Transcript Ch 3 ppt
Chapter 3 Plate Tectonics: A Unifying Theory or, “How the map was made” Life on a Plate Boundary Dec. 26, 2003 Earthquake Bam, Iran Life on a Plate Boundary Dec. 26, 2004 Indonesia Tsunami Life on a Plate Boundary Haiti January 12, 2010 Earthquake > Life on a Plate Boundary In California, a stream bends as plates move on either side Long Lakes fill the area along the fault Source: wikimedia Hollister, CA on the San Andreas fault Hollister UC Berkeley, CA Hawyard fault line Univ. of Calif. At Berkeley In the Middle East, the Red Sea Gulf of Aden East African Rift form a triple junction of spreading and divergent boundaries Volcanism beneath the Red Sea and Gulf are forming new ocean basins On the continent, in the rift zone, volcanoes line the valley Mt. Pinatubo, Phillippines June 15, 1991 Mt. St. Helens, Washington Mt. St. Helens today Volcanism along a mid-ocean ridge 2004 Icelandic volcanic eruption under a glacier courtesy BBC News 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. Three phases of the theory: • Continental drift, considered by many but investigated by Alfred Wegener • Seafloor Spreading, discovered in the 1960s by Harry Hess and others– showed why the continents appear to be moving • Theory of Plate Tectonics: still being tested and developed What Is the Driving Mechanism of Plate Tectonics? • In addition to a thermal convection system, – some geologists think that movement may be aided by – “slab-pull” • the slab is cold and dense and pulls the plate – “ridge-push” • rising magma pushes the ridges up • and gravity pushes the oceanic lithosphere away from the ridge and toward the trench Plate Tectonics • Plate tectonic theory is based on the simple model that – the lithosphere is rigid – it consists of oceanic and continental crust with upper mantle – it consists of variable-sized pieces called plates – with plate regions containing continental crust • up to 250 km thick – and plate regions containing oceanic crust • up to 100 km thick Plate Map Numbers represent average rates of relative movement, cm/yr What Is the Driving Mechanism of Plate Tectonics? • Most geologists accept some type of convective heat system hypothesis – as the basic cause – of plate motion • In one possible model, – thermal convection cells – are restricted to the asthenosphere Plate Movement • Plates carrying both ocean and continental crust move over hotter and weaker semiplastic asthenosphere • Movement of the plates – results from some type of heat-transfer system within the asthenosphere • As plates move over the asthenosphere – they separate, mostly at oceanic ridges – they collide, in areas such as oceanic trenches – they move past each other There are 7 types of plate boundaries Divergent (2 types) 1. Continental: extensional forces move continental crust apart. Faulting, stretching, and thinning of crust occurs. Magma may intrude. 2. Oceanic: spreading ridges, occur beneath the ocean Faulting and thinning of crust occurs. Lava flows onto the ocean floor. new oceanic lithosphere is forming. Magma intrudes into faults or erupts as lava Lavas are high temperature, low viscosity, basaltic Convergent Boundaries (3) 3. Continent - continent When two continental plates collide, each is relatively buoyant and similar in density. Minor subduction may occur but typically continental crust is deformed, pushed together and uplifted forming thick crustal mountain ranges. Example: India and Asia – Himalayan Mountains 4. Ocean - continent When ocean crust converges with continental crust, the more dense ocean crust subducts into a deep trench beneath the less dense continental crust. As the oceanic “slab” moves down into the mantle, it melts at about 100 km depth. The melted rock or magma moves up within the crust and eventually erupts forming a continental volcanic arc. Example: Cascades (Oregon & Washington State, U.S.) Convergent Boundaries cont’d 5. Ocean – Ocean: When ocean crust coverges with ocean crust, both are similar density. Typically, the colder, older, more dense ocean crust subducts. The subducting ocean slab melts at about 100 km depth and forms an underwater volcano. When this volcano reaches a height above sea level, a volcanic island arc forms. • Convergent boundaries are characterized by explosive volcanism, rocks typically andesite and felsic composition, earthquakes that are high magnitude and varying in depth from shallow to deep. Tsunamis may originate from shallow subduction zone earthquakes. • Mountain ranges at convergent boundaries form generally linear belts or curved arcs. Metamorphism is common as pressure, heat and/or hydrothermal fluids are present. Mineral deposits are associated with mountain building and subduction zones. • • The process of subduction accommodates the new crust formed at divergent boundaries and keeps the earth’s size constant. Transform plate boundaries (2) 6. Mid Ocean Ridge transform faults: connect two oceanic ridge segments – and are marked by perpendicular fracture zones causing a discontinuous pattern along the ridge 7. Continental transform faults: characterized by strike-slip faults as two plates “slide” past each other. Transform faults are characterized by build-up of stress and strain and inevitable shallow earthquakes which can be of significant magnitude Divergent Boundaries 1. Continental Divergent occur during the early stages of continental breakup the crust is initially elevated, stretched and thinned 1. cont’d Contintal “Rift Valley” forms magma typically intrudes into the fractures and flows onto the valley floor • East African Rift Valley Narrow Sea – the continental crust eventually breaks – a narrow linear sea is