves are the last to arrive. The land surface undulates like ripples across a pond. These waves usually last longer than the other kinds. R-waves cause extensive damage.
