How the Earth works lecture 8: Deformation in the Earth's crust

Question 1

List the three tectonic forces/boundaries

Answer 1

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

Question 2

Define stress

Answer 2

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.

Question 3

Define strain

Answer 3

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.

Question 4

Define compression and when it takes place

Answer 4

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).

Question 5

What are the twp types of deformation

Answer 5

Two major deformation styles: brittle and ductile.

The type of deformation depends on temperature and pressure conditions.

Question 6

Define the Brittledeformation

Answer 6

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

Question 7

Define ductile deformation

Answer 7

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.

Question 8

What are some controlling factors which determine what kind of deformation occurs

Answer 8

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

Question 9

Define a fault

Answer 9

Faults are planar fractures that show offset called displacement

Question 10

Define a drip slip fault

Answer 10

Dip slip faults – block moves up or down.

Diagonal fracture between blocks

Question 11

Define a reverse fault

Answer 11

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.

Question 12

Define a thrust fault

Answer 12

A thrust fault is a special type of reverse fault with a dip below 30 degrees.

Question 13

Define an antiform

Answer 13

Antiform: limbs dip out and away from the hinge

Question 14

Define a Synform

Answer 14

Synform- limbs dip inward towards the hinge

Question 15

Define a joint

Answer 15

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.

Question 16

Define a Vein

Answer 16

Veins- mineralized extensional planar features are called veins- these record records of paleo-fluid flow

Question 17

Define strike slip fault

Answer 17

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.

Question 18

Explain an Earthquake

Answer 18

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

Question 19

Explain stick-slip behavior

Answer 19

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

Question 20

Define the hypocentre/focus of an earthquake

Answer 20

The hypocenter (or, focus) is the location where fault slip occurs. It is usually on a fault surface.

Question 21

Define the epicentre of an earthquake

Answer 21

The epicenter is the land surface directly above the hypocenter. Maps often portray the location of epicenters.

Question 22

What are the three seismic waves associated with earthquakes?

Answer 22

P waves
S waves (Secondary waves)
L waves (Love waves)
R waves (Rayleigh waves)

Question 23

Define seismic P waves

Answer 23

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.

Question 24

Define seismic S waves

Answer 24

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.

Question 25

Define seismic L waves

Question 26

Define Surface waves

Answer 26

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

Question 27

Describe Love waves

Answer 27

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.

Question 28

Describe R waves (Rayleigh waves)

Answer 28

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.