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Isostasy and continental_drift

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geology of earth

Publié dans : Ingénierie
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Isostasy and continental_drift

  1. 1. Isostasy and Continental Drift
  2. 2. Isostasy • Isostasy (Greek ísos "equal", stasis "standstill") • Term used in geology to refer to the state of gravitational equilibrium between the earth’s lithosphere and asthenosphere. Isostasy: a state of gravitational equilibrium in which an area of crust “floats” in a balanced way on the denser rock of the mantle below. The elevation of any part of the Earth’s crust is a function of the THICKNESS and DENSITY of the crust.
  3. 3. Isostasy • Isostasy is the vertical movement of the crust to attain “buoyancy” in the mantle. • The height a block of wood floats in water depends on it’s density and thickness. • The “height” of the earth’s crust also depends on it’s density and thickness. 3
  4. 4. Isostasy The deflection of plumb bob near mountain chains is less than expected. Calculations show that the actual deflection may be explained if the excess mass is canceled by an equal mass deficiency at greater depth. 4 A plumb-bib
  5. 5. The Concept of Isostasy These figures show how either thickness differences or density differences determine how high the wood blocks will float.
  6. 6. • Isostasy used to denote the ideal state of balance b/w different parts of the earth crust due to difference of densities of two crustal material lithosphere and asthenosphere. • This state of balance tends to maintain the certain level known as level of compensation.
  7. 7. Hypothesis of Isostasy PRATT ISOSASTY Mountain range supported by density variations within the crust. AIRY ISOSTASY Mountain range supported by changes in crustal thickness; crustal density relatively constant. 7 Either way, the plate is relatively light and 'floats' on a fluid layer beneath.
  8. 8. APPLICATIONS The principle of isostasy suggests that the earth's crust should adjust to any changes in mass that occur at the earth's surface (we call these "isostatic adjustments"). There are basically two types of responses: 1. SUBSIDENCE a. Definition: the slow, sinking of the earth's crust b. Cause: the addition of mass to the crust c. Example: the advance of glacial ice sheets 2. REBOUND a. Definition: the slow, vertical rise in earth's crust b. Cause: the removal of mass from the earth's crust c. Example: post-glacial rebound (Scandinavia) (U.S.) It should also be noted that isostasy explains why continental plates cannot be subducted: the forces that drive subduction cannot overcome the "buoyancy" of these low density plates. Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced or sinks due to gravity into the mantle. Regions where this process occurs are known as subduction zones.
  9. 9. “see” isostatic adjustment today from load of glaciers on crust during last glaciation and unloading from melting (possible because response of asthenosphere is slow) process is called post- glacial rebound
  10. 10. Continental Drift • In 1915, the German geologist Alfred Wegener first proposed the theory of continental drift, which states that parts of the Earth's crust slowly drift atop a liquid core. • Wegner revived the early idea of continental drift, contending that all of the present-day continents were connected side by side as long as far (~300Mya). • He called the Super Continent mass “Pangaea”, Greek for all Lands. • The gradual movement of the continents across the earth's surface through geological time. • the lateral movement of continents resulting from the motion of crustal plates.
  11. 11. • Earth's continents have been joined together and have moved away from each other at different times in the Earth's history. A term, no longer used by geologists, that refers to the fact that continents are not stationary, but move across the Earth 's surface. Continental drift is one feature of the modern theory of plate tectonics. • the hypothetical landmass that existed when all continents were joined, from about 300 to 200 million years ago. (Pangea • a hypothetical protocontinent of the remote geologic past that rifted apart to form the continents of today.
  12. 12. • Wegner summary was based on the number of careful observation: – Matching Rock , Fossils , Glacier , and structural relations among different parts of different continents. • Pangaea started to break up into two smaller supercontinents, called Laurasia and Gondwanaland, during the Jurassic period. • By the end of the Cretaceous period, the continents were separating into land masses that look like our modern-day continents.
  13. 13. Wegener’s Evidence: The presence of fossils only over small areas of now separate continents (how did they get from continent to continent?).
  14. 14. Cause of Continental drift • Continental drift occurs under the earth crust by convection current in the mantle which drive the plates, happening since development of earth. • Deep material, hotter than their surroundings would tend to flow upward. In approaching the cool surface of earth, the mantle would loose its thermal energy, cool and sink. • Motion of mantle put in to action by convection thus becomes mechanism for moving rigid pieces of crust over some more actively flowing mantle material.