Earthquakes and Volcanoes
Earthquakes
An earthquake is the shaking and trembling that results from the sudden movement of part of the Earth’s crust.The most common cause of earthquakes is faulting. During faulting, energy is released. Rocks continue to move until the energy is used up.
Seismic Risk Map of the U.S.
San Andreas Fault
The San Andreas fault extends 960 km from Mexico to the north of California.The land to the west is moving north. The land to the east of the fault is moving south. All the rocks do not move at the same time so earthquakes occur in one area and then another.
San Francisco Quake
In 1906, movement along the San Andreas fault caused an earthquake in San Francisco.Tsunamis
Earthquakes which occur on the ocean floor produce giant sea waves called tsunamis. Tsunamis can travel at speeds of 700 to 800 km per hour. As they approach the coast, they can reach heights of greater than 20 meters.Focus
Most faults occur between the surface and a depth of 70 kilometers.The point beneath the surface where the rocks break and move is called the focus. The focus is the underground origin of an earthquake.
Epicenter
Directly above the focus, on the Earth’s surface is the epicenter. Earthquake waves reach the epicenter first. During an earthquake, the most violent shaking is found at the epicenter.Seismic Waves
There are three main types of seismic waves.Primary
Secondary
Surface
Primary Waves
Seismic waves that travel fastest are P waves. They travel through solids, liquids and gases.They move at different speeds depending on the density of the material through which they are moving. As they move deeper in the Earth they move faster.
P waves are push-pull waves.
Secondary Waves
Seismic waves that do not travel through the Earth as fast as P waves do are called secondary or S waves.S waves travel through solids but not liquids or gases.
S waves cause particles to move from side to side. They move at right angles to the direction of the wave.
Surface Waves
The slowest moving seismic waves are called surface waves or L waves.L waves originate on the Earth’s surface at the epicenter. They move along the surface the way waves travel in the ocean. The Earth’s surface moves up and down with each L wave.
L waves cause most of the damage.
The Seismograph
Invented in 1893 by John Milne, a seismograph detects and measures seismic waves.A weight attached to a spring remains nearly still even when the Earth moves. A pen attached to the weight records any movement on a roll of paper on a constantly rotating drum. The drum moves with the Earth and affects the line.
Seismologists
Seismologists study earthquakes. They can determine the strength of an earthquake by the height of the wavy line recorded on the paper.The seismograph record of waves is called a seismogram.
The Richter scale is used to calculate the strength of an earthquake.
The Richter Scale
The amount of damage created by an earthquake depends on several factors.The earthquake’s strength
The kind of rock and soil that underlie an area
The population of the area
The kind of buildings in the area
The time at which the earthquake occurs
Predicting Earthquakes
Scientists have identified warning signals the help predict earthquakes with greater accuracy.Often changes occur in the speed of P and S waves before an earthquake occurs.
Sometimes slight changes in the tilt of the Earth’s surface can be detected.
Some scientists believe animals behavior is affected.
Formation of a Volcano
Magma
Deep within the Earth, under tremendous pressure and at great temperatures, rock exists as a hot liquid called magma. This molten rock is found in pockets called magma chambers.Lava
When the magma reaches the surface, it is called lava. The place in the Earth’s surface through which magma and other materials reach the surface is called a volcano. In some places, lava can build up to forma cone-shaped mountain.The opening from which lava erupts is the vent. Volcanoes often have more than one vent.
Dark-Colored Lava
There are four types of lava.One is dark-colored and contains a lot of water. This is rich in iron and magnesium and cools to form igneous rocks such as basalt. This lava is thin and runny and most tends to flow. The islands of Hawaii and Iceland were formed by many lava flows.
Light-Colored Lava
The second type of lava is light in color. This lava, contains little water and is rich in silicon and aluminum. Light-colored lava causes explosive eruptions. Silicon tends to harden in the vents and form rocks. Steam and new lava build up under the rocks. When the pressure becomes great, a violent explosion occurs. When this type of lava cools it form the igneous rock, rhyolite, which resembles granite.Combination Lava
The third type of lava has a chemical composition similar to that of both the dark-colored type and the light-colored type. Different varieties of igneous rocks in the Earth’s crust, such as andesite, are form from this type of lava.Gaseous Lava
The fourth type of lava contains large amounts of gases such as steam and carbon dioxide. When this lava hardens, it forms rocks with many holes in them, due to gas bubbles. Pumice and scoria are igneous rocks formed from this type of lava.Volcanic Eruptions
During volcanic eruptions, many rock fragments are blown into the air. The smallest particles are called volcanic dust. (less than 0.25 mm)Volcanic Ash ( 0.25 -5mm) falls to the Earth and forms small rocks.
Volcanic bombs (a few cm to several meters) are molten and harden as they travel through the air.
Types of Volcanoes
Different types of volcanic eruptions form different types of volcanoes.Cinder cones
Shield volcanoes
Composite volcanoes
Three Types
Cinder Cones
Volcanoes made mostly of cinders and other rock particles that have been blown into the air are called cinder cones. Cinder cones form from explosive eruptions. Because the material is loosely arranged, the cones are not high. The have a narrow base and steep sides such as Paricutin in Mexico.Cinder Cone Volcano
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