Presentation on theme: "Chapter 2 Plate Tectonics: A Scientific Revolution Unfolds"— Presentation transcript:
1 Chapter 2 Plate Tectonics: A Scientific Revolution Unfolds
2 Continental Drift: An Idea Before its Time Alfred WegenerFirst proposed his continental drift hypothesis in 1915 before the theory of Plate TectonicsContinental drift hypothesisSupercontinent called Pangaea began breaking apart about 200 million years agoContinents "drifted" to present positions
3 Continental Drift: An Idea Before its Time, cont’d Evidence used in support of continental drift hypothesisFit of the continentsFossil evidenceRock type and structural similaritiesPaleoclimatic evidence
4 The Great Debate Objections to the continental drift hypothesis Lack of a mechanism for moving continentsWegener incorrectly suggested that continents broke through the ocean crust, much like ice breakers cut through iceStrong opposition to the hypothesis from all areas of the scientific community
5 Continental Drift and Paleomagnetism Renewed interest in continental drift initially came from rock magnetismMagnetized minerals in rocksDeclinations show the direction to Earth’s magnetic poles when rocks formedInclinations provide a means of determining the rocks latitude of origin and how much it moved north or south since it was created.
6 Continental Drift and Paleomagnetism Polar wanderingThe apparent movement of the magnetic poles indicates that the continents have movedIndicates Europe was much closer to the equator when coal-producing swamps existed
7 Continental Drift and Paleomagnetism, cont’d Polar wanderingCurves for North America and Europe have similar paths but are separated by about 24 of longitudeDifferences between the paths can be reconciled if the continents are placed next to one another
8 A Scientific Revolution Begins During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floorSeafloor spreading hypothesis was proposed by Harry Hess in the early 1960sOceanic crust created and spread apart at shallow mid-oceanic ridgesOceanic crust destroyed at deep trenches
9 A Scientific Revolution Begins, cont’d Geomagnetic reversalsEarth's magnetic field periodically reverses polarity – the north magnetic pole becomes the south magnetic pole, and vice versa. This flips the remnant magnetic inclination of the rocks 180 degrees.Dates of geomagnetic reversals determined from lava flows.
10 A Scientific Revolution Begins, cont’d Geomagnetic reversalsGeomagnetic reversals are recorded in the ocean crust as alternating linear stripes of high and low magnetic anomalies (highs for normal polarity and lows for reversed polarity).These linear anomalies parallel the mid-ocean ridges and are symmetrical with respect to them.In 1963 Vine and Matthews tied the discovery of these “magnetic stripes” in the ocean crust near mid-ocean ridges to Hess’s concept of seafloor spreading.
11 A Scientific Revolution Begins, cont’d Earthquakes and Subduction ZonesWadati and Benioff mapped out the depths and locations of earthquakes along trenches and beneath arcuate trends of volcanoes that parallel them.They found shallower earthquakes closer to the trench, and progressively deeper earthquakes towards the volcanic arcs and beyond them.Postulated that the oceanic crust and upper-most mantle is sinking or subducting down into the rest of the mantle along these zones and is being destroyed.
12 Plate Tectonics: The New Paradigm Earth’s major tectonic platesEarth is divided into seven major lithospheric plates and several other minor plates that are relatively rigid.Plates are composed of the oceanic and/or continental crust and the uppermost mantle.Several plates include an entire continent plus a large area of seafloor.
13 Plate Tectonics: The New Paradigm, cont;d Earth’s major tectonic plates cont.Plates are in constant slow motion with respect to one another and are continually changing in shape and size.Earthquakes, deformation and volcanic eruptions occur where these plates meet.The interior of plates generally have fewer earthquakes, less deformation and less volcanic activity compared to the margins.
14 Plate Tectonics: The New Paradigm, cont’d Plate boundariesInteractions between individual plates occur along their boundariesTypes of plate boundariesDivergent plate boundaries (constructive margins formed by extension)Convergent plate boundaries (destructive margins formed by compression)Transform fault boundaries (conservative margins formed by side-to-side shear)
15 Divergent Plate Boundaries Most are located along the crests of oceanic ridgesOceanic ridges and seafloor spreadingThe seafloor is elevated along well-developed divergent plate boundaries, forming oceanic ridgesGeologic features includecrustal and lithospheric thinningrift zones formed by extensional faultingvolcanic activityshallow earthquakes
16 Divergent Plate Boundaries, cont’d Continental riftingSplits landmasses into two or more smaller segments along a continental riftExamples include the East African rift valleys and the Rhine Valley in northern EuropeProduced by extensional forces acting on lithospheric plates
17 Convergent Plate Boundaries Older portions of oceanic plates are returned to the mantle or subducted in these destructive plate marginsSurface expression of the descending plate is an ocean trenchOceanic plate moves downward along subduction zonesGeologic features includevolcanic arcsshallow-deep earthquakesmountain belts
18 Types of Convergent Plate Boundaries Oceanic-continental convergenceDenser oceanic slab sinks into the asthenosphereAlong the descending plate partial melting of mantle rock generates magmaResulting volcanic mountain chain is called a continental volcanic arc (Andes and Cascades)
19 Types of Convergent Plate Boundaries, cont’d Oceanic-oceanic convergenceWhen two oceanic slabs converge, one descends beneath the otherOften forms volcanoes on the ocean floorIf the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)
20 Types of Convergent Plate Boundaries, cont’d Continental-continental convergenceContinued subduction can bring two continents togetherLess dense, buoyant continental lithosphere does not subductResulting collision between two continental blocks produces mountains (Himalayas, Alps, Appalachians)
21 Transform Fault Boundaries Plates slide past one another and no new lithosphere is created or destroyedMost join two segments of a mid-ocean ridge along breaks in the oceanic crust known as fracture zonesA few (e.g. the San Andreas fault) cut through continental crustGeologic Features includestrike slip faultslack of volcanic activityshallow earthquakesinactive fracture zones which extend outward from oceanic transform faults (active fracture zones) and separate lithosphere of different age and density
22 Testing the Plate Tectonics Model Hot spots and mantle plumesCaused by rising plumes of heat or hot mantle materialVolcanoes and calderas can form over them (Hawaiian Island chain, Yellowstone caldera)Mantle plumesLong-lived structures that are fixed in one subsurface location while the plates drift over themSome originate at great depth, perhaps at the core-mantle boundary
23 Measuring Plate Motion Paleomagnetism and plate motionsPaleomagnetism stored in rocks on the ocean floor provides a method for determining plate motionsBoth the direction and rate of seafloor spreading can be established
24 Measuring Plate Motion, cont’d Measuring plate velocities from a fixed reference frame in spaceAccomplished by establishing exact locations on opposite sides of a plate boundary and measuring relative motionsTwo methods are usedVery Long Baseline Interferometry (VLBI)Global Positioning System (GPS)
25 What Drives Plate Motions Many, but not all researchers agree that convective flow in the mantle is the basic driving force of plate tectonicsTheory I: Causes elevation and density differences due to temperature variations, which create gravitational forcesTheory II: Causes frictional drag on the plates above the convecting mantle