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Discontinuity of Rock structure

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  2. 2. FAULTS In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement along the fractures as a result of earth movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults.
  3. 3. Mechanisms of faulting Because of friction and the rigidity of the rock, the rocks cannot glide or flow past each other.  Stress builds up in rocks and when it reaches a level that exceeds the strain threshold, the accumulated potential energy is dissipated by the release of strain, which is focused into a plane along which relative motion is accommodated—the fault.
  4. 4. Deformation  The process by which the shape of a rock changes because of stress .  There are two types of stress that occur in rocks:  Compression  Tension
  5. 5. Compression  Compression occurs when rock is squeezed.  Compression happens when two plates collide at a convergent boundary.  Compression occurs and forms large mountain ranges.
  6. 6. Compression/ Mountain Building The Himalayas, for example, were raised by the compression that accompanied collision of the Indian plate with the Eurasian plate. Another example is Europe's Alps and Jura mountains which were also formed by horizontal compression, generated in their case by collision with the African plate and the Eurasian plate.
  7. 7. Tension  Tension occurs when an object is stretched.  Tension occurs when plates move away from each other at plate boundaries.  At the Mid-Atlantic ridge the seafloor is spreading at a rate of about 3cm per year. The frequency of earthquakes at a mid-ocean ridge will depend on how much tension is happening at that point. The more tension means the more seafloor spreading, resulting in a higher frequency of earthquakes at a particular midocean ridge.
  8. 8. Faults  Some rock layers break when stress is applied to them.  The surface along which rocks break is called a fault.  The blocks of crust on each side of the fault are called fault blocks.  A fault has a foot wall and a hanging wall.
  9. 9. Footwall Footwall You could walk up this face of the fault, on foot, hence the name footwall.
  10. 10. Hanging Wall  You could hang from this wall. Hanging Wall This is a normal fault. Notice how the rock layers are the same at the red lines.
  11. 11. Hanging wall and Footwall
  12. 12. 3 Types of Faults  Normal Fault  Reverse Fault  Slip-Strike Fault
  13. 13. Normal Fault  Rocks are pulled apart because of tension. Footwall In a normal Fault, the hanging wall slides down the footwall. Hanging wall
  14. 14. Reverse Fault In a reverse fault the hanging wall is pushed upward due to compression.
  15. 15. Strike-Slip Fault Opposing forces cause rocks to move horizontally. Earthquakes occur along these faults.
  16. 16. San Andreas Fault, California Strike-Slip Fault
  17. 17. 3 Most Common Types of Mountains  Mountains exist because tectonic plates are continually moving around and colliding with each other.  There are 3 types of mountains, named for how they form:  Folded Mountains  Fault-Block Mountain  Volcanic Mountain
  18. 18. Folded Mountains  Folding is a process in which the Earth's plates are pushed together in a roller coaster like series of high points and low points. Folding bends many layers of rocks without breaking them. The Appalachian Mountains and Rocky Mountains of the United States, and the Alps of Europe are examples of mountain ranges that were formed by folding. Folded Strata (Layers)
  19. 19. Fault- Block Mountains  Mountains sometimes form when many layers of the Earth's crust are moved vertically upward at fault lines by pressures caused by plates colliding. Fault lines are great cracks in the crust. The mountains that are formed in this way are called fault-block mountains. The Sierra Nevada mountains in California and Nevada, and the Grand Teton range of Wyoming are examples of fault-block mountains. Fault block mountains form when tension causes large blocks of crust to drop down relative to other blocks.
  20. 20. Volcanic Mountains  Volcanic mountains are forms when magma erupts from a divergent boundary and hardens.  Many volcanic mountains exist under the sea.  Some volcanic mountains rise above the surface of the ocean to from islands, Hawaii for example.
  21. 21. Hawaiian Islands
  22. 22. Summary  Faulting has a major influence on the way the earth looks. Mountains form and disappear over time, as well as large rift valleys and other features. This has an impact on where and how we live.