Internal Geologic Process
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Internal Geological Processes
The term internal geological processes describe the natural deep seated forces that partly shape the earth. The plate tectonics theory explains the main internal force that significantly affects the earth’s surface and accounts for its major geomorphic features. These forces are the main cause of earthquake, volcanicity and tsunamis. The theory of plate tectonics suggests that the earth consists of eight major plates, in addition to a number of minor plates making up the earth crust and is called the lithosphere (1). The lithosphere consists of two layers: the continental crust and below it the oceanic crust, which is composed of denser rock. The oceanic crust is exposed under the oceans. The lithosphere plates are floating on the asthenosphere which is formed of molten rocks. These plates are constantly moving around the planet in the range of centimeters per year. The Red Sea is believed to be opening in the range of two centimeters per year, a range more or less equivalent to the growth of a toe nail.
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| When the floating plates spread apart, it is called a spreading center or divergence. When they are forced toether, it is called a zone of convergence. One of the plates usually moves or subduct under the other in a zone of convergence. Subduction may take place when an oceanic plate collides with a continental plate (2) or when two continental plates collide (3). As the plate moves down into the asthenosphere it begins to melt. The place where they meet has a crack or a trench. Some of the deepest parts of the oceans are these trenches. |
| Plate tectonics explains why earthquakes and volcanic activity tend to occur around the crustal plate boundaries. These are the areas of greatest geological activity, where convergence and subduction or the movement of continental plates against one another can result in violent events.
Earthquakes
Stress buildup in some areas in the earth crust especially at the crustal plate boundaries may fail as a fault and is felt as earthquake. There are two types of earthquakes, either deep or shallow:
a) Deep-focus Earthquakes - In general, the term "deep-focus earthquakes" is applied to earthquakes deeper than 70 km. It usually occurs when the stress fails in the subduction zone while two plates converge.
b) Shallow Earthquakes - Shallow earthquakes occur at a depth between 0 and 70 km from the surface and usually along an existing fault when the buildup of stress fails. Good examples of the shallow earthquakes are those occurring along the San Andrea’s fault in California (4). Shallow earthquake can also be associated with active volcanoes. Shallow earthquakes sometimes are created by man when a dam is built over an active fault or the fault gets activated by the neutral stresses created by the water in the reservoir behind a dam.
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Volcanicity
Volcanoes are formed in volcanic active areas when the lava works its way from the mantle to the ground surface through faults or deep fissures or cracks. There are four types of volcanoes.
a) Cinder Cones
Cinder cones are simple volcanoes usually created by one eruption where lave is blown in the air, solidify, break into pieces and fall to form steep sided cone with a bowl shaped crater (5).
b) Composite volcanoes
They are steep sided volcanoes built of alternating layers of viscous lava flows, volcanic ash and cinders (5). They are also known as strato-volcanoes.
c) Shield Volcanoes
They are wide in nature with long gentle slopes and with a large crater on the top (5). They are covered by a large number of basaltic lave flows and no pyroclastics.
d) Fissure or Lava Volcanoes
When lava is viscous and flows through a fissure, it forms a pile of lava covering the original fissure (5). The fissure volcanoes have no crater and do not have the shape of a volcano (6).
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