GOL 106 LAB 3 ROCK UNITS AND TIME ROCK UNITS Group ________ Member Names (1)_________________________________________________ (2)_________________________________________________ (3)_________________________________________________ Page 75 76 78 Figure Question(s) Fig. 8.5 All All All Chapter 3 The Theory of Plate Tectonics 1 Unifying Theory • A unifying theory is one that helps – explain a broad range of diverse observations – interpret many aspects of a science on a grand scale – and relate many seemingly unrelated phenomena • Plate tectonics is a unifying theory for geology. 2 Plate Tectonics • Plate tectonics helps to explain – earthquakes – volcanic eruptions – formation of mountains – location of continents – location of ocean basins • Tectonic interactions affect – atmospheric and oceanic circulation and climate – geographic distribution, evolution and extinction of organisms – distribution and formation of resources 3 Early Ideas about Continental Drift • Edward Suess • Austrian, late 1800s – noted similarities between the Late Paleozoic plant fossils • Glossopteris flora – and evidence for glaciation in rock sequences of • • • • India Australia South Africa South America • He proposed the name Gondwanaland (or Gondwana) – for a supercontinent composed of these 4 continents Alfred Wegener and the Continental Drift Hypothesis • German meteorologist • Credited with hypothesis of continental drift 5 Alfred Wegener and the Continental Drift Hypothesis • He proposed that all landmasses – were originally united into a supercontinent named Pangaea • He presented a series of maps – showing the breakup of Pangaea • He amassed a tremendous amount of geologic, 6 paleontologic, and climatologic evidence Jigsaw-Puzzle Fit of Continents • Continental Fit 7 Jigsaw-Puzzle Fit of Continents • Matching mountain ranges • Matching glacial evidence 8 Matching Fossils 9 Additional Support for Continental Drift • Alexander du Toit (South African geologist, 1937) – Proposed that a northern landmass, Laurasia, that consisted of present-day • • • • North America Greenland Europe and Asia (except India). Provided additional fossil evidence for Continental drift 10 The Perceived Problem with Continental Drift • Most geologists did not accept the idea of moving continents because: – There was no suitable mechanism to explain how continents could move over Earth’s surface • Interest in continental drift revived when – new evidence from studies of Earth’s magnetic field – and oceanographic research – showed that the ocean basins were geologically young features 11 Revival of Continental Drift Hypothesis • Paleomagnetism is – – – – a remnant magnetism in ancient rocks recording the direction and the strength of Earth’s magnetic field – at the time of the rock’s formation • When magma cools – below the Curie point temperature – magnetic iron-bearing minerals align – with Earth’s magnetic field 12 Polar Wandering • In 1950s, research revealed – that paleomagnetism of ancient rocks showed orientations different from the present magnetic field • Magnetic poles apparently moved. – The apparent movement was called polar wandering. – Different continents had different paths. • The best explanation – is stationary poles – and moving continents 13 Mapping Ocean Basins • Ocean mapping revealed – a ridge system more than 65,000 km long, the most extensive mountain range in the world Mid-Atlantic Ridge • The Mid-Atlantic Ridge – is the best known part of the system – and divides the Atlantic Ocean basin in two nearly equal parts 14 Seafloor Spreading • Harry Hess, in 1962, proposed the theory of seafloor spreading: – Continents and oceanic crust move together – Seafloor separates at oceanic ridges • where new crust forms from upwelling and cooling magma, and • the new crust moves laterally away from the ridge – The mechanism that drives seafloor spreading was thermal convection cells in the mantle • hot magma rises from mantle to form new crust • cold crust subducts into the mantle at oceanic trenches, where it is heated and recycled 15 Confirmation of Hess’s Hypothesis • Ocean research revealed magnetic anomalies on the sea floor • A magnetic anomaly is a deviation from the average strength of Earth’s magnetic field • The stripes are parallel and symmetrical with the 16 oceanic ridges Age of Ocean Basins 17 Plate Tectonics • Plate tectonic theory is based on a simple model of Earth that the lithosphere is rigid and consists of oceanic & continental crust with upper mantle – it consists of variablesized slabs called plates – with plate regions containing continental crust • up to 100 km thick – and plate regions containing oceanic crust • up to 10 km thick 18 Plate Map 19 Numbers represent average rates of relative movement in cm/yr Atlantic Ocean Basin North America Europe Atlantic Ocean basin South America Africa 20 An Example of Ancient Rifting • What features in the rock record can geologists use to recognize ancient rifting? – – – – – faults dikes sills lava flows