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Chapter 2 Lecture McKnight's Physical Geography Lectures Chapter 14 Earth Internal Processes Andrew Mercer Mississippi State University © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Learning Goals of Chapter 14 • Explain Wegener's continental drift hypothesis. • Explain the theory of plate tectonics. • Identify and describe the three kinds of plate boundaries. • Explain hot spots, accreted terranes, four types of volcanoes and types of volcanic eruptions and their materials. • Explain measures of earthquake energy and effects. © 2014 Pearson Education, Inc. From Rigid Earth to Plate Tectonics • Until mid-twentieth century, scientists believed Earth’s continents rigid • Continental drift – pangaea • Evidence includes similar geologic features on coasts of different continents • Continents actually once fit together © 2014 Pearson Education, Inc. From Rigid Earth to Plate Tectonics • Paleontology records support continental drift concept. • Glaciated continents reconstructed made sense. • Rejected by most scientists until the last 60 years. © 2014 Pearson Education, Inc. The Theory of Plate Tectonics – Plate boundaries • Earthquakes occur along major fracture boundaries • Correspond with locations of trenches and ridges in the sea floor © 2014 Pearson Education, Inc. The Theory of Plate Tectonics Sea floor spreading • Mid-ocean ridges formed by magma rising up from the mantle • New basaltic ocean floor created; moves away from ridge • At trenches, older lithosphere descends into the asthenosphere where it is recycled – subduction © 2014 Pearson Education, Inc. The Theory of Plate Tectonics – Paleomagnetism • Iron in cooled magma orients itself with the magnetic poles of Earth • Provides a record of past magnetic fields • Magnetic field has changed orientation at least 170 times • Should be symmetry in magnetic orientation • Used to verify age of ocean floor rock and sea floor spreading © 2014 Pearson Education, Inc. The Theory of Plate Tectonics • Plate tectonic theory – – – – Theory behind motion of lithospheric plates Plates float on asthenosphere (thin molten ‘blood layer’) 7 major plates, 7 intermediate plates, 12 smaller plates Smaller plates are large plates that are being subducted • Plate tectonic activity – Slow convection in Earth’s mantle – Convection can push plates away from each other – Most motion results from plates pulled by subduction of dense oceanic lithosphere – Heavier subducting plates are like hot taffy sliding off the table. © 2014 Pearson Education, Inc. Major Plate Boundaries © 2014 Pearson Education, Inc. Divergent boundaries • Plates move away from each other • Asthenosphere wells up in the plate opening • Represented by a midoceanic ridge • Associated with shallow-focus earthquakes and volcanic activity • Constructive • Continental rift valley, proto-ocean © 2014 Pearson Education, Inc. Convergent boundaries • Collisions between plates • Destructive • Three primary collisions: – Oceanic–continental – oceanic plate sinks since more dense; subduction » Forms oceanic trench and continental mountains (e.g., Cascades, Andes) » Earthquakes occur along margin » Volcano formation along the plates – continental volcanic arc » Forms metamorphic rocks – blueschist (above) © 2014 Pearson Education, Inc. More about convergent boundaries • Three primary collisions (cont.): – Oceanic–oceanic » Subduction results in undersea trench formation » Deep and shallow earthquakes » Island volcanic arc – Continental–continental » No subduction since two plates are highly buoyant » Builds huge mountain ranges » Volcanoes are rare » Shallow earthquakes are relatively common © 2014 Pearson Education, Inc. More About Plate Boundaries Transform boundaries • Two boundaries slip past each other laterally • Transform faults • Neither creates nor destroys crust • Commonly produce shallow focus earthquakes • San Andreas fault © 2014 Pearson Education, Inc. The Theory of Plate Tectonics • The rearrangement – 450 million years ago, one supercontinent existed – Broke up 200 million years ago • Laurasia • Gondwanaland – Arrangement to the current continental configuration © 2014 Pearson Education, Inc. The Theory of Plate Tectonics • The Pacific Ring of Fire – Plate boundaries exist all around the Pacific Rim – Primarily subduction zones – 75 percent of all volcanoes lie in the Ring of Fire © 2014 Pearson Education, Inc. The Theory of Plate Tectonics • Additions to basic plate tectonic theory – Mantle plumes • • • • localized hot areas not associated with plate boundaries Move with the plate, so eventually become inactive Hot spot trail Hawaiian islands © 2014 Pearson Education, Inc. Volcanism • Magma chemistry and styles of eruption – Type of eruption is determined by: • Amount of gas involved – gas pressure • Stickiness of rocks – how much pressure the rock can hold back to produce a large eruption – high-silicate rocks resist more pressure than low-silicate rocks – High silica eruptions – pyroclastic – explosive and dangerous – Low silica eruptions – quiet, non-explosive © 2014 Pearson