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Building Earth’s surface

Building Earth’s surface . Chapter 19. Interpreting Earth’s surface. Principle of uniformity “The present is the key to the past.” Rocks are changed today by the same processes that changed them in the past Replaced catastrophic models of previous thinkers

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Building Earth’s surface

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  1. Building Earth’s surface Chapter 19

  2. Interpreting Earth’s surface Principle of uniformity “The present is the key to the past.” Rocks are changed today by the same processes that changed them in the past Replaced catastrophic models of previous thinkers Catastrophic events contribute nonetheless Volcanoes, earthquakes, meteorite impacts, …

  3. Diastrophism The process of deformation that changes the Earth’s surface Produces structures such as plateaus, mountains and folds in the crust Related to volcanism (the movement of magma) and earthquakes Basic working theory is plate tectonics

  4. Stress and strain Stress Force tending to compress, pull apart or deform a rock Three stress forces Compressive stress Plates moving together Tensional stress Plates moving apart Shear stress Plates sliding past each other Strain Adjustment to stress Three strain types Elastic strain Returns to original shape Plastic strain Molded or bent Do not return to original shape Fracture strain Rock cracks or breaks

  5. Folding Sedimentary rocks Originate from flat sediment deposits Layers usually horizontal Folds Bends in layered bedrock Result of stress produced plastic strain Widespread horizontal stress can produce domes and basins Anticline: arch-shaped structure Syncline: trough-shaped

  6. Faulting Fault Produced by relative movement on opposite sides of a crack Footwall: mass of rock below the fault Hanging wall: mass of rock above the fault Fault plane: surface between the footwall and hanging wall

  7. Classes of faults Normal fault Hanging wall has moved down relative to the footwall Related features Graben Block surrounded by normal faults drops down Horst Block surrounded by normal faults is uplifted

  8. Other faults Reverse fault Hanging wall moved upward relative to footwall Result of horizontal compressive stress Thrust fault Reverse fault with a low-angle fault plane Faults provide information on the stresses producing the formation

  9. Earthquakes Quaking, shaking, vibrating or upheaval of the ground Result from sudden release of energy from stress on rocks Vibrations are seismic waves Most occur along fault planes when one side is displaced with respect to the other

  10. Causes of earthquakes Elastic rebound theory Two plates press tightly together Friction restricts motion Stress builds until friction or rock rupture strength is overcome Stressed rock snaps suddenly into new position

  11. Locating and measuring earthquakes Focus Actual origin of seismic waves Epicenter Location on Earth’s surface directly above the focus Seismograph Instrument used to detect and measure earthquakes Detects three kinds of waves P-wave (longitudinal) S-wave (transverse) Surface wave (up and down)

  12. Seismic data P-waves travel faster than S-waves Difference in arrival times correlates to distance from earthquake Triangulation used to pinpoint epicenter and focus

  13. Classification of earthquakes Based upon depth of focus Shallow-focus earthquakes Down to 70 km deep 85% of all earthquakes (surface rocks more brittle; more plate friction near surface) Intermediate-focus earthquakes 70 to 300 km deep Upper part of the mantle Deep-focus earthquakes 350 to 700 km deep Lower part of upper mantle About 3% of all earthquakes

  14. Measuring earthquake strength Effects: structural damage to buildings, fires, landslides, displacement of land surfaces, tsunami (tidal wave) Mercalli scale Relative intensity I(not felt) to XII(total destruction with visible ground waves) Richter scale Based on swings in seismograph recordings Logarithmic scale 3(not felt); 9(largest measured so far)

  15. Origin of mountains Mountains Elevated parts of Earth’s crust rising abruptly above the surrounding surface Created by folding and faulting of crust Three basic origins Folding Faulting Volcanic activity

  16. Folded and faulted mountains Domed mountains Begin as a broad arching fold Overlying sedimentary rocks weather away, leaving more resistant granite peaks Fault block mountains Rise sharply along steeply inclined fault planes Weathering erodes sharp edges

  17. Volcanic mountains Volcano A hill or mountain formed by the extrusions of lava or rock fragments from magma below Structure: vent, crater, lava flow

  18. Types of volcanoes Shield volcano Constructed of solidified lava flows Broad, gently sloping cones Cinder cone volcano Constructed of rock fragments (cinders) Steeper and smaller than shield volcanoes Composite volcano Alternating layers of cinders, ash and lava flows with volcanic mud

  19. Other features Most magma remains underground Cools and solidifies to form intrusive rocks Batholiths Large amount of crystallized magma Stock: small protrusion from a batholiths Batholiths intrusions can cause hogbacks Related processes: dikes, sills, laccoliths,…

  20. Overall picture Mountain ranges are composites of many different processes, each uniquely structured Folding Faulting Volcanic activity Especially apparent along converging plate boundaries

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