planet earthsection 2 what are earthquakes? 〉 where do most earthquakes occur? 〉 by looking at...

Planet Earth Section 2

What are Earthquakes?

〉Where do most earthquakes occur?

〉By looking at maps showing past seismic activity, one can see that earthquakes occur mostly at the boundaries of tectonic plates, where the plates move with respect to each other.


What are Earthquakes? continued

– Earthquakes are vibrations resulting from rocks sliding past each other at a fault.

• Seismic waves are waves of energy released during an earthquake.

• focus: the location within Earth along a fault at which the first motion of an earthquake occurs

• epicenter: the point on Earth’s surface directly above an earthquake’s starting point, or focus


Focus and Epicenter



• Energy from earthquakes is transferred by waves.– The energy released from an earthquake is measured

as shock waves.– Earthquakes generate three types of waves:

• Longitudinal waves, also known as P waves• Transverse waves, also known as S waves• Surface waves



• Waves move through Earth and along its surface.– Both P waves and S waves spread out from the focus

in all directions through the earth.– Surface waves move only on Earth’s surface.

• surface wave: a seismic wave that travels along the surface of a medium and has a stronger effect near the surface of the medium than it has in the interior


Measuring Earthquakes

〉How do scientists learn about earthquakes and the Earth’s interior?

〉Because energy from earthquakes is transferred by waves, scientists can measure the waves to learn about earthquakes and about the interior of Earth through which the waves travel.


Measuring Earthquakes, continued

• Seismologists detect and measure earthquakes.– Seismologists use sensitive equipment called

seismographs to record data about earthquakes.– Records of seismic activity are called seismograms.

• seismology: the study of earthquakes including their origin, propagation, energy, and prediction



• Three seismograph stations are necessary to locate the epicenter of an earthquake.– There are more than 1000 seismograph stations

across the world.– Because P waves travel faster, the difference

between the arrival of P waves and the arrival of S waves allows scientists to calculate how far away the focus is.



• Geologists use seismographs to investigate Earth’s interior.– The way P and S waves travel through Earth’s interior

helps scientists make a model of Earth with layers of different densities.

– Scientists have used this information to develop a model of Earth’s interior structure.



• The Richter scale is used to measure earthquakes.– Although the Richter scale was used popularly for

much of the 20th century, scientists today more often use other scales, such as the moment-magnitude scale.

• Richter scale: a scale that expresses the magnitude of earthquakes


Volcanoes

〉What is a volcano?

〉A volcano is any opening in Earth’s crust through which magma has reached Earth’s surface.

• crust: an opening in the surface of Earth through which volcanic material passes


Volcanoes


Volcanoes, continued

• Most volcanoes occur at convergent plate boundaries.– 75% of the active volcanoes on Earth are located in

an area known as the Ring of Fire.– The Ring of Fire is located along the edges of the

Pacific ocean, where oceanic tectonic plates are colliding with continental plates.


Ring of Fire


Volcanoes, continued

• Underwater volcanoes occur at divergent plate boundaries.– As plates move apart at divergent boundaries,

magma rises to fill the gap. – This magma creates the volcanic mountains that form

ocean ridges.– Iceland is a volcanic island on the Mid-Atlantic ridge

that 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 hot

rock that rise from deep inside the mantle, melt as they rise, and erupt from volcanoes at hot spots at the surface.

– The plumes remain in the same place as the tectonic plate moves, creating a trail of volcanoes.

– The Hawaiian Islands are an example of this type of volcanic activity.

planet earthsection 2 what are earthquakes? 〉 where do most earthquakes occur? 〉 by looking at...

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