Earthquakes An earthquake is the vibration of Earth produced by
the rapid release of energy Focus and Epicenter Focus is the point
within Earth where the earthquake starts. Epicenter is the location
on the surface directly above the focus. Faults Faults are
fractures in Earth where movement has occurred.
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Earthquake
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Cause of Earthquakes: Elastic Rebound Hypothesis Most
earthquakes are produced by the rapid release of elastic energy
stored in rock that has been subjected to great forces. When the
strength of the rock is exceeded, it suddenly breaks, causing the
vibrations of an earthquake.
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Measuring Earthquakes Seismographs are instruments that record
earthquake waves. Seismograms are traces of amplified,
electronically recorded ground motion made by seismographs.
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Earthquake Waves Three types of Earthquake Waves Surface waves,
P waves, and S waves Surface waves are seismic waves that travel
along Earths outer layer.
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P waves Are push-pull waves that push (compress) and pull
(expand) in the direction that the waves travel Travel through
solids, liquids, and gases Have the greatest velocity of all
earthquake waves
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S waves Seismic waves that travel along Earths outer layer
Shake particles at right angles to the direction that they travel
Travel only through solids Slower velocity than P waves
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Movement of Waves
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Locating an Earthquake Earthquake Distance The epicenter is
located using the difference in the arrival times between P and S
wave recordings, which are related to distance. Earthquake
Direction Travel-time graphs from three or more seismographs can be
used to find the exact location of an earthquake epicenter.
Earthquake Zones About 95 percent of the major earthquakes occur in
a few narrow zones.
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Locating an Earthquake
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Measuring Earthquakes Richter Scale Based on the amplitude of
the largest seismic wave Each unit of Richter magnitude equates to
roughly a 32-fold energy increase Does not estimate adequately the
size of very large earthquakes Momentum Magnitude Derived from the
amount of displacement that occurs along the fault zone Moment
magnitude is the most widely used measurement for earthquakes
because it is the only magnitude scale that estimates the energy
released by earthquakes. Measures very large earthquakes
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Damage from Earthquakes The damage to buildings and other
structures from earthquake waves depends on several factors. These
factors include: the intensity and duration of the vibrations, the
nature of the material on which the structure is built, and the
design of the structure.
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Damage from Earthquakes Building Design Factors that determine
structural damage Intensity of the earthquake Unreinforced stone or
brick buildings are the most serious safety threats Nature of the
material upon which the structure rests The design of the
structure
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Layers of the Earth
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Earths interior consists of three major zones defined by their
chemical compositionthe crust, mantle, and core.
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Crust Thin, rocky outer layer Varies in thickness Roughly 7 km
in oceanic regions Continental crust averages 840 km Exceeds 70 km
in mountainous regions Early seismic data and drilling technology
indicate that the continental crust is mostly made of lighter,
granitic rocks.
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Mantle Below crust to a depth of 2900 kilometers Composition of
the uppermost mantle is the igneous rock peridotite (changes at
greater depths). Composition is more speculative. Some of the lava
that reaches Earths surface comes from asthenosphere within.
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Core Below mantle Sphere with a radius of 3486 kilometers
Composed of an iron-nickel alloy Average density of nearly 11 g/cm
3 Earths core is thought to be mainly dense iron and nickel,
similar to metallic meteorites. The surrounding mantle is believed
to be composed of rocks similar to stony meteorites.
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Layered Earth
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Plate Tectonics
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Evidence
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Plate Tectonics According to the plate tectonics theory, the
uppermost mantle, along with the overlying crust, behaves as a
strong, rigid layer. This layer is known as the lithosphere. A
plate is one of numerous rigid sections of the lithosphere that
move as a unit over the asthenosphere.
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Plate Boundaries Divergent boundaries (also called spreading
centers) are the place where two plates move apart. Convergent
boundaries form where two plates move together. Transform fault
boundaries are margins where two plates grind past each other
without the production or destruction of the lithosphere.
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Plate Boundaries
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Divergent Boundaries
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Convergent Boundaries Types of Convergent Boundaries
Oceanic-Continental Oceanic-Oceanic Continental-Continental A
subduction zone occurs when one oceanic plate is forced down into
the mantle beneath a second plate.
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Oceanic-Continental
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Oceanic-Oceanic
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Continental-Continental
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Transform-Fault Boundaries
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What makes the plates move?
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Volcanic Activity
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Factors that affect Volcanic Eruptions Viscosity of the magma
Composition of the magma Temperature of the magma Dissolved gases
in the magma
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Viscosity Viscosity is the measure of a material's resistance
to flow. Factors affecting viscosity Temperature (hotter magmas are
less viscous) Composition (silica content) High silicahigh
viscosity Low silicamore fluid
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Dissolved Gases Mainly water vapor and carbon dioxide Violence
of an eruption is related to how easily gases escape from magma
Gases escape easily from fluid magma which produces a less violent
eruption Viscous magma produces a more violent eruption.
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Volcanic Materials Lava Flows Basaltic lavas are more fluid.
Types of lava Pahoehoe lava (resembles braids in ropes) Aa lava
(rough, jagged blocks)
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Volcanic Material Gases One to 5 percent of magma by weight
Pyroclastic Materials Pyroclastic materials is the name given to
particles produced in volcanic eruptions The fragments ejected
during eruptions range in size from very fine dust and volcanic ash
(less than 2 millimeters) to pieces that weigh several tons.
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Volcanic Material Types of pyroclastic material Ash and
dustfine, glassy fragments Pumicefrothy, air-filled lava
Lapilliwalnut-sized particles Cinderspea-sized particles
Blockshardened lava Bombsejected as hot lava
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Volcanoes The three main volcanic types are shield volcanoes,
cinder cones, and composite cones. A volcano is a mountain formed
of lava and/or pyroclastic material. A crater is the depression at
the summit of a volcano or that which is produced by a meteorite
impact. A conduit, or pipe, carries gas-rich magma to the
surface.
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Volcanoes
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Shield Volcano
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Cinder Cone
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Composite Cone
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Mt. Saint Helens Before and After May 18, 1980 Eruption
