Geology Unit 3

Three types of stress

Compression (squeezing together), Tension (Pulling apart), and shear (Adjacent sides move opposite to each other)

Anticline

Limbs inclined away from each other; oldest rocks in center

Syncline

linbs inclined toward each other; youngest rocks in center

Why anticlines and synclines form together

Compressional stress is equal in both directions

Overturned limbs

Forms when compressional stress is greater in one direction, very common in mountain belts

Joints

Simple Fractures in rock, with no movement; caused by tension compression, or shear. Very common, shows that crust is always under slight stress.

Columnar Jointing

A special type of jointing that occurs in some lavas and shallow intrusive rocks. Joints form to relieve stress; commonly form in six-sided shapes.

Faults

Fractures across which movement has occurred, due to tension, compression, or shear

What does the classification of faults depend on

Relative displacement across the fault

Earthquakes

Vibrations caused by sudden movement of large masses of rock along a fault

Fault scarp

A fault that has reached the surface

When do earthquakes occur

When a part of the earth is subjected to stress, which begins to cause deformation; If the strength of the rock is exceeded, a rupture occurs causing an earthquake

Elastic rebound

How earthquakes occur; Three steps:

1: Rock is subjected to stress, begins to deform

2: Deformation involves bending

3: Strain builds up, strength of rock is exceeded, rock breaks, stored up energy is released as vibrations, creating earthquakes

Types of seismic waves

Body waves (travel beneath surfaces) and surface waves (travel along the earth’s surface and cause the most destruction)

Two types of body waves

Primary waves (P-wave, travel by compressing and expanding, like a spring), and secondary waves (S-wave, travel by wave motion like shaking a rope, slower than P-waves)

Where are earthquakes more likely to occur

On active plate boundaries involving strike-slip or thrust motion; also more likely to occur on parts of a fault that have not moved for a long time.

Resonance

Different Earth materials have different levels of vibration. If these match the vibrations of the seismic waves, they will greatly amplify the destruction

Liquefaction

In areas where unconsolidated material (e.g. sand) are water-saturated, seismic waves can turn the ground into a mobile fluid that cannot support buildings.

Normal fault

A fault where the hanging wall moves down relative to foot wall; stretching forms dues to the tension

Rift valleys

Areas where the crust is being pulled apart, magma commonly rises from below (e.g. East African Rift Valleys)

Reverse fault

A fault where the hanging wall moves up relative to the foot wall, forming shortening due to compression; common in collisional mountain belts

Thrust fault

A low angle reverse fault due to intense compression in mountain belts; has a very high amount of shortening

Strike-slip fault

Fault where displacement is horizontal, parallel to the fault (e.g. San Andreas Fault)

Folds

Formed by plastic deformation (ductile deformation) - shape of rock mass is changed without breaking; caused by compressional stress; occurs very slowly so that rocks don’t break; commonin mountain belts

Metamorphic Rocks

Formed by metamorphism of older rocks; Occurs when older igneous or sedimentary rocks undergo changes in mineral content and texture due to changes in pressure/temperature in order to maintain equilibrium; occurs by solid state recrystallization

Contact Metamorphism

“Baked” rocks near igneous intrusions; occurs in relatively narrow zones along intrusive contacts.

Regional Metamorphism

Occurs over large regions in centers of mountain belts; large masses of rocks are carried down causing and increase in temp and pressure, leading to metamorphic recrystallization from stress

How does marble form

Regional or contact metamorphism of limestone

Metamorphic process of shale

Shale - Slate - Schist - Gneiss

Schist

Alignment of coarser grained micas and other minerals that are visible to the naked eye; garnet will also crystalize as isolated crystals

Gneiss

Forms at high temp in shale, also in igneous rocks; typically shows metamorphic banding defined by mafic and felsic layers

Migmatite

Rock formed at a very high temp due to partial melting; partly igneous and partly metamorphic, can be sources for large granite intrusions

Tsunamis and how they form

Waves caused by sudden displacement of large amounts of water, most commonly caused by earthquakes. Can also be caused by eruption of island volcanoes or submarine landslides; most are caused by the vertical displacement along a submarine fault and travel at up to 950 km/h

How tsunami waves get so big

Wave passes undetected in the open ocean because the height is >1 m. As the tsunami approaches land, friction against sea floor slows the front of the wave, and rear of wave catches up, adds water to the wave

Slaty cleavage

Slate breaking parallel to micas; micas grow and line up in response to stress