Section 2: Earthquakes and Volcanoes
Key Ideas
〉Where do most earthquakes occur?
〉How do scientists learn about earthquakes and theEarth’s interior?
〉What is a volcano?
Bellringer
1.Imagine a corked bottle of soda pop that is standing in a panof hot water.What do you think will happen as the soda pop heats up?
2.What happens when the pressure builds up in the soda pop?
3.Molten rock in Earth’s mantle is like the soda pop.What happens when pressure builds up in Earth’s mantle?
What are Earthquakes?
〉Where do most earthquakes occur?
〉By looking at maps showing past seismic activity, onecan see that earthquakes occur mostly at theboundaries of tectonic plates, where the plates movewith respect to each other.
What are Earthquakes? continued
•Earthquakes occur at plate boundaries.
–Earthquakes are vibrations resulting from rockssliding past each other at a fault.
–Seismic waves are waves of energy releasedduring an earthquake.
•focus: the location within Earth along a fault at whichthe first motion of an earthquake occurs
•epicenter: the point on Earth’s surface directly abovean earthquake’s starting point, or focus
What are Earthquakes? continued
•Energy from earthquakes is transferred by waves.
–The energy released from an earthquake is measuredas shock waves.
–Earthquakes generate three types of waves:
•Longitudinal waves, also known as P waves
•Transverse waves, also known as S waves
•Surface waves
What are Earthquakes? continued
–Longitudinal waves travel by compressing andstretching crust. They are also called primary waves(P waves).
–Transverse waves travel in an up and downwardmovement. They are also called secondary waves(S waves).
What are Earthquakes? continued
•Waves move through Earth and along its surface.
–Both P waves and S waves spread out from the focusin all directions through the earth.
–Surface waves move only on Earth’s surface.
•surface wave: a seismic wave that travels along thesurface of a medium and has a stronger effect near thesurface of the medium than it has in the interior
Measuring Earthquakes
〉How do scientists learn about earthquakes and theEarth’s interior?
〉Because energy from earthquakes is transferred bywaves, scientists can measure the waves to learn aboutearthquakes and about the interior of Earth throughwhich the waves travel.
Measuring Earthquakes, continued
•Seismologists detect and measure earthquakes.
–Seismologists use sensitive equipment calledseismographs to record data about earthquakes.
–Records of seismic activity are called seismograms.
•seismology: the study of earthquakes including theirorigin, propagation, energy, and prediction
Measuring Earthquakes, continued
•Three seismograph stations are necessary to locate theepicenter of an earthquake.
–There are more than 1000 seismograph stationsacross the world.
–Because P waves travel faster, the differencebetween the arrival of P waves and the arrival of Swaves allows scientists to calculate how far away thefocus is.
Measuring Earthquakes, continued
•Geologists use seismographs to investigate Earth’sinterior.
–The way P and S waves travel through Earth’s interiorhelps scientists make a model of Earth with layers ofdifferent densities.
–Scientists have used this information to develop amodel of Earth’s interior structure.
Measuring Earthquakes, continued
•The Richter scale is used to measure earthquakes.
–Although the Richter scale was used popularly formuch of the 20th century, scientists today moreoften use other scales, such as the moment-magnitude scale.
•Richter scale: a scale that expresses the magnitude ofearthquakes
Measuring Earthquakes, continued
•Magnitude scales alone cannot predict how severe anearthquake will be.
–Many factors, such as distance from the focus andlocal construction techniques, affect how muchdamage an earthquake will cause.
•Scientists are trying to predict earthquakes.
–Scientists are trying to measure changes in Earth’scrust that might signal an earthquake.
–The ability to predict an earthquake could savethousands of lives.
Volcanoes
〉What is a volcano?
〉A volcano is any opening in Earth’s crust through whichmagma has reached Earth’s surface.
•crust: an opening in the surface of Earth through whichvolcanic material passes
Volcanoes
•Shield volcanoes have mild eruptions.
–Shield volcanoes are some of the largest volcanoes.
•Composite volcanoes have trapped gas.
–Composite volcanoes are made of alternating layersof ash, cinders, and lava.
–Composite volcanoes are usually very steep.
•Cinder cones are the most abundant volcanoes.
–Cinder cones are the smallest and most abundantvolcanoes.
Volcanoes, continued
•Most volcanoes occur at convergent plate boundaries.
–75% of the active volcanoes on Earth are located inan area known as the Ring of Fire.
–The Ring of Fire is located along the edges of thePacific ocean, where oceanic tectonic plates arecolliding with continental plates.
Volcanoes, continued
•Underwater volcanoes occur at divergent plateboundaries.
–As plates move apart at divergent boundaries,magma rises to fill the gap.
–This magma creates the volcanic mountains that formocean ridges.
–Iceland is a volcanic island on the Mid-Atlantic ridgethat is growing outward in opposite directions.
Volcanoes, continued
•Volcanoes occur at hotspots.
–Some volcanoes occur in the middle of plates.
–Mantle plumes are mushroom shaped trails of hotrock that rise from deep inside the mantle, melt asthey rise, and erupt from volcanoes at hot spots at thesurface.
–The plumes remain in the same place as the tectonicplate moves, creating a trail of volcanoes.
–The Hawaiian Islands are an example of this type ofvolcanic activity.
