Earthquakes are shakings of the earth's crust due to disturbances within the earth's interior. The shaking may be gentle and perceptible only to delicate instruments, or it may be violent and cause great disasters.
The principal explanation of earthquakes lies in the yielding of the earth's crust to some strain from beneath, resulting in a displacement of the surface along an existing fault line, or the breaking of the crust, forming a new fracture.
Causes of an Earthquake:
1. Tectonic Earthquake- the shifting or movement of the crustal plates
2. Volcanic Earthquakes- when magma moves along a fissure, they cause the Earth's crust to tremble. The magma may reach the surface and when this happens, a volcanic eruption occurs.
When an earthquake occurs, some places experience stronger tremors than others. Areas where stronger vibrations are felt are usually near the epicenter of the earthquake.
An epicenter is the point on the surface of the Earth located directly above the center of an earthquake's origin. The center or source of the earthquake is called focus.
MEASUREMENT
Intensity- it is the extent of damage caused by an earthquake. When the crust shakes, such tremor causes damages to people, buildings, bridges, roads and vegetation.
Magnitude- is the amount of energy released by an earthquake. An American scientists named Charles F. Richter developed a scale to indicate the magnitude of an earthquake. This scale is popularly known as the Richter Magnitude Scale that uses numbers from 1 to 10. The stronger the earthquake is the higher number on the scale. The more feeble or weaker the earthquake is, the lower is the number.
Scientists have invented a device that measures the movement of the Earth's crust. It is called seismograph. A seismograph can detect and record the vibrations on the Earth's surface.
VOLCANISM
Volcanism is the movement of liquid rock inside or outside the Earth's crust.
A volcano is a mountain or hill with a vent extending from the top down to the Earth's interior.
The molten material in a volcano is called magma. When an eruption takes place the magma expels volcanic gases and the molten mass which flows out is known as lava. The flowing lava is at white heat when eruption takes place, but it cools comparatively quickly when it is exposed to the air, and in cooling it solidifies, forming igneous rock.
Volcanoes may either be active or inactive. Active volcanoes are those that have erupted or shown signs of activity for the past 600 years. Examples of this type are Mt. Pinatubo, Mt. Mayon and Taal Volcano. Volcanoes that have not erupted for 600 years or more are considered inactive. Mt. Apo, Mt. Arayat and Mt. Makiling are some of the inactive volcanoes in the Philippines.
MOUNTAIN
Mountains- a large mass of earth and rock, rising above the common level of the earth and adjacent land; earth and rock forming an isolated peck or a ridge; an eminence higher than a hill.
When faulting occurs, as we have already seen, two areas formerly in the same horizontal plane are forced out of alignment, one being raised, the other lowered. When this occurs on a large scale and involves several nearly parallel fractures, mountains and valleys may be formed as blocks are raised and lowered.
