3.6DEFORMATION AND

RESHAPING THE EARTH

How Rock Deforms

Mountain ranges provide us with continuous evidence that the Earth’s surface is constantly changing shape.› These changes result from

deformation, or the bending, tilting, and breaking of Earth’s crust. Happens when the

lithosphere thickens or thins out.

How Rock Deforms

Vertical movement of the lithosphere depends on two opposing forces:› Gravitational Force

Weight of the lithosphere pressing down on the asthenosphere.

› Buoyant Force The asthenosphere pressing up on the lithosphere.

A condition of gravitational and buoyant equilibrium between Earth’s lithosphere and asthenosphere is called isostasy.

Isostasy

Stress

As Earth’s lithosphere moves, the rock in the crust is squeezes, stretched, and twisted.› The amount of force per unit

area that acts on a rock is called stress.

3 types include:

1. Compression2. Tension

3. Shear Stress

Compression

The type of stress that squeezes and shortens a body of rock.

Common along convergent plate boundaries where tectonic plates collide with one another.

Tension

The type of stress that stretches and pulls a body of rock apart.

Common along divergent plate boundaries where tectonic plates are being pulled apart.

Shear Stress

The type of stress distorting a body of rock by pushing parts of the body in opposite directions.

Common along transform plate boundaries where tectonic plates slide horizontally past one another.

Folds

A bend in rock layers resulting from stress is known as a fold.› The sloping sides of a fold are

called limbs.

› The limbs meet at the bend in the rock layers, which is called the hinge.

› If the fold’s structure is such that a plane could slice the fold into two symmetrical halves, the fold is symmetrical and the plane is called the axial plane.

Upward-arching folds are called anticlines.

Anticlines

Downward trough-like folds are called synclines.

Synclines

Folds where rock layers are folded so both ends of the fold are horizontal are called monoclines.

Monoclines

Faults Recall, a fault is a break in the

Earth’s crust along which blocks of the crust slide relative to one another.

A normal fault occurs when the hanging wall moves down relative to the footwall.› Results from tension.

Normal Fault

A reverse fault occurs when the hanging wall moves up relative to the footwall.› Results from compression.

Reverse Fault

Faults that occur when opposing forces cause rock to break and move horizontally are called strike-slip faults.› Results from shear stress.

Strike-Slip Faults

Earthquake Anatomy An earthquake is the

movement or trembling of the ground caused by a sudden release of energy when rocks along a fault move.› Each year, over 30,000

earthquakes occur worldwide strong enough to be felt.

› However, only about 100 major earthquakes take place each year.

Earthquake Anatomy Earthquakes are a result of elastic rebound, which is

the sudden return of elastically deformed rock to its un-deformed shape.

Earthquake Anatomy

The location within the Earth along a fault at which the first motion of an earthquake occurs is called the focus.› By the time vibrations from an earthquake having a

deeper foci reach the surface, much of their energy has dissipated.

› Therefore, earthquakes causing the most damage tend to have shallow foci.

The point on Earth’s surface directly above the focus is called the epicenter.

Seismic Waves As rocks along a fault slip into new positions, the rocks

release energy in the form of vibrations called seismic waves.

Two main types of seismic waves:› Body – seismic waves traveling through the body of a medium

(fastest-moving seismic wave category). P waves S waves

› Surface – seismic waves traveling along the surface of a body rather than through the middle of it (slowest-moving seismic wave category; most destructive). Rayleigh waves Love waves

P Waves Body waves traveling through solids and liquids are called P waves.

“Primary” waves “Pressure” waves

Fastest seismic wave.› Avg. speed in crust = 6.1 km/s

Particles of rock move in a back-and-forth direction.

S Waves Body waves traveling through only solids are called S waves.

“Secondary” Waves “Shear” Waves

Second fastest seismic wave.

› Avg. speed in crust = 4.1 km/s

Particles of rock move in a side-to-side direction.

Rayleigh Waves & Love Waves

Rayleigh waves are surface waves causing the ground to move with an elliptical, rolling motion.

Love waves are surface waves causing the ground to move with a side to side motion perpendicular to the direction of the traveling wave.

Recording Earthquakes Vibrations in the ground can be

detected and recorded using an instrument known as a seismograph.

› First seismograph by Chinese astronomer, Chan Heng. Each of the 8 dragons had bronze ball

in its mouth fell with tremor.

A tracing of an earthquake motion recorded by a seismograph is called a seismogram.

A person who studies earthquakes is a seismologist and the study of earthquakes is called seismology.

Seismograph

Seismogram

Locating an Earthquake To determine the distance to an epicenter, scientists analyze the arrival

times of the P waves and the S waves.› The longer the lag time between the arrival of the waves, the farther away the

earthquake occurred.

