Answer briefly BUT completely. GIve me 4-5 sentences on each. 1. How is the epic
ID: 118388 • Letter: A
Question
Answer briefly BUT completely. GIve me 4-5 sentences on each.
1. How is the epicenter of an earthquake determined? Discuss fully.
2. What is the low-velocity zone and why does it exist?
3. What are the four principal layers of the Earth? Describe the thickness and general chemical make-up (composition) of each layer.
4.What factors control whether a rock will be brittle or ductile? Explain.
5. What is the elastic rebound theory? Explain.
6. How is the Earth's magnetic field generated?
7. What is isostasy? What is isostatic adjustment? Explain.
8. Define strike and dip. How do we use these in the study of geologic structures?
Explanation / Answer
1) P & S waves each shake the ground in different ways as they travel through the Earth .There is a predictable distance between the arrival of a P wave and the slower S wave are determine using standard travel-time curves. Using the arrival times of the P and S waves from 3 different stations distances to epicentre can be determined. The intersection of the 3 cirles gives epicentre location.
2) The zone within the upper mantle beneath the oceans within which seismic P-waves are slowed and S-waves are slowed and partially absorbed. The top of the zone is some 40–60 km deep near the oceanic spreading ridges, and this depth increases to 120–160 km beneath the older oceanic crust. The bottom of the zone is poorly defined in the region of 250–300 km in depth. Beneath the continents, a restricted low-velocity zone occurs beneath crust areas subjected to orogenesis during the last 600 million years or so, but is not found beneath cratonic areas.
It is attributed to the presence of a 0.1% fluid phase and commonly ascribed to the partial melting of mantle rocks at these depths. It is often considered coincident with the asthenosphere, but probably
3) Crust : Crust is the outer thin layer with a total thickness normally between 30-50 km. The thickness of the crust varies under the oceanic and continental areas. Oceanic crust is thinner (5-30 km thick) as compared to the continental crust (50-70 km thick).The continental crust is thicker in the areas of major mountain systems. It is as much as 70 -100 km thick in the Himalayan region. It forms 5-1.0 per cent of the earth’s volume. The continents are composed of lighter silicates—silica + aluminium (also called ‘sial’) while the oceans have the heavier silicates—silica + magnesium (also called ‘sima’). The outer covering of the crust is of sedimentary material (granitic rocks) and below that lie crystalline, igneous and metamorphic rocks which are acidic in nature.
Mantle : The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km.The lower mantle extends beyond the asthenosphere. It is in solid state.The density of mantle varies between 2.9 and 3.3.It forms 83 per cent of the earth’s volume.The outer layer of the mantle is partly simatic while the inner layer is composed of wholly simatic ultra-basic rock. It is composed of solid rock and magma
Core: Lies between 2900 km and 6400 km below the earth’s surface.Accounts for 16 per cent of the earth’s volume.Core has the heaviest mineral materials of highest density.It is composed of nickel and iron [nife].The outer core is liquid while the inner core is solid.A zone of mixed heavy metals + silicates separates the core from outer layers
4) Rocks are defined as brittle or ductile on the basis of the way they are deformed by forces . In brittle deformation, a continuous, force is applied to a rock. As the force is gradually increased, little change occurs in the rock until suddenly it fractures. In ductile deformation, a gradually increasing force will cause the rock to undergo smooth and continuous plastic deformation. The rock will contort and change shape without fracturing. The type of rock also determines the type of deformation. Under similar confining pressures, halite (rock salt) is more susceptible to ductile deformation than is granite, which will more likely fracture.
5) Over time stresses in the Earth build up (often caused by the slow movements of tectonic plates). At some point the stresses become so great that the Earth breaks... an earthquake rupture occurs and relieves some of the stresses (but generally not all).These stresses release in form of energy during earthquake.The stored energy(stresses) is released partly as heat, partly in alteration of the rock, and partly as a seismic wave.
In earthquakes these ruptures generally happen along fault planes, or lines of weakness in the Earth's crust
6)
planet’s magnetic field is believed to be generated deep down in the Earth’s core.
Differences in temperature, pressure and composition within the outer core cause convection currents in the molten metal as cool, dense matter sinks whilst warm, less dense matter rises. The Coriolis force, resulting from the Earth’s spin, also causes swirling whirlpools.
This flow of liquid iron generates electric currents, which in turn produce magnetic fields. Charged metals passing through these fields go on to create electric currents of their own, and so the cycle continues. This self-sustaining loop is known as the geodynamo.
7)
Isostasy is the state of gravitational equilibrium between Earth's crust and mantle such that the crust "floats" at an elevation that depends on its thickness and density. Isostasy is essentially the Archimedian principle of hydrostatic balance between floating bodies
Isostatic adjustment refers to the transient (102104 years) or long term (> 105 years) nonelastic response of the earth's lithosphere to loading and unloading due to erosion, deposition, water loading, desiccation, ice accumulation, and deglaciation
8) The orientations of rock layers, folds, fractures and faults can all be measured in three dimensional space using strike and dip. The strike of a surface is the direction of a line formed by the intersection of a rock layer with a horizonal surface. The strike is described in terms of direction such as N 10o W. The dip is measured at right angles to the strike and is a measure of the angle at which the surface tilts relative to a horizontal surface. The dip is indicated in terms of angle and direction (e.g. 35o E).