Earthquakes

• Earthquakes occur at plate boundaries.

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

• Seismic waves are waves of energy released during in earthquake

• Focus the area along a fault at which the first motion of an earthquake occurs

• Epicenter the point on Earth’s surface directly above an earthquake’s focus
• Energy from earthquakes is transferred by waves.
• Earthquakes generate three types of waves:
• Longitudinal waves
• Transverse waves
• Surface waves

• Longitudinal waves travel by compressing and stretching crust, also called primary waves (P waves)
• Transverse waves travel in an up and downward movement, also called secondary waves (S waves)
• Surface waves seismic waves that can move only through solids, move in a rolling circular motion
• 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.
• Seismologists detect and measure earthquakes.

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

• Seismologists use sensitive equipment called seismographs to record data about earthquakes.
• 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 help scientists make a model of Earth with layers of different densities.
• The Richter scale is a measure of the magnitude of earthquakes.

• Richter scale a scale that expresses the magnitude of an earthquake

• The intensity of an earthquake is measured by the modified Mercalli scale. Intensity depends on many factors.
• Earthquakes that occur deeper below the Earth’s surface will not be as intense at the surface.
• The hardness of the rock above and around an earthquake affects the intensity.
• Scientists are trying to predict earthquakes.

• Scientists are trying to measure changes in Earth’s crust that might signal an earthquake.

• The ability to predict an earthquake could save thousands of lives in the future.
• A volcano is any opening in Earth’s crust through which magma has reached Earth’s surface.

• Vent an opening at the surface of Earth through which volcanic material passes

• Volcanoes generally have one central vent, but they can also have several smaller vents.
• Magma that reaches Earth’s surface is called lava.
• Shield volcanoes have mild eruptions.

• Lava from shield volcanoes is very fluid and forms a gently sloping mountain.

• Shield volcanoes are some of the largest volcanoes.
• Composite volcanoes have trapped gas.

• Composite volcanoes are made up of alternating layers of ash, cinders, and lava.

• The lave is thicker than that of shield volcanoes.

• Gases are trapped in the magma, causing eruptions that alternate between flows and explosive activity that produces cinders and ash.

• Composite volcanoes are typically tall with steep sides.
• Cinder cones are the most abundant volcano.

• Cinder cones are the smallest and most common volcanoes.

• Large amounts of gas are trapped in the magma, and violent eruptions of hot ash and lava occur.

• Cinder cones tend to be active for only a short time and then become dormant.
• 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.
• 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 occur at hot spots.
• 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.