Scientists use computers to calculate how far an earthquake is from a given seismograph station.

What is the lag time for the earthquake recorded on the

seismogram to the left?

56 seconds

Earthquake Measurement

A measure of the strength of an earthquake is called magnitude.› Determined by measuring the amount of ground motion

caused by an earthquake.

The Richter Scale was used throughout the 20th century to study magnitude.› It is a logarithmic scale, meaning the numbers on the

scale measure factors of 10. Therefore, an earthquake measuring a 4.0 on the Richter

Scale is 10 times larger than one measuring at a 3.0.

Richter Scale Synopsis

Earthquake Measurement

Scientists now prefer to measure the magnitude of earthquakes with the Moment Magnitude Scale.› It is a measurement of earthquake

strength based on the following: Size of the area of the fault.

Average distance the fault blocks move.

Rigidity of the rocks in the fault zone.

Although this scale gives similar values as the Richter Scale, this one is more accurate.

Earthquake Measurement

Before the development of the magnitude scales, the size of an earthquake was determined based on the earthquake’s effects on the area.

A measure of the effects of an earthquake is called the intensity.

The Modified Mercalli Scale expressed intensity in Roman Numerals I to XII (highest- total destruction).

Modified Mercalli Scale Synopsis

Volcanism Any activity including the

movement of magma onto Earth’s surface is called volcanism.› Molten rock beneath Earth’s

surface is called magma. › Once it erupts onto the surface, it

is termed lava.

The vent in Earth’s surface through which magma and gases are expelled is called a volcano.› Most active volcanoes occur in

zones near both convergent and divergent plate boundaries.

Subduction Zones Many volcanoes are located along subduction zones, where one tectonic

plate moves under the other.› As the subducting plate gets deeper into the asthenosphere, the rock melts and

forms magma.

› The magma rises through the lithosphere (less dense) and erupts on Earth’s surface.

Hot Spots Not all volcanoes develop along

plate boundaries.› Areas of volcanism within the

interiors of the lithosphere are called hot spots. Most form where columns of solid,

hot material from the deep mantle, called mantle plumes, rise and reach the lithosphere.

The plume will spread out and magma will break through the Earth’s crust resulting in the formation of a hot spot volcano (HI Island formation).

Non-explosive Eruptions Mafic (dark-colored; rich in Fe and Mg) magmas are common with

non-explosive volcanic eruptions since gases can easily escape generally quiet.

Most common type of eruption.

Produce relatively calm flows of lava, but can produce huge amounts of it.

Explosive Eruptions Felsic (light-colored; high Si

content) magmas are common with explosive eruptions since large amounts of gases are trapped inside.

› Effects can be incredibly destructive.

During explosive eruptions, clouds of hot debris, ash, and gas shoot rapidly out from a volcanic vent.› Fragments of rock forming during a

volcanic eruption is called pyroclastic material.

Types of Lava The viscosity of magma, or its resistance

to flow, affects the force with which a particular volcano will erupt.› A glass of milk has LOW viscosity, flowing

more quickly. Common with mafic magmas.

› A milkshake has HIGH viscosity, flowing slowly. Common with felsic magmas.

Four main types of lava: Pahoehoe

Aa Blocky Lava Pillow Lava

Pahoehoe Lava flowing similar to wax dripping from a

candle (LOW viscosity). As it cools, it forms a smooth, rope-like texture.

Aa Lava flowing quickly (LOW viscosity), forming a

brittle crust. The crust is torn into jagged pieces as molten lava continues to flow underneath.

Blocky Lava Higher Si content than Aa; cooler, stiff lava usually

oozing and flowing slowly (HIGH viscosity) from a volcano after an explosion. Cooled lava eventually breaks into blocks.

Pillow Lava Forms when lava erupts underwater. This lava

flows quickly (LOW viscosity) and has rounded lumps resembling pillows.

Types of Volcanoes

Magma rises to the surface, like an air bubble in a honey jar.› This happens because the magma is

less dense than surrounding rock.

Three main types of volcanoes:› Shield Volcanoes› Cinder Cone Volcanoes› Composite Volcanoes

Mauna Kea – HawaiiShield Volcano

Sunset Crater - ArizonaCinder Cone

Mt. St. Helens- Washington

Composite (Stratovolcano)

Calderas

When the magma chamber below a volcano empties, the volcanic cone may collapse and leave a large, basin-shaped depression called a caldera.

Calderas may later fill with water to form lakes.

› Common example: Crater Lake, OR

Aniakchak - AlaskaCaldera