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Copyright © 2008 Thomson Delmar Learning, a division of Thomson Learning, Inc.

ALL RIGHTS RESERVED.

Earthquakes!Earthquakes!

• Chapter 10



EarthquakesEarthquakes

• One of most destructive forces on Earth

• Natural rapid shaking of the ground due to friction from sliding faults

(Ex: rubbing your hands)– Results in displaced rocks

– Causes energy to dissipate outward

– Energy travels through Earth in seismic waves

– When reaches the surface, moves the ground!

– On ocean floor, form tidal waves or tsunamis









Seismic WavesSeismic Waves

• Seismic Waves can be detected using a seismograph– Uses series of springs and weights

• Types of seismic waves:1) P-waves (compressional Primary wave)

• Move like sound waves

• Fastest type of seismic wave

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http://earthsci.org/education/teacher/basicge

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2) S-waves (Secondary waves)

– Travel in an up-and-down pattern

– Can only travel through solid material

Surface wave —> interaction of P- and S- waves

– Results in a rolling motion of the ground

http://earthquakes.usgs.gov/learning/faq.php?categoryID=5&faqID=26



http://www.astro.virginia.edu/class/oconnell/astr121/im/seismic-waves-crossec-CM.jpg

Source of waves is called the focus

Area directly above focus called

epicenter



Epicenter LocationEpicenter Location

• Determined by arrival of P- and S-waves– Using a seismograph

– Locates the distance to the epicenter

– Use difference in arrival times

– Data from three seismograph stations needed



Earthquake MeasurementEarthquake Measurement

Strength measured using:

– Richter scale — measures equivalent energy produced

• Scale of 0 to 9

• Every increase in number means ten fold

(10 x) increased magnitude

• 5 is equivalent of medium-sized atomic bomb!





http://data.gns.cri.nz/geoatlas/images/mercalli.jpg

EarthquakeEarthquake

MeasurementMeasurement

• Or the MM scale, which measures destruction.

• The Modified MercalliIntensity (MM) scale is a means of categorizing the effects of shaking on people, structures and the environment.

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Volcanoes !Volcanoes !



Formation of a VolcanoFormation of a Volcano

• Where molten rock flows from beneath crust– Molten rock is called lava

• Location associated with convergent tectonicplate boundaries– Where subducting plate begins to descendunderneath another plate.

– Sinking plate brings in seawater making steam





Types of VolcanoesTypes of Volcanoes

• Volcanoes are classified by type of eruption

• Eruption — release of lava, ash, steam, and gases





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Quiet Eruption VolcanoesQuiet Eruption Volcanoes

• Quiet eruptions from volcanoes called shield cones– Formed when lava flows from a central vent

• Lava cools at surface and solidifiesforming a shield



• Fissure eruption

– Long crack in Earth’s crust

– Produces large amounts of lava

– Cool to form huge sheets or plateaus

– Deccan lava flows in India

– Columbia River plateau



Explosive Eruption VolcanoesExplosive Eruption Volcanoes

• Much more violent

• Greatest threat to human life and property

• Composite cone — mountain-like structures

– Fire-broken rocks that were blasted apart

– Usually surround one central vent

– Near subduction zones with convergent plate boundaries



Explosive eruption with lightning at

Sakurajima Volcano

Cinders were issued up to >2,500 m high

(23:03 on18 May 1991; taken by SVO)



• Caldera — volcanic crater (a small canyon)

– Left behind after composite cone is destroyed

– Crater Lake in Oregon

– One of world’s largest in Yellowstone Park

– Heats water

• Produces geysers, mud pots, and hot springs

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Caldera A large, usually circular depression at the summit of a

volcano formed when magma is withdrawn or erupted from a

shallow underground magma reservoir.



Geyser in Yellowstone



• Lava dome

– Smaller mounds, hardened lava covering a vent

– Grow slowly

– Build up great pressure

– After Mount St. Helens in 1980

• Formed small lava dome in its crater



• Cinder cone

– Small mound of lava, ash,

and pyroclastic material

– Smallest type of volcano

– When lava breaks through for first time

– Resulting eruption spews lava into air

– Forms a volcanic bomb

• Lava hardens before hitting the ground



Volcanic HazardsVolcanic Hazards

• Most dangerous result is pyroclastic flow

– Moves at speeds of more than 100 mph

– Contains hot gases, water, ash, and debris

– Destroys everything in its path

– Mount Vesuvius in A.D. 79

• Destroyed Pompeii

• Volcanic ash enters atmosphere

and travels miles!



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• Ash mixes with water, causes mudflows

• Mount Pinatubo in 1991

– Mudflow 650 feet deep



• Release of toxic gas

– Sulfur dioxide and water forms sulfuric acid

• Debris flows called lahars

– When snow melts suddenly at high elevations



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