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Chapter 11notes
 

Chapter 11notes

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    Chapter 11notes Chapter 11notes Presentation Transcript

    • Chapter 11 Mountain Building
    • Rock Deformation
      • It is theorized that all continents were once mountainous masses and grow by adding mountains to their edges.
      • If this is so, then how do mountains grow in the middle of continents?
    • Factors Affecting Deformation
      • Every rock has a point at which it will bend and/or break.
      • Deformation is the term that refers to all changes in the original shape and/or size of a rock body.
      • Most deformation occurs at plate margins.
      • Stress is the force per unit area acting on a solid. Under great stress, rocks tend to deform usually by:
      • Folding
      • Faulting
      • Flowing
      • Fracturing
      • The change in shape or volume of a body of rock as a result of stress is called strain .
      • Rocks can be bent into folds if stress is applied gradually and not going beyond the breaking point.
      • We already know that rocks under stress have elastic properties and will go back to their original shape and size when the force is removed. (elastic rebound)
      • Once the elastic limit or strength of a rock is surpassed, it either flows or fractures .
      • The factors that influence the strength of a rock and how it will deform include:
      • Temperature
      • Confining pressure
      • Rock type
      • Time
    • Temperature and Pressure
      • Rocks deform permanently two ways:
      • Brittle deformation
      • Ductile deformation
      • Rocks near the surface, where temperatures and confining pressures are low, usually behave like brittle solids and fracture once their strength is exceeded.
      • This type of deformation is called brittle failure or brittle deformation .
      • Examples: china, bones, pencils, glass
      • At depth, where temperatures and confining pressures are high, rocks show ductile behavior.
      • Ductile deformation is a type of solid-state flow that produces a change is size and shape of an object without fracturing the object.
      • Examples: modeling clay, bee’s wax, caramel candy, and most metals
    • Question ????????????
      • Placing a penny on a railroad track and having a train run over it, causing the penny to flatten out is an example of?
      • ductile deformation
    • Rock Type
      • The mineral composition and texture of a rock also affect how it will deform.
      • Granite and basalt contain minerals with strong internal molecular bonds will fracture .
      • Sedimentary rocks that are weakly cemented or metamorphic rocks that are foliated are more likely to deform by ductile flow .
      • Rocks that exhibit ductile flow may include:
      • Soft
      • Halite (rock salt)
      • Gypsum
      • Shale
      • Intermediate strength
      • Limestone
      • Schist
      • Marble
    • Time
      • Natural stress applied over a long period of time will fold rocks.
      • Forces that are unable to deform rock when first applied may cause rock to flow if the force is maintained over a long period of time .
    • Types of Stresses
      • Three types of stresses rocks undergo include:
      • Tensional forces – rocks being pulled in opposite directions
      • Compressional force – rocks are squeezed and shortened
      • Shear stress – when a body of rock is distorted
    • Stresses
    • Folds
      • During mountain building, flat lying rocks (both sedimentary and igneous) are bent in wave-like ripples called folds .
      • There are three types of folds:
      • Anticline – an upward fold (arch)
      • Syncline – a downward fold (trough)
      • Monocline – closely associated with faults, monoclines are large step–like folds in otherwise horizontal layers of sedimentary strata.
      • Prominent features of the Colorado Plateau which coves Colorado, New Mexico, Utah, and Arizona.
    • Anticline and Syncline Process
    •  
    • Circular Anticline
    • Syncline
    •  
    •  
      • Joints – fractures in rock strata where no movement has taken place
      • So faults are……………
    •  
    • Three types of faults
      • Normal – caused by tensional stress
      • Reverse (or thrust) – caused by compressional stress
      • Strike-slip – commonly caused by shear stress – San Andreas Fault
    • Normal
    • Reverse or Thrust
    • Strike-Slip
    • What type of fault?
      • Strike-slip
    • What type of fault?
      • Normal
    • What type of fault?
      • Reverse or Thrust
    •  
    •  
    • Thrust fault
    • Types of Mountains
      • The collective processes that produce a mountain belt are called orogenesis .
      • The dominant processes that have formed them classify mountains.
    • Folded Mountains
      • Mountains formed primarily by folding are called folded mountains .
      • Thrust faults are also important in the formation of folded mountains.
      • These mountains are called fold and thrust belts .
      • Examples: Appalachian Mountains, the northern Rocky Mountains, and the Alps of Europe
    •  
    • Fault-Block Mountains
      • Large-scale normal faults are associated with structures called fault-block mountains.
      • These mountains form when large blocks of crust are uplifted and tilted along normal faults . 
      • Examples: Teton Range of Wyoming and the Sierra Nevada Range of California.
    •  
      • Normal faulting occurs when tensional stresses cause the crust to be stretched or extended.
      • As the crust is stretched, a block called a graben , which is bounded by normal faults, drops down.
      • Graben is German for ditch or trench.
      • Grabens produce an elongated valley bordered by relatively uplifted structures called horsts .
      • Examples: Basin and Range Province of Nevada, Utah, and California.
