Introduction to Plate Tectonics

Plate Tectonics New Evidence1No mechanism to move the continents from one place to anotherScientists did not know much about the ocean floordismissed as being eccentric, preposterous, and improbableAlfred Wegener & Continental Drift2

Early StudiesFirst Bathymetric Chart in 1855 by U.S. Navy Lieutenant Matthew Maury underwater mountains in Atlanticconfirmed when laying the trans-Atlantic telegraph cable.

World War I (1914-18)echo-sounding devices (primitive sonar systems) measure ocean depthOcean floor more rugged than thought. Continuity of submarine mountain chain

3World War II & Sea-Floor TopographyShips and Submarines used SONAR to map the ocean-floor topographySubmarines used topography to hide from ships and had to avoid under-sea topographyShips used SONAR to find submarines

1950s -- oceanographic surveys a great mountain range on the ocean floor, the global mid-ocean ridge, encircled the Earth.more than 50,000 kilometers (km) long and up to 1000 km across zig-zags between the continentsRising about 4,500 meters(m) above the sea floor, Taller than all mountains in the US except for Mount McKinley (Denali) at 6,194 m The most prominent topographic feature on the surface of our planet. Deep Sea trenches -- incredibly deep4Age of the Ocean Floor

6Youngest rocks at RidgeOldest furthest away from Ridge1968 -- a research vessel, the Glomar Challenger, designed specifically for marine geology studiesDeep-Sea Drilling Project -samples from ocean floor.Criss-crossed the Mid-Atlantic Ridge between South America and Africa the ages of the samplespaleontologic isotopic dating studies

7

Earths Magnetic Field & Polar Reversals Early 1900s Paleomagnetists Bernard Brunhes in France (in 1906)Motonari Matuyama in Japan (in the 1920s)rocks generally belong to two groups according to their magnetic properties

Normal = the magnetic minerals in the rock having the same polarity as that of the Earth's present magnetic field (North is North)

Reversed = the magnetic minerals in the rock having the opposite polarity as that of the Earth's present magnetic field (North is South)9How can this happen?Grains of magnetite behave like little magnets and align with the orientation of the Earth's magnetic field. Magma cools to form solid rockAlignment of the magnetite grains is "locked in" The grains record the Earth's magnetic orientation or polarity (normal or reversed) at the time of cooling.

Currents in Earths Liquid Outer Core create the magnetic fieldConvection currents of liquid iron and nickel generate an electrical current dynamo.How did they figure this out?World War II magnetometers used to detect submarines

1950s, scientists used magnetometers to study the ocean floor Odd magnetic variations across the ocean floorOcean floor rock, basalt, contains magnetite (a magnetic iron mineral) Magnetite can distort compass readings The magnetic variations are not random or isolated!

Magnetic striping = a zebra-like patternalternating stripes of normal & reversed rock laid out in rows on either side of the mid-ocean ridge11New oceanic crust forms continuously at the crest of the mid-ocean ridgeAs you move away from the ridge Crust is coolerCrust is olderCrust moves away from the ridge crest with seafloor spreading a. the spreading ridge about 5 million years agob. about 2 to 3 million years agoc. now

Sea-Floor Spreading: Putting it all togetherEvidenceSea-Floor TopographyYoung crust at ridges, old crust far from ridgesMagnetic Striping1961 New Questions:Are mid-ocean ridges structurally weak zones where the ocean floor was being ripped in two lengthwise along the ridge crest?Could new magma from deep within the Earth rises easily through these weak zones and erupts along the crest of the ridges to create new oceanic crust?13Harry H. Hess & Robert S. Dietz = seafloor spreading. 1. Crust is added at the ridgesnew oceanic crust continuously spread away from the ridges in a conveyor belt-like motion2. Crust is destroyed at the trenchesAfter millions of years of moving away from the ridges, oceanic crust descends into very deep, narrow canyons along the rim of the Pacific Ocean basin called trenches

Sea-Floor Spreading: Putting it all together14AssumptionsThe Atlantic Ocean was expanding.The Pacific Ocean was shrinking. The ocean basins were perpetually being recycled the creation of new crust at mid-ocean ridgesthe destruction of old oceanic lithosphere at subduction zones

Thus, Hess' ideas neatly explained why the Earth does not get bigger with sea floor spreadingwhy there is so little sediment accumulation on the ocean floor why oceanic rocks are much younger than continental rocks.And, the oceanic crust is a natural "tape recording" of the history of the reversals in the Earth's magnetic field.Sea-Floor Spreading: Putting it all together16Continental Drift+ Sea Floor Spreading = Plate Tectonics!!!!17The Theory of Plate TectonicsThe outer layer of the Earth is broken up into rigid platesThese plates are made of the lithosphere = Crust + the Uppermost Mantle which is stuck to the bottom of the crust (peanut butter on bread)Plates move around on the hot, weak, mobile asthenosphereWeak zone in the mantle below the lithosphere (jelly)The PlatesRigid - movement is minimal within a platePlates move relative to each other resulting in their edges havingEarthquakesVolcanoesMountainsThe intensity of earthquakes, types of volcanoes, and types of mountains are dependent on how the plates interact3 primary types of plate boundariesDivergent BoundariesPlates move away from each otherMostly found at mid-ocean ridges Crust is made as plates move away from each other and magma oozes up at the ridge

Earthquakes are common at ridge but shallowVolcanic Activity is prevalent at ridgeExamples: Mid-Atlantic Rise and the Red Sea

Transform BoundaryPlates move past each otherAccommodate the different types of movement on a sphere

Earthquakes very common and can be severeVolcanoes absentExamples: San Andreas Fault and Mendicino Boundary

Convergent BoundaryPlates move toward each otherThree different typesOcean-ContinentOcean-OceanContinent-ContinentOcean-Continent ConvergentMore dense oceanic lithosphere is subducted under less dense continental lithosphere forming a trenchContinental Volcanic Arcs result as the water in the subducted oceanic lithosphere is released and melting occurs in the mantle below the continent.

Earthquakes are severe, common and vary in depth.Continental Volcanic ArcsExamples: Cascade and Andes MountainsOcean-Ocean ConvergenceThe older, colder more dense oceanic lithosphere is subducted beneath the younger, warmer less dense segment.Deep trenches form at edge of subduction zone

Volcanic Island Arcs form as water is released into the mantle causing meltingEarthquakes are common and severe, varying in depthOften result in Tsunamis.Examples: Japan and IndonesiaContinent-Continent ConvergenceContinental lithosphere cannot be subducted because it is not dense enough.As the continents are forced together, they form huge mountains

Earthquakes are severe and common.Volcanism is rare.Examples: Himalaya, Alps, and Appalachia

What Drives the Plates?

Heating from Below

Heating from Within

Heating from within and below