formed, – separating two continental blocks – Examples: • Red Sea • Gulf of California Ocean • As a newly created narrow sea – continues to spread, – it may eventually become – an expansive ocean basin Modern Divergence View looking down the Great Rift Valley of Africa. Little Magadi soda lake 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 2. Divergent Mid Ocean Ridge North America Europe Atlantic Ocean basin South America Africa Elevation map of Mid Atlantic Ridge Divergent Boundaries • Mid Ocean Ridges also have – high heat flow – and basaltic flows or pillow lavas • Pillow lavas have – a distinctive bulbous shape resulting from underwater eruptions Convergent Plate Boundaries Fig. 2.8 Keller Convergent Boundary: Continent-Continent Boundary • When 2 continents collide – the continental lithosphere cannot subduct • Its density is too low, – although one continent may partly slide under the other Continent-Continent Boundary • When the 2 continents collide – they weld together at a continent-continent plate boundary, (suture zone) – An mountain belt forms on the continent • Earthquakes occur here Continental-Continental Boundary 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 Mt. Everest Himalays • From its summit to the top of the Yellow Band, about 8,600 m above sea level, the top of Mount Everest consists of the Qomolangma Formation,. It consists of grayish bedded limestone with dolomite and siltstone. There are finely fragmented fossils of trilobites, crinoids, and ostracods – marine organisms. 4. Oceanic-Continental Boundary • Formation of Continental Volcanic Arc: 4. 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 5. Oceanic-Oceanic Boundary subduction complex has volcanic island arc, deep trench and back arc basin Japan Island Arc 5. • • • • Volcanic Island Arc Forearc composed of slices of folded and faulted sediments and oceanic lithosphere scraped off the descending plate Back arc caused by stretching and thinning allowing magma to reach the surface beneath a shallow sea Sea of Japan 6. Transform Boundaries • Movement results in – zone of intensely shattered rock – numerous shallow earthquakes • The majority of transform faults – connect two oceanic ridge segments – and are marked by fracture zones fracture zone 6.Transform Boundaries • Other kinds of transform plate boundaries – connect two trenches – or connect a ridge to a trench – or even a ridge or trench to another transform fault • Transforms can also extend into continents 7. Transform Boundaries connect other plate boundary types Continental: 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 What Is the Driving Mechanism of Plate Tectonics? • In a second model, the entire mantle is involved in thermal convection. • In both models, – spreading ridges mark the rising limbs of neighboring convection cells – trenches occur where the convection cells descend back into Earth’s interior Plate Movement at Hot Spot Map of Plate Boundaries and Rate of Plate Motion Fig. 2.4a Map of Volcanoes and Earthquakes Fig. 2.4 b Map and cross section of Hawaiian Ridge and Emporer Seamounts Fig. 2.16 a Fig. 2.16 b Hot Spots and Mantle Plumes These are NOT plate boundaries! • Hot spots are locations where – stationary columns of magma – originating deep within the mantle, • called mantle plumes – slowly rise to the surface • Mantle plumes remain stationary • although some evidence suggests they may move • When plates move over them – hot spots leave trails • of extinct, progressively older volcanoes • called aseismic ridges • which record the movement of the plates Hot Spots and Mantle Plumes • Example: Emperor SeamountHawaiian Island chain Age increases plate movement How Is Plate Motion Determined? • Rates of plate movement can be calculated in several ways – Sediment • determine the age of sediment* that is immediately above any portion of oceanic crust • divide the distance from the spreading ridge by the age • gives average rate of movement relative to the ridge * LEAST ACCURATE METHOD relies on incomplete fossil record Plate Movement Measurements – Seafloor magnetic anomalies • measure the distance of the magnetic anomaly in seafloor crust from the spreading ridge • divide by the age of the anomaly – The present average rate of movement, relative motion, and the average rate of motion in the past can be determined. Plate Position Reconstruction • Reconstructing plate positions – to determine the plate and continent positions at the time of an anomaly – move the anomaly back to the spreading ridge • Since subduction destroys oceanic crust • this kind of reconstruction cannot be done earlier than the oldest oceanic crust Plate Movement Measurements • Satellite-laser ranging – bounce laser beams from a station on one plate – off a satellite, to a station on another plate – measure the elapsed time – measure the elapsed time at a later date. – use the difference in elapsed times to calculate the rate of movement between the two plates • Hot spots – determine the age of rocks and their distance from a hot spot – divide the distance by the age – this gives the motion relative to the hot spot so – (possibly) the absolute motion of the plate Plate Tectonics and the Distribution of Natural Resources • Plate movements influence the formation and distribution of some natural resources such as – Petroleum – folded & faulted plate boundaries form “traps” for oil and gas – natural gas – some mineral deposits related to volcanism • Metal resources related to igneous and associated hydrothermal activity include – Copper, gold, lead Summary • Plate tectonic theory replaced the idea of continental “drift” – became widely accepted by the 1970s – because of