Education, Inc. Volcanism – Relatively temporary features on the landscape – Much of Earth’s water originated from water vapor from volcanic eruptions – Magma contains major elements required for plant growth – Provides soil fertility © 2014 Pearson Education, Inc. [Insert Fig. 14-28 p. 328] Lava flows – Lava generally flows horizontally, parallel to the surface along which it flows. – It eventually cools in horizontal orientation, strata. – Streams flowing through lava flows result in irregular or fragmented surface. – Uniform cooling results in hexagonal structure (see figure). © 2014 Pearson Education, Inc. [Insert Fig. 14-30 p. 329] Flood basalt – Most extensive lava flows come from hot spots. – Flood basalt is a vast accumulation of lava build up. – It is correlated with mass extinctions. Yellowstone Park contains a “hot spot” © 2014 Pearson Education, Inc. Volcanic peaks The Hawaiian Islands and Yellowstone Park are examples of ‘hot spots’. The Hawaiian version is a very large, but mild shield volcano – not explosive: less sticky silicate rock and less gas. Hawaiian Islands are actually the tallest mountains on earth, rising from the bottom of the ocean to 13,500 feet above sea level. © 2014 Pearson Education, Inc. Volcanism Composite Volcano – ‘strato’ (layered) • More silica lavas (andesite lava) • Form symmetric, steep-sided volcanoes • Pyroclastics from explosive lava • Pyroclastic flows produce steep slopes, lava holds it together © 2014 Pearson Education, Inc. More volcanic peaks Lava domes • Masses of very viscous lava that do not flow far • Lava bulges from the vent, dome grows by expansion from below and lava within • Some lava domes form inside of composite volcanoes © 2014 Pearson Education, Inc. Cinder Cones • • • • Smallest volcanic mountains Basaltic magma is common Slopes form from pyroclastic materials Generally found in association with other volcanoes © 2014 Pearson Education, Inc. Calderas • Result from a volcano that explodes, collapses, or both • Immense, basin-shaped depression; larger than original crater • Crater Lake in Oregon is an example • Yellowstone Park is a major example – a ‘hot spot’ that exploded, forming a caldera basin. © 2014 Pearson Education, Inc. Volcanic hazards – Volcanic gases – mainly water vapor, but can cause acid rain and alter global climate – Lava flows – cause immense property damage – Eruption clouds – gas and ash material clouds that extend up to 16 km into the atmosphere, drop large rock fragments called “bombs” © 2014 Pearson Education, Inc. Volcanic hazards – Pyroclastic flows – avalanche of hot gases and material, up to 100 mph – Volcanic mud flows (lahars) – result from heavy rain and/or snow melt during an eruption © 2014 Pearson Education, Inc. Monitoring volcanic hazards – Research to locate previous pyroclastic flows and lahars – Tiltmeters, measure the slope of a volcano to look for swelling – Monitor earthquake activity The U.S. west coast portion of the “Ring of Fire” exhibits many volcanoes. © 2014 Pearson Education, Inc. Volcanism – Igneous intrusion Some volcanic activity does not reach the surface – cools as dense, hard rock crystals like granite and rhyolite. Sometimes, the earth Erodes down to expose these intrusions that never made it to the surface on their own © 2014 Pearson Education, Inc. [Insert Fig. 14-49 p. 339] Plutonic intrusions later exposed above ground – Dikes – vertical sheet of magma thrust upward into preexisting rock; long and narrow – Veins – Molten material forces itself into smaller fractures in preexisting rock; take irregular shapes © 2014 Pearson Education, Inc. Folding Tectonic force can squeeze or stretch layers of rock, creating rough surfaces that erode in complex ways. © 2014 Pearson Education, Inc. Faulting • Occurs when rock breaks accompanied by displacement • Occurs along zones of weakness in the crust, fault zones • Begin as sudden ruptures or slow creep after breaking Call these “block” faults or “dip-slip” © 2014 Pearson Education, Inc. Common block faults come in many varieties © 2014 Pearson Education, Inc. Faulting – Horst – uplift of a land block between two parallel faults Graben - collapsed area of fractured rock – Graben – downthrown land block between two parallel faults Horst - lifted rock © 2014 Pearson Education, Inc. Transform or slip-strike fault zones Slide past each other in opposite directions Example: San Andreas Fault – a major fault and quake zone © 2014 Pearson Education, Inc. Earthquakes – sudden slip along a fracture line © 2014 Pearson Education, Inc. Quake magnitude measured exponentially – Logarithmic scale, 32nd power – Richter scale – Strongest recorded earthquake – 9.5 in Chile • Shaking intensity – Intensity of ground shaking not consistent during an earthquake – Mercalli intensity scale © 2014 Pearson Education, Inc. [Insert Table 14-2 p. 347] Shaking and damage are worse on softer, wetter earth Liquefaction - very wet soils can ‘liquify’ when shaken – buildings fall apart or move © 2014 Pearson Education, Inc. • Hard rock is best • Sand is shaken easily • Mud is the worst - very hazardous when shaken