2.4.1 Plate Tectonics At A Glance
The outermost solid layer of the earth is the 50- to 100-kilometer-thick lithosphere, which is broken up into a series of rigid plates. The lithosphere is
underlain by a plastic, partly molten layer of the mantle, asthenosphere, over which the plates can move.
Figure 2.6. Lithosphere plates movements Source: Asikin, 2003
This plate motion give rise to earthquakes and volcanic activity at the plate boundaries. At seafloor spreading ridge, which are divergent boundaries, new sea
floor is created from magma rising from asthenosphere. The sea floor moves in conveyor-belt fashion, ultimately to be destroyed in subduction zones, a type of
convergent plate boundary, where it is carried down into the asthenosphere and eventually remelted. Convergence of continents from high mountain ranges.
According to Montgomery 1991, the evidence for seafloor spreading includes the distribution of ages of seafloor rocks, and magnetic stripes on the
ocean floor. Continental drift can be demonstrated by such means as polar-wander curves and evidence of ancient climates as revealed in the rock record. Past
“supercontinents” can be reconstructed by fitting together modern continental
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margins and matching up similar geologic features and fossil deposits from continent to continent.
Present rates of plate movement average a few centimeters a year. A mechanism for moving the plates has not been proven definitively. The most
likely driving force is slow convection in the asthenosphere and perhaps in the deeper mantle. Although plate motions are less readily determined in ancient
rocks, plate- tectonic processes have probably been more or less active for much of the earth’s history. They play an integral part in the rock cycle as shown in
Figure 2.7.
Figure 2.7. The rock cycle, interpreted in plate-tectonic terms. Source: Montgomery, 1991, page: 140
2.4.2 Sensitive Area
The term of sensitive area in this research is areas which are geologically can generate hazard when on those respected areas used as settlement areas or
human other activities. Its includes areas which are dominant controlled by structure geology such mountain range, plateau and plain, arid lands particularly
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areas which are formed above clay and limestone, volcanic and geothermal area and of course an opened coastal areas surrounds by bay, which entirely, in
agreement with on going geomorphic processes which shape the Earth’s surface.
2.4.3 Geological Risk Map