LIST OF APPENDICES
II. LITERATURE REVIEW
2.4. Geological Hazards
The majority of geological hazard that happening in Indonesia, especially take place along volcanic belt mostly in Indonesian islands. This indicates that Indonesian Islands located and controlled by a set of major tectonic activities.
Most of Indonesia's volcanoes are part of the Sunda arc, a 3,000-km-long line of volcanoes extending from northern Sumatra to the Banda Sea. Most of these volcanoes are the result of subduction of the Australian Plate beneath the Eurasian Plate. Volcanoes in the Banda Sea are the result from subduction of the Pacific Plate under the Eurasia Plate. On the Figure 2.3 shows the black "teeth" are on the overriding plate and the arrows showing the direction of movement along major transform faults.
Figure 2.3. General Tectonic Pattern of Indonesia (Source: USGS)
In congeniality of geomorphic processes, the landscape changes is a response to geologic and climatic stimuli. Its appearance at any one time
represents a fleeting stage in a continuing conflict between internal processes which tend to elevate the lands and external processes which tend to wear them down. Although the results of such changes is generally imperceptible and becomes visible in the landscape only after centuries or millennia, however, individual local events such earthquakes, tsunamis, volcanoes, avalanches, landslides and floods may take place very rapidly and constitute serious environmental hazards.
Those geological hazards, are mostly, unpredictable. On the other hand, human often induce change or accelerate the process of changes with their needs for existence. Their problems is not to bring environmental change to a halt, a generally impossible task, but to adapt to the environmental and to occupy it with the least physical and aesthetic damage. Thereby, as a consequence, one of them, as does a victim of geologic disaster that occurred in our country is primarily caused by poorly planned placement of settlement locations. To do so, people must be familiar with earth processes so that they may avoid or minimize damage to the terrain as well as to life and property, equally, to reduce its detrimental effects, however, we should understand the condition of our environment geologically, mainly the major of geological aspect that operates in selected areas.
Refering to the scope of the research on the previous chapter, geological hazards is disaster generated by effect of direct or indirect corresponding natural phenomenon with geologic processes including man. There are two types of geological hazards generated by the direct effect, first earthquake and the second is vulcanism.
Figure 2.4. Location of modern volcanoes and earthquakes around the world (Source: After Montgomery, 1991, page: 126)
Earthquakes result from sudden slippage or failure of rocks along fault zones in response to stress. Most earthquakes occur at plate boundaries and are related to plate-tectonic processes. The pent-up energy is released through seismic waves, which include both compressional and shear body waves, plus surface waves, which cause the most structural damage. Earthquake hazards include damage from ground rupture and shaking, fire, liquefaction, landslide, and tsunamis (Montgomery, 1991).
We cannot hope to stop earthquakes, but we can try to limit their destructive effects. Physical damage could be limited by the following: seeking ways to cause locked faults to slip gradually and harmlessly, perhaps by using fluid injection to reduce frictional resistance to shear; designing structures in active fault zones to be more resistant to earthquake damage; identifying and, wherever possible, avoiding developments in areas at particular risk from earthquake-related hazards.
Casualties could be reduced by increasing public awareness of and by improving our understanding of earthquake precursor phenomena so that accurate and timely predictions of earthquake occurrence can be made.
Furthermore, most volcanic activity is concentrated near plate boundaries.
Volcanoes differ widely in eruptive style and thus in the kinds of dangers they represent. Spreading ridges and hot spots are characterized by the more fluid, basaltic lava's. Subduction-zone volcanoes typically produce much more viscous, silica-rich, gas-charged andesitic magma, so in addition to lava they may emit large quantities of pyroclastics and other deadly products like nuées ardentes.
Lava is perhaps the least serious hazard associated with volcanoes: it moves slowly, it can sometimes be diverted, and its path can be predicted.
Figure 2.5. Volcanism and Plate Tectonic (Source: Montgomery, 1991, page: 180)
The result of explosive eruption are less predictable, and the eruptions themselves more sudden. According to Montgomery (1991) an early sign of potential volcanic activity includes bulging and warming of the ground surface and increased seismic activity. Volcanologists cannot yet predict precisely the definite time or type of eruption, except insofar as they can anticipate eruptive style on the basis of historic records, the nature of the products of previous eruptions, and tectonic settings.
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
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
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
The first step in the study of collective geological hazards is the plotting of specific information on maps at the same scale. A geological map, for example, present the areal distribution of rock structure and type. The scale chosen and the emphasis on particularr features may be selected to optimize the use of information for a particular need.
In California, a new 1:750,000 scale geological map was produced in 1972 to give an over-view of the geological properties of the State with sufficient detail to be useful for preliminary land-use planning. Published in color, it emphasizes recent volcanic rocks and volcanoes, earthquake fault and the major folds in the layered rocks. Maps with much more detail than feasible on the usual 1:250,000 to 1:1,000,000 scale maps are needed for specific hazard evaluations. For urban areas, specializied mapping for land-use planning and engineering design must show considerable detail and even include geophysical and boreholes studies of local subsurface structure. The required scale may be of the order of 1:20,000.
Recent examples are slope maps produced by the U.S. Geological Survey with scale of 1:24,000. These maps indicaete the per cent of slope of hills and mountains by means of color code so that assessment of hillside erosion and stability conditions can be made. Likewise, U.S.G.S and Corps of Engineers flood hazard maps at about this scale show the elevations attained by major historical floods and floods of a specific frequency of occurences (Bolt et al 1975: 288).
There are several unsatisfactory features of the usual geological map published in most countries. First, these maps often emphasize the formations (igneous, basin deposits, etc.) rather than the rock types involved. Alluvium consists of fine- and coarse-grained material may have depth and horizontal facies changes that lead to major seismic response consequences. Again, it is not sufficient to say that a given formation consists largely of sandstone and shale without mapping bed boundaries. The Geological Survey of New South Wales in Australia has tried to solve the problem by indicating overburden and underlying rock units by appropriate symbols. In this way, the map color defines the underlying rock, while the map symbol tags the type of overburden. In New Zealand, the Soil Bureau of the Department of Scientific and Industrial Researche produces maps of soil type that may be read in conjunction with standard geological maps. In the New England States, USA, one series of maps delineates bedrock and another the superficial glacial deposits (Bolt et al 1975: 288).
Another weakness is lack of detail when mapping the weathered conditions of the rock types. The depth of weathering may be of considerable importance in estimating the response of the ground to strong earthquake motion. In the same way, locations of unobscured bedrock exposures deserve plotting on the basic geological maps so that when detailed investigations are needed these outcrops can be revisited quickly Alluvial deposits often require sub-division, appropriate to the scale used (e.g. 1:250,000) showing flood plains, lake deposits, colluvial, residual soils, and so on. In this way, parts of a particular surficial deposit, consisting of fine-grained material with braided stream channels of coarser material, could be identified from the map.
In many country and also in Indonesia, a recent imaginative development is the use of computers to calculate and draw predictive hazard maps. Once the controling parameters of the hazard are known these can be combined into mathematical form and programmed once and for all.
The differences between this research compared with another that mentioned above, principally in geological and geomorphological interpretation point of view. This research thoroughly used GIS and Remote Sensing Technology for determining geological hazard sensitive area through integrating remote sensing capability especially principal component analysis (PCA) procedures to obtain common picture of present rocks and minerals distribution which indicating past as well as endogenetic and exogenetic processes.