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.
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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.