Earthquakes and Volcanoes Earthquakes and volcanic eruptions are the most dramatic and observable examples of the movement of the Earths crust. An earthquake or an eruption suddenly changes the surface of the Earth, often with devastation effects on human property and life. Earthquakes An earthquake is the shaking and trembling that results from the sudden movement of part of the Earths crust. The most common cause of earthquakes is faulting. Earthquakes also occur on the floor of the ocean. Ocean earthquakes often produce giant sea waves called tsunamis. A tsunami can produce winds of up to 700-800 km/hr and can produce waves up to 20 meters (6 story building) high. The point beneath the Earths surface where the rocks break and move is called the focus of the earthquake. Directly above the focus, on the Earths surface, is the epicenter. Earthquake waves are known as seismic waves. The three main types of seismic waves are primary waves, secondary waves, and surface waves. 1. The fastest seismic waves are called primary waves, or P waves. They travel through solids, liquids and gasses. As they travel, they push or compress rock particles ahead of them. 2. Secondary waves, or S waves, are slower than P waves. S waves travel through solids but not through liquids and gases. S waves move rock particles from side to side, at right angles to the direction the waves are heading. 3. The slowest seismic waves are called surface waves, or L waves. They arrive last at any given pint, after primary and secondary waves. They originate at the epicenter of an earthquake and move along the surface the way waves travel in the ocean. They move the surface up and down and cause the most damage during an earthquake. A seismograph is an instrument that detects and measures seismic waves. It consists of a weight attached to a spring or wire. The weight is not attached directly to Earth, so it remains nearly still even when the Earth moves. When the Earth moves, a pen attached to the weight records the movement on a sheet of paper wound around a constantly rotating drum. The record is a wavy line called a seismogram. Seismologists are scientists who study earthquakes and determine the strength of an earthquake by studying the height of the wavy lines. Seismo= to shake Logy= study of The strength of earthquakes is measured on the Richter scale. The Richter scale measures the amount of energy released by an earthquake by assigning it a number from 1 to 10. The more energy an earthquake releases, the stronger the earthquake is, and the higher the number on the Richter scale. Formation of Volcano Deep within the Earth, rock exists as a hot liquid called magma. When magma reaches the Earths surface it is called lava. The place in the Earths surface through which magma and other materials reach the surface is called a volcano. The opening from which lava erupts is called a vent. During volcanic eruptions, many rock fragments are blown into the air. The smallest articles are called volcanic dust. Rock particles more than 0.25 millimeters but less than 5 millimeters in diameter are called volcanic ash. Larger rock particles, up to several meters in diameter, are called volcanic bombs. Small volcanic bombs about the size of golf balls are called cinders. Three types of volcanoes formed by different types of volcanic eruptions: Cinder cone volcano— volcanoes made mostly of ciders and other rock particles that have been blown into the air. These cones are low and have a narrow base and steep sides. Shield volcano—volcanoes composed of quiet lave flows. These volcanoes have gently sloping sides and are dome-shaped. EX: Mauna Loa, Hawaii Composite volcano—volcanoes built up of alternating layers of rock particles and lave. They are generally large cone-shaped mountains. EX: Mt. Vesuvius A depression at the top of a volcanic cone is called a crater. If the crater becomes very large as a result of the collapse of its walls, it is called a caldera. Scientists classify volcanoes as active, dormant, or extinct, according to their volcanic activity. An active volcano is one that erupts either continually or periodically. EX: Mt. St. Helens, WA A volcano that has been known to erupt within modern times but is now inactive classified as a dormant volcano. EX: Mr. Rainier, WA and Mt. Shasta, CA A volcano not known to have erupted within modern history is classified as an extinct volcano. EX: Mt. Pompeii Volcano and Earthquake Zones Most of the major earthquakes and volcanic eruptions in the world occur in three zones. Scientists believe that there is a great deal of movement and activity in the Earths crust in these zones. One major zone extends nearly all the way around the edge of the Pacific Ocean and is called the Ring of Fire. A second major zone is located near the Mediterranean Sea. The third major zone extends from Iceland to the middle of the Atlantic Ocean along a long range of underwater volcanic mountains called the Mid-Atlantic Ridge. Scientists believe that the volcano and earthquake activity in this zone is due to the formation of new parts of the Earths crust along the ridge.