thick sedimentary sequences within rift valleys • Example: – Triassic fault-block basins in eastern US 21 Ancient Rifting • These Triassic fault basins – mark the zone of rifting between North America and Africa sill Palisades of Hudson River – They contain thousands of meters of continental sediment – and are riddled with dikes and sills 22 Convergent Boundaries • Older crust must be destroyed and recycled – at convergent boundaries – so that Earth’s surface area remains the same • Where two plates collide, – subduction occurs • when an oceanic plate • descends beneath the margin of another plate – The subducting plate • moves into the asthenosphere • is heated • and eventually incorporated into the mantle 23 Convergent Boundaries • Convergent boundaries are characterized by – – – – – – deformation volcanism mountain building metamorphism earthquake activity valuable mineral deposits • Convergent boundaries are of three types: – oceanic-oceanic – oceanic-continental – continental-continental 24 Oceanic-Oceanic Boundary • When two oceanic plates converge, – one is subducted beneath the other along an oceanicoceanic plate boundary – forming an oceanic trench and a subduction complex • composed of slices of folded and faulted sediments • and oceanic lithosphere scraped off the descending plate 25 Volcanic Island Arc • As the plate subducts into the mantle, – – – – it is heated and partially melted generating magma of andesitic composition that rises to the surface because it is less dense than the surrounding mantle rocks • At the surface of the nonsubducting plate, – the magma forms a volcanic island arc 26 Oceanic-Oceanic Plate Boundary • A back-arc basin forms in some cases of fast subduction. – The lithosphere on the landward side of the island arc – is stretched and thinned • Example: Sea of Japan 27 Oceanic-Continental Boundary • An oceanic-continental plate boundary – occurs when a denser oceanic plate – subducts under less dense continental lithosphere • Magma generated by subduction – rises into the continental crust to form large igneous bodies – or erupts to form a volcanic arc of andesitic volcanoes – Example: Pacific coast of South 28 America Oceanic-Continental Boundary • Where the Nazca plate in the Pacific Ocean is subducting under South America – the Peru-Chile Trench marks subduction site – and the Andes Mountains are the volcanic arc Andes Mountains 29 Continent-Continent Boundary • Two approaching continents are initially – separated by ocean floor that is being subducted – under one of them, which, thus, has a volcanic arc • When the 2 continents collide – the continental lithosphere cannot subduct • Its density is too low, – although one continent may partly slide under the other 30 Continent-Continent Boundary • When the 2 continents collide – they weld together at a continent-continent plate boundary, where an interior mountain belt forms consisting of • deformed sedimentary rocks • igneous intrusions • metamorphic rocks • fragments of oceanic crust • Earthquakes occur here 31 Continental-Continental Boundary • Example: Himalayas in central Asia – – – – Earth’s youngest and highest mountain system resulted from collision between India and Asia began 40 to 50 million years ago and is still continuing Himalayas 32 Recognizing Ancient Convergent Boundaries • How can former subduction zones be recognized in the rock record? – Andesitic magma erupted, forming island arc volcanoes and continental volcanoes – The subduction complex results in a zone of intensely deformed rocks between the trench and the area of igneous activity – Sediments and submarine rocks are folded, faulted and metamorphosed making a chaotic mixture of rocks termed a mélange – Slices of oceanic lithosphere may be accreted to the continent edge and are called ophiolites 33 Ophiolite • Ophiolites consist of layers – representing parts of the oceanic crust and upper mantle. • The sediments include – graywackes – black shales – cherts • Ophiolites are key to detecting old subduction zones 34 Transform Boundaries • Transform plate boundary – where plates slide laterally past each other – roughly parallel to the direction of plate movement • Movement results in – zone of intensely shattered rock – numerous shallow earthquakes fracture zone • The majority of transform faults – connect two oceanic ridge segments – and are marked by fracture zones 35 Transform Boundaries • Example: San Andreas Fault, California – separates the Pacific plate from the North American plate – connects ridges in • Gulf of California • with the Juan de Fuca and Pacific plates – Many of the earthquakes in California result from movement along this fault 36 Hot Spots and Mantle Plumes • Hot spots are locations where – stationary columns of magma originating deep within the mantle, • called mantle plumes – slowly rise to the surface • Mantle plumes apparently remain stationary • When plates move over them – hot spots leave trail
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