1 Lecture Outlines Physical Geology, 12/ePlummer & Carlson Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Plate Tectonics Physical Geology 12/e, Chapter 19

3 Intro: Plate TectonicsBasic idea of plate tectonics Earth’s surface is composed of a few large, thick plates that move slowly and change in size Intense geologic activity is concentrated at plate boundaries, where plates move away, toward, or past each other Combination of continental drift and seafloor spreading hypotheses in late 1960s

4 I. Early Case for Continental DriftPuzzle-piece fit of coastlines of Africa and South America has long been known In early 1900s, Alfred Wegener noted South America, Africa, India, Antarctica, and Australia have almost identical late Paleozoic rocks and fossils Glossopteris (plant), Lystrosaurus and Cynognathus (animals) fossils found on all five continents Mesosaurus (reptile) fossils found in Brazil and South Africa only

5 Early Case for Continental DriftWegener reassembled continents into the supercontinent Pangaea Pangea initially separated into Laurasia and Gondwanaland Laurasia - northern supercontinent containing North America and Asia (excluding India) Gondwanaland - southern supercontinent containing South America, Africa, India, Antarctica, and Australia Late Paleozoic glaciation patterns on southern continents best explained by their reconstruction into Gondwanaland

6 Early Case for Continental DriftCoal beds of North America and Europe support reconstruction into Laurasia Reconstructed paleoclimate belts suggested polar wandering, potential evidence for Continental Drift SKEPTICISM: Continental Drift hypothesis initially rejected Wegener could not come up with viable driving force continents should not be able to “plow through” sea floor rocks while crumpling themselves but not the sea floor

7 Paleomagnetism and Continental Drift RevivedStudies of rock magnetism allowed determination of magnetic pole locations (close to geographic poles) through time Paleomagnetism uses mineral magnetic alignment direction and dip angle to determine the direction and distance to the magnetic pole when rocks formed Steeper dip angles indicate rocks formed closer to the magnetic poles Rocks with increasing age point to pole locations increasingly far from present magnetic pole positions

8 Paleomagnetism and Continental Drift RevivedApparent polar wander curves for different continents suggest real movement relative to one another Reconstruction of supercontinents using paleomagnetic information fits Africa and South America like puzzle pieces Improved fit results in rock units (and glacial ice flow directions) precisely matching up across continent margins

9 Evidence: Seafloor SpreadingIn 1962, Harry Hess proposed seafloor spreading Seafloor moves away from the mid-oceanic ridge due to mantle convection Convection is circulation driven by rising hot material and/or sinking cooler material Hot mantle rock rises under mid-oceanic ridge Ridge elevation, high heat flow, and abundant basaltic volcanism are evidence of this

10 Seafloor Spreading Seafloor rocks, and mantle rocks beneath them, cool and become more dense with distance from mid-oceanic ridge When sufficiently cool and dense, these rocks may sink back into the mantle at subduction zones Downward plunge of cold rocks gives rise to oceanic trenches Overall young age for sea floor rocks (everywhere <200 million years) is explained by this model

11 Plates and Plate MotionTectonic plates are composed of the relatively rigid lithosphere Lithospheric thickness and age of seafloor increase with distance from mid-oceanic ridge Plates “float” upon ductile asthenosphere Plates interact at their boundaries, which are classified by relative plate motion Plates move apart at divergent boundaries, together at convergent boundaries, and slide past one another at transform boundaries

12 How do we know?? Marine Magnetic AnomaliesMarine magnetic anomalies - bands of stronger and weaker than average magnetic field strength Parallel mid-oceanic ridges Field strength related to basalts magnetized with same and opposite polarities as current magnetic field Symmetric “bar-code” anomaly pattern reflects plate motion away from ridge coupled with magnetic field reversals Matches pattern of reversals seen in continental rocks (Vine and Matthews)

13 Evidence of Plate MotionSeafloor age increases with distance from mid-oceanic ridge Rate of plate motion equals distance from ridge divided by age of rocks Symmetric age pattern reflects plate motion away from ridge

14 Evidence: Plate MotionMid-oceanic ridges are offset along fracture zones Fracture zone segment between offset ridge crests is a transform fault Relative motion along fault is result of seafloor spreading from adjacent ridges Plate motion can be measured using satellites, radar, lasers and global positioning systems Measurements accurate to within 1 cm Motion rates closely match those predicted using seafloor magnetic anomalies

15 II. Divergent Plate BoundariesAt divergent plate boundaries, plates move away from each other Can occur in the middle of the ocean or within a continent Divergent motion eventually creates a new ocean basin Marked by rifting, basaltic volcanism, and eventual ridge uplift During rifting, crust is stretched and thinned Graben valleys mark rift zones Volcanism common as magma rises through thinner crust along normal faults Ridge uplift by thermal expansion of hot rock

16 Transform Plate BoundariesAt transform plate boundaries, plates slide horizontally past one another Marked by transform faults Transform faults may connect: Two offset segments of mid-oceanic ridge A mid-oceanic ridge and a trench Two trenches Transform offsets of mid-oceanic ridges allow series of straight-line segments to approximate curved boundaries required by spheroidal Earth

17 Convergent Plate BoundariesAt convergent plate boundaries, plates move toward one another Nature of boundary depends on plates involved (oceanic vs. continental) Ocean-ocean plate convergence Marked by ocean trench, Benioff zone, and volcanic island arc Ocean-continent plate convergence Marked by ocean trench, Benioff zone, volcanic arc, and mountain belt Continent-Continent plate convergence Marked by mountain belts and thrust faults

18 Movement of Plate BoundariesPlate boundaries can move over time Mid-oceanic ridge crests can migrate toward or away from subduction zones or abruptly jump to new positions Convergent boundaries can migrate if subduction angle steepens or overlying plate has a trenchward motion of its own Back-arc spreading may occur, but is poorly understood Transform boundaries can shift as slivers of plate shear off San Andreas fault shifted eastward about five million years ago and may do so again

19 Plate Size North American Plate Nazca Plate Pacific PlateGrowing because of no subduction and the spreading within the Atlantic Ocean (MAR) Nazca Plate Shrinking because the plate is subducting at the Peru-Chile Trench Pacific Plate Shrinking also because of the Eurasian plate sliding over the pacific plate.

20 III. “Attractiveness” of Plate TectonicsGeologists love this idea to explain Earth’s features and their distribution. Volcanoes and types Earthquakes Mountains Trenches MOR

21 What Causes Plate Motions?Causes of plate motion are not yet fully understood, but any proposed mechanism must explain why: Mid-oceanic ridges are hot and elevated, while trenches are cold and deep Ridge crests have tensional cracks The leading edges of some plates are subducting sea floor, while others are continents (which cannot subduct) Mantle convection may be the cause or an effect of circulation set up by ridge-push and/or slab-pull

22 Evidence: Mantle Plumes and Hot SpotsMantle plumes - narrow columns of hot mantle rock rise through the mantle Stationary with respect to moving plates Large mantle plumes may spread out and tear apart the overlying plate Flood basalt eruptions Rifting apart of continental land masses New divergent boundaries may form

23 Mantle Plumes and Hot SpotsMantle plumes may form “hot spots” of active volcanism at Earth’s surface Approximately 45 known hotspots Hot spots in the interior of a plate produce volcanic chains Orientation of the volcanic chain shows direction of plate motion over time Age of volcanic rocks can be used to determine rate of plate movement Hawaiian islands are a good example

24 End of Chapter 19