    •  
    •  
    • Rift or Grabben
      • Graben or Rift
      • example is: Death Valley 250 ft. below sea level
      Dead Sea is 1200 ft below sea level
    • Domes and Basins
      • These mountains are produced by broad upwarping in the basement rock deforming the overlying sedimentary strata.
      • A dome is when upwarping produces a circular or elongated structure or dome .
      • How are these domes uncovered?
      • Erosion strips away the highest portion of the sedimentary beds exposing older igneous and metamorphic rocks in the center.
      • The oldest rocks form the core of these mountains. 
      • Example: Black Hills
      • Downwarped structures having a circular shape are called basins .
      • These structures have gently sloping bed similar to saucers.
      • Basins are thought to be formed by the accumulation of sediment, whose weight caused the crust to subside .
      • Example: Basins of Michigan & Illinois
      • In the case of a basin the oldest rocks are found in the center with the youngest rocks on the flanks.
    •  
    • Mountain Formation
      • Just how old are mountains?
      • Appalachians are 100s of million of years old
      • Himalayas are about 45 million years old… Just a youngster!
    • Mountain Building at Convergent Boundaries
      • Most mountain building occurs at convergent plate boundaries.
      • Colliding plates provide the compressional forces that fold, fault, and metamorphose the thick layers of sediment deposited on the edges of landmasses.
    • Ocean-Ocean Convergence
      • Converging oceanic plates result in subduction that creates magma leading to the growth of a volcanic island arc once it grows above sea level.
      • Examples: Aleutian Islands in Alaska, Japan,
      • Ocean-ocean convergence mainly produces volcanic mountains.
    •  
    •  
    •  
    • Ocean-Continental Convergence
      • Example: West coast of South America – the Andes Mountains
      • This produces a continental volcanic arc .
      • During subduction, sediment scraped from the subducting plate is stuck against the landward side of the trench.
      • This accumulation of sediment and metamorphic rock is called an accretionary wedge .
      • Ocean-continent convergence produces mountains in two roughly parallel belts.
      • The continental volcanic arc develops on the continental block and the accretion wedge is on the seaward belt.
      • The types of mountains formed by ocean-continent convergence are volcanic mountains and folded mountains .
    •  
    • Continent-Continent Convergence
      • A convergent boundary between two plates carrying continental crust, a collision between the continental fragments will result and form folded mountains .  
      • Example: Himalayan Mountains and the Tibetan Plateau, European continent collided with Asia producing the Ural Mountains in Russia.
    •  
    • Mountain Building at Divergent Boundaries
      • These mountains are usually formed on the ocean floor
      • Example: Mid-ocean ridges that extend 65,000 kilometers.
      • The mountains that form along ocean ridges at divergent plate boundaries are fault-block type mountains.
    •  
    • Non-Boundary Mountains
      • Example: Hawaiian Islands
      • Continental Accretion
      • Accretion is when small crustal fragments collide and merge with continental margins.
    •  
      • Terrane – is any coastal fragment that has a geological history distinct from that of the adjoining terranes.
      • Some may be no larger than volcanic islands, while other are immense, such as the entire Indian subcontinent.
      • Some may have been microcontinents similar to present day Madagascar.
      • Many others were island arcs like Japan and the Philippines.
      • All these materials add to the continent width and thickness displacing other fragments further inland.
    •  
    • Mountains of Accretion
      • Example: Mountains in western North America and western Canada contain rock, fossils, and structures different from the surrounding area.
      • These materials have been accreted (added to) the western margin of North America.
    • Principle of Isostasy
      • This is a gradual up and down motion of materials that make up the interior of continents away from continental margins.
      • Earth’s crust floats on top of denser more flexible rocks in the mantle.
      • The concept of a floating crust in gravitational balance is isostasy .
      • Mountain belts float on top of more dense crustal roots that extend into the mantle.
      • The denser mantle supports the mountains from below.
      • If more material were added to the top of the mountains the combined block would sink until a new isostatic balance was reached.
      • However, the top of the combined block would be higher than before and the bottom would be lower.
      • This process of establishing a new level of gravitational equilibrium is called isostatic adjustment .
    •  
    •  
    • Isostatic Adjustment of Mountains
      • Continental ice sheets provided evidence of crustal subsidence followed by rebound.
      • The weight of 3 kilometers of ice depressed Earth’s crust by hundreds of meters.
      • In 8000 years since the last ice sheets melted, uplift of as much as 330 meters has occurred in Canada’s Hudson Bay region.
      • Most mountain building causes the crust to shorten and thicken.
      • Because of isostasy, deformed and thickened crust will undergo regional uplift both during mountain building and for a long period afterward.
      • As the crust rises (rebounds) the processes of erosion increases and the deformed rock layers are carved into mountains.
      • Erosion removes material from the summit reducing the load causing the crust to rise. This will continue until the mountain block reaches its “normal” thickness.