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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