overwhelming evidence supporting it – Especially the new theory of sea floor spreading • and because it provides a powerful explanation and becomes the unifying theory for – – – – – – volcanism, earthquake activity, mountain building, global climate changes, distribution of the world’s biota and distribution of resources Alfred Wegener and the Continental Drift Hypothesis • German meteorologist • Credited with hypothesis of continental drift: he studied continental glaciers and iceberg movement in Greenland as a model for earth’s crust Reasons to propose that the continents were moving • • • • • Fit of the continents like a puzzle Fossil evidence Glacial evidence Patterns of mountain ranges Apparent polar wandering Early Ideas about Continental Drift • Another scientist, Edward Suess (Austrian, late 1800s) noted Late Paleozoic plant fossils Glossopteris flora found on many continents • This plant • Grows in subpolar climates but is found today in warmer areas Suess (cont’d) 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 modern continents – It was located near the South Pole Alfred Wegener and the Continental Drift Hypothesis • He proposed Pangaea: a supercontinent – Greek for “all land” – showing the breakup of Pangaea – Using geologic, paleontologic, and climatologic evidence Wegener’s Evidence • Shorelines of continents fit together – matching rock types: marine, nonmarine and glacial – from Pennsylvanian to Jurassic age – for all five Gondwana continents • including Antarctica • Mountain ranges and glacial deposits – match up when continents are united – into a single landmass Jigsaw-Puzzle Fit of Continents • Continental Fit Jigsaw-Puzzle Fit of Continents • Matching mountain ranges • Matching glacial evidence Matching Fossils The Perceived Problem with Continental Drift – There was no suitable mechanism to explain – how continents could move over Earth’s surface • Interest in continental drift only revived in mid 1900s – new evidence from studies of Earth’s magnetic field and oceanographic research – ocean basins as geologically young features Earth’s Magnetic Field • Earth as a giant dipole magnet – magnetic poles essentially coincide – with the geographic poles – and may result from different rotation speeds – of outer core and mantle Magnetic Field Varies • Strength and orientation of the magnetic field varies – weak and horizontal at the equator – strong and vertical at the poles Paleomagnetism • Paleomagnetism is – – – – – A remanent magnetism recorded in rocks Recording the direction of Earth’s magnetic field Recording the strength of the field over time Recording the latitude of the rocks All at the time of the rock’s formation When magma cools – – – – below the Curie point temperature 770o C magnetic iron-bearing minerals align with Earth’s magnetic field For example, magnetite When iron sediments settle in liquid water (i.e. oceans, lakes, they can record the same history Polar Wandering • In 1950s, research revealed – that paleomagnetism of ancient rocks showed – orientations different from the present magnetic field • Two different “paths” were recorded in N. America and Europe • The best explanation – is stationary poles – and moving continents Magnetic Reversals • Earth’s present magnetic field: Normal – with magnetic north near the north geographic pole – and magnetic south near the south geographic pole • At various times in the past, Reversed – with magnetic south near the north geographic pole – and magnetic north near the south geographic pole • a series of magnetic reversal is recorded in newly lava on the continent or sea floor continent – alternating layers of reversed orientation sea floor – pattern or “stripes” of reversals parallel to the Mid Ocean Ridge Magnetic Reversals • Measuring paleomagnetism and dating continental lava flows led to – the realization that magnetic reversals existed – the establishment of a magnetic reversal time scale Mapping Ocean Basins: a modern discovery • Ocean mapping revealed – a ridge system – more than 65,000 km long (over 40,000 mi – the most extensive mountain range in the world • The Mid-Atlantic Ridge – is the best known part of the system – and divides the Atlantic Ocean basin – in two nearly equal parts Atlantic Ocean Basin Mid-Atlantic Ridge Seafloor Spreading Hypothesis Harry Hess, 1962 – 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 is 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 Evidence for Hess’s Hypothesis • In addition to mapping mid-ocean ridges, – ocean research also revealed – magnetic anomalies on the sea floor • A magnetic anomaly is a deviation – from the average strength – of Earth’s Magnetic field Confirmation of Hess’s Hypothesis • The magnetic anomalies were discovered to be rougly parallel to and symmetrical with the oceanic ridges Evidence for Plate Tectonics • Magnetic anomaly • Presence of volcanic mid ocean ridges • Age of the ocean crust – Vs. age of the continental crust • Symmetrical Patterns • “Disappearance” of sea floor at edges of some continents • Linear mountain ranges of active volcanoes – continental volcanic arcs and volcanic island arcs Oceanic Crust Is Young • Seafloor spreading theory indicates that – oceanic crust is geologically young because – it forms during spreading – and is destroyed during subduction, therefore it does not survive as long as continental crust that does not subduct • Radiometric dating confirms – the oldest oceanic crust is less than 180 million years old • whereas oldest continental crust – is 3.96 billion yeas old Age of Ocean Basins Historical Geologists: 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 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