Planet EarthSection 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.

Planet EarthSection 2 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

Planet EarthSection 2 Focus and Epicenter

Planet EarthSection 2 What are Earthquakes? continued 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

Planet EarthSection 2 What are Earthquakes? continued 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

Planet EarthSection 2 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.

Planet EarthSection 2 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

Planet EarthSection 2 Measuring Earthquakes, continued 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.

Planet EarthSection 2 Measuring Earthquakes, continued 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.

Planet EarthSection 2 Measuring Earthquakes, continued The Richter scale is used to measure earthquakes. –Although the Richter scale was used popularly for much of the 20 th century, scientists today more often use other scales, such as the moment- magnitude scale. Richter scale: a scale that expresses the magnitude of earthquakes

Planet EarthSection 2 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

Planet EarthSection 2 Volcanoes

Planet EarthSection 2 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.

Planet EarthSection 2 Ring of Fire

Planet EarthSection 2 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.

Planet EarthSection 2 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.