Earthquakes and Volcanoes

Forces in the Earth’s Crust The movement of the Earth’s plates creates

enormous forces that squeeze or pull the rock in the crust

These forces are examples of stress Can change the shape or volume of the rock

making up the crust Stress stores energy in the rock that is released

when the rock changes shape or breaks

Types of Stress Three different kinds of stress can occur

Tension- pulls on the crust, stretching the rock so that it becomes thinner in the middle

Occurs where two plates are moving apart Divergent boundaries

Compression- squeezes the rock until it folds or breaks Happens when two plates push against each other Convergent boundaries

Shearing- pushes a mass of rock in two opposite direction Can cause rock to break and slip apart Transform fault boundaries

Faults A fault is a break in the crust where rock

surfaces slip past each other The rocks on each side of the fault can

move up or down or sideways Most faults occur at plate boundaries

where the forces of plate motion push or pull the crust so much that it breaks

Three types: normal fault, reverse fault, and strike-slip

Changing Earth’s Surface The forces of plate movement can change a flat plain into

landforms such as anticlines and synclines, folded mountains, fault-block mountains, and plateaus Anticline- A fold that bends upward into an arch Syncline- A fold that bends downward to form a valley Folded mountains- Caused by the compression and

folding of the crust over a wide area Fault-block mountains- Forms when a block of rock

moves upward between two normal faults Plateaus- Often forms when forces within the Earth push

up a large, flat block of rock

Earthquakes

Earthquakes- Shaking and trembling that results from the sudden movement of part of the Earth’s crust

The most common cause is faulting When part of the crust is pushed together or pulled

apart Occur when the stress along a fault overcomes the

force of friction and releases stored energy Can also be triggered by volcanic eruptions, collapse of

caverns, and meteor impacts

Where Earthquakes Occur Most earthquakes occur at plate boundaries Focus- Point beneath the Earth’s surface where

the rock breaks and moves Depth of the focus depends on where it occurs Earthquakes at divergent boundaries are shallower than

those that occur at subduction zones

Epicenter- Point on the Earth’s

surface directly above the focus

Seismic Waves

Seismic waves- Shock waves produces by earthquakes ( 3 types) Primary Waves (P Waves) Secondary Waves (S Waves) Surface Waves (L Waves)

Primary WavesPrimary waves (P waves)- Push–pull seismic waves that can travel through solids, liquids, & gasses Travels from the focus by

compressing and expanding

the material it passes through Fastest of the earthquake waves

Secondary Waves Secondary waves (S waves)- Side-to-side

moving earthquake waves which can travel through solids but not liquids or gasses Rock particles move at right angles to the direction of the wave Travels through the interior from the focus Slower than P waves, but faster than L waves

Surface Waves

Surface waves (L waves)- Up-and-down earthquake waves Move along the Earth’s surface like waves travel

in the ocean Originate at the epicenter Bend and twist the Earth’s

surface, causing most of the

damage during an earthquake

Locating Earthquakes Seismographs- (Instruments used to detect and measure seismic waves) are used to locate earthquakes Data about each type of seismic wave is taken from the

seismograph and plotted on a time-travel graph The epicenter is located by taking the distances from

three different reporting stations and finding the point where they intersect (also called triangulation)

The depth of the focus is determined by measuring the lag time of the L waves (the longer the lag time, the deeper the focus)

Measuring an Earthquake

Seismographs are used to measure the strength, or magnitude, of an earthquake

Magnitude is determined by measuring the amplitude (height) of the largest wave recorded by a seismograph

Three commonly used methods of measuring earthquakes are the Mercalli scale, the Richter scale, and the moment magnitude scale

The Mercalli Intensity Scale Measures the intensity of an earthquake Based on the damage done to different

types of structures Identifies what someone might experience

(see, hear, or feel) during the earthquake Scale ranges from I to XII, where I is hardly

felt and XII indicates total destruction

The Richter Scale Used to describe the magnitude or strength of an

earthquake Measures the amount of energy released Each number on the scale indicates an

earthquake that is ten times stronger than the next lower number A magnitude 5.0 earthquake is ten times stronger than a

4.0 quake Major earthquakes have magnitudes of 7.0 or higher

The Moment Magnitude Scale Measures the total energy of an earthquake, called

the seismic moment The seismic moment of an earthquake is determined

based on three factors The distance that rock slides along a fault surface after it

breaks, called the fault slip The area of the fault surface that is actually broken by the

earthquake How rigid the rocks are near the broken fault Seismologists multiply the fault slip, fault area, and rigidity

together to determine the actual seismic moment

Earthquake Damage The amount of damage mostly depends on the earthquake’s

magnitude and its proximity to populated areas Other factors that determine the amount of destruction

include: Duration of the quake Time at which the earthquake occurs Types of buildings Material on which structures are built (can produce liquefaction) Fire caused by broken gas mains Broken waterlines hampering firefighters Tsunamis along coastal areas

Volcanoes Volcano- A weak spot in the Earth’s

crust where molten rock and other materials reach the surface

Inside a volcano Crater- Depression at the summit of a volcanic cone Magma chamber- Large reservoir of magma below the Earth’s crust Pipe- Tube that connects the

magma chamber to the Earth’s surface

Vent- Opening from which volcanic material is ejected

Volcanism Releases Magma Magma- Melted rock below Earth’s surface Magma forms where temperatures are high enough to

melt rock Asthenosphere Plate boundaries

Magma rises to the surface because it is less dense than the surrounding material

The rate at which magma flows (its viscosity) is determined primarily by its silica content and temperature

Two Types of Magma Felsic Magma

Also called granitic magma

High silica content Viscous or thick Slow moving Contains a lot of water Creates explosive

volcanic eruptions

Mafic Magma Also called basaltic

magma Low silica content Less viscous or thin Flows easily Contains very little water Produces quiet volcanic

eruptions

Gases in Magma Magma contains dissolved gases that are

released during an eruption Gases are primarily water vapor, carbon

dioxide, and sulfur Magmas containing higher amounts of

dissolved gases produce more explosive eruptions than those with smaller amounts

Temperature of Magma Magma ranges in temperature from about

1000°C to 1200°C The hotter the magma, the easier it flows

Hotter magma is less viscous than cooler magma

Hotter magmas trap less gas Hotter magmas are associated with quieter

eruptions

Lava Lava- Magma that reaches the surface

Two types: AA Pahoehoe

How lava differs from magma Composition is slightly different Some gases have escaped New material is often added when the magma comes in

contact with other rock Temperature is lower

Volcanic Eruptions Three factors determine the nature of a

volcanic eruption1. Composition of the magma

2. Temperature of the magma

3. Amount of dissolved gases

Different types of eruptions form different types of volcanoes

Kinds of Volcanic Eruptions Geologists classify volcanic eruptions as quiet or explosive

Quiet Eruptions- Low silica, low viscosity magma that flows easily Gases bubble out gently Lava oozes quietly from the vent

Explosive Eruptions- High Silica, high viscosity magma that clogs the volcano’s vent Trapped gases build up pressure until they explode Eruption breaks lava into fragments that quickly cool and harden into

pieces of different sizes Results in a pyroclastic flow of hot gases, ash, cinders and volcanic

bombs

Three Main Types of Volcanoes Shield Volcanoes

Cinder Cones

Composite Volcanoes

Shield Volcanoes Large, gently sloping dome-shaped volcanic

mountains Made from fluid, basaltic lava (mafic magma) Produced by quiet

eruptions Formed at “hot spots” Example: Mauna Loa

(Hawaiian volcano)

Cinder Cones Small, steep-sided volcanoes Produced by violent, pyroclastic ejections of

material from a central vent Made of cinders and other rock particles (felsic

magma) Usually occur in groups Found along convergent boundaries Example: Paricutin, Mexico

Composite Volcanoes Large, steep-sided, cone-shaped volcanic

mountains Built of alternating layers of rock particles

(pyroclastic material) and fluid lava Produced by very violent eruptions Found along convergent boundaries Example: Mt. St. Helens

Where Volcanic Activity Occurs Divergent boundaries- produces rift zone

eruptions Convergent boundary- creates subduction

zone eruptions At hot spots, in the middle of lithospheric

plates- produces hot spot eruptions

Rift Eruptions Occur along narrow fractures in the crust (usually

along divergent boundaries) Mid-Atlantic Ridge East African Ridge

Magma wells up to fill the gap

as the crust splits Eruptions are typically quiet

Magma is basaltic Magma contains little gas

Subduction Boundary Eruptions Occur at convergent boundaries where one plate

is driven below another Magma tends to be thick (viscous) and contain

large amounts of dissolved gas Eruptions are usually explosive Form steep-sided volcanoes (cinder cones or

composite) Most volcanoes occur at subduction boundaries

along the edge of the Pacific Ocean (the Ring of Fire)

Hot Spots Areas of volcanic activity which occur in the middle of

plates (also called intraplate volcanism) Form volcanoes with broad, gently sloping sides (shield

volcanoes) Magma is thin and flows easily, similar to that of rift

eruptions Produces quiet eruptions Thought to be caused by hot plumes of magma rising from deep within the Earth Example: The Hawaiian Islands

Life Cycle of a Volcano The terms active, dormant, and extinct are used

to describe a volcano’s stage of activity Active- A volcano that is erupting or has shown signs

that it may erupt in the near future Dormant- A volcano that is not currently active, but

may become active in the near future Extinct- A volcano that is no longer active and is

unlikely to erupt again