Plate Tectonics

 

 

 

 

Alfred Wegener, German meteorologist, proposed continental drift in his 1915 paper, The Origin of the Continents and Oceans.

 

Geologists did not agree, asking what forces could move continents?

 

During the next 50+ years evidence supporting the idea of continental drift accumulated

  • during the 1960s the ideas of seafloor spreading and subduction developed
  • by 1968 a fairly complete model was developed
  • Earth's lithosphere is composed of 20 plates that move relative to each other

 

 

 

Plate Tectonic Theory

 

Is a grand unifying theory of geology, because it can explain a great many geologic phenomena

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Wegener's Evidence for Continental Drift

 

He suggested that a vast super continent, Pangaea, existed from about 250 million years ago (mya) until about 70 million years ago.

 

Wegener said Pangaea then broke apart and the landmasses then moved away from each other to form the continents we see today.

 

Evidence:

  • looking at a map, the fit of the continents
  • locations of past glaciations
  • past distribution of climate zones
  • distribution of fossils
  • matching geologic units

 

 

 

 

 

Past Glaciations

  • Wegener was an Arctic climate scientist
  • it bothered him that glacial evidence (till & striations) occurred in the following places that are widely separated and generally warm:
    • southern South America
    • southern Africa
    • southern India
    • Antarctica
    • southern Australia
  • scientists had determined that an ice age occurred from about 280 to about 260 mya.
  • Wegener found that that all glaciated areas lay adjacent to each other on his map of Pangaea!
  • when he plotted the direction of glacial striations, they all pointed roughly outward from a location in southeastern Africa
  • Wegener determined that the distribution of glaciation at the end of the Paleozoic could easily be explained if the continents had been united in Pangaea

 

 

Distribution of Climate Zones

  • if the southern part of Pangaea had straddled the South Pole at the end of the Paleozoic, then during this same time interval, the following locations would have straddled the equator:
    • North America
    • southern Europe
    • northwestern Africa
  • these locations would have had tropical or subtropical climates
  • Wegener found climate evidence from:
    • coal deposits
    • desert sand dunes
    • salt deposits from evaporative lakes
    • coral reefs

 

 

 

 

 

The Distribution of Fossils

 

 

 

 

 

 

 

Matching Geologic Units

 

 

 

 

 

 

 

 

 

 

Criticism of Wegener's Ideas

  • Wegener's model of continental drift explained the distribution of ancient:
    • glaciers
    • coal
    • sand dunes
    • rock assemblages
    • fossils
  • strong circumstantial evidence Wegener could not adequately explain how or why continents drifted
  • Wegener died on a Greenland resupply mission in 1930
  • it would take another 30 years of research before there was sufficient data to test Wegener's hypotheses properly

 

 

 

 

Paleomagnetism - Proving That Continents Move

 

  • dipole: Earth's magnetic field resembles that produced by a bar magnet.

 

 

 

 

 

 

 

What is Paleomagnetism?

 

 

 

Apparent Polar Wander -- A Proof That Continents Move

  •  why doesn't the paleomagnetic dipole in ancient rocks point to the present-day magnetic field?
  • when geologists first attempted to answer this question, they assumed continents were in fixed positions and the Earth's magnetic pole locations changed
  • paleopole: the supposed position of the Earth's magnetic north pole in the past
  • geologists wanted to track the position of the paleopole over time.
  • to do this, they measured the paleomagnetism in a succession of rocks of different ages from the same general location on a continent
  • they plotted the derived position of the paleopole on a map
  • apparent polar-wander path

 

 

 

  • when geologists plotted polar-wander paths from many different continents, they found that each continent has a different apparent polar-wander path!
  • the hypothesis that continents are fixed in position cannot explain this observation
  • if the magnetic pole moved while all the continents stayed fixed, measurements from all continents should produce the same apparent polar-wander paths
  • Geologists realized it's not the pole that moves relative to fixed continents, but rather the continents move relative to a fixed pole!!!

 

 

 

 

 

 

  • since each continent has its own unique polar-wander path, the continents must move with respect to each other!!

 

 

 

 

 

The Discovery of Seafloor Spreading

 

 

 

 

 

  • bathymetric maps reveal several important features:

 

 

  • abyssal plains: broad, relatively flat regions that lie 12 - 16,000 feet below the surface
  • mid-ocean ridges: submarine mountain ranges whose peaks are only 7-8000 feet below the surface
    • are generally symmetrical
  • fracture zones: narrow bands of vertical cracks and broken up rock that lie roughly perpendicular to mid-ocean ridges
  • deep-ocean trenches: very deep ocean floor found along the perimeter of the Pacific Ocean and some other locations.
    • they define long troughs referred to as trenches that border volcanic arcs

 

 

 

  • seamount chains: numerous volcanic islands poking up from the ocean floor:
    • for example, Hawaiian Islands rise above sea level
    • seamounts are submarine mountains. Once erupting volcanoes, they are now inactive
    • typically, only the end island of a seamount chain remains active volcanically

 

 

 

New Observations on the Nature of Ocean Crust

  • by the mid-1950s geologists had discovered many important characteristics of the seafloor crust:

     

     

 

 

  • some belts follow
    • trenches
    • mid-ocean ridge axes
    • portions of fracture zones
  • since earthquakes define locations where rocks break and move, geologists realized that these features are places where motion is taking place

 

 

 

Harry Hess

  • In the late 1950s, Prof. Harry Hess made several important deductions:

 

  • In 1960, Hess brought these ideas together and suggested:
    • magma rose upward at mid-ocean ridges
    • this material solidified to form the basalt of oceanic crust
    • the new seafloor then moved away from the ridge axis leading to the widening of the ocean basin, seafloor spreading

     

 

 

 

 

 

Evidence for Seafloor Spreading

  • for a hypothesis to become a theory, researchers must demonstrate that the idea really works
  • during the 1960s, geologists found that the seafloor-spreading hypothesis successfully explains several previously baffling observations:
    • the existence of orderly variations in the strength of the measured magnetic field over the seafloor, producing a pattern of stripes called marine magnetic anomalies
    • the age of the oldest deep-sea sediment, relative to the position of the ridge axis

 

 

Marine Magnetic Anomalies

Recognizing Anomalies

  • geologists can measure the strength of Earth's magnetic field with an instrument called a magnetometer
  • at any given location on the surface of the earth, the magnetic field that you measure includes two parts:
    • one produced by the main dipole of the Earth generated by circulation of molten iron in the outer core
    • the magnetism of near-surface rock
  • magnetic anomaly:
    • the difference between the expected strength of the Earth's main dipole at a certain location
    • the actual measured strength of the magnetic field at that location

 

  • positive anomalies: places where the field strength is stronger than the expected
  • negative anomalies: places where the field strength is weaker than expected

 

  • geologists towed magnetometers back and forth across the ocean to map variations in magnetic field strength:

 

 

 

 

 

 

 

 

 

  • What causes positive anomalies and negative?
  • magnetic forces of the Earth and in rock can enhance or subtract from each other:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Evidence From Deep-Sea Drilling

  • in the late 1960s, a research drilling ship, the Glomar Challenger set out to sail around the ocean drilling holes into the seafloor

 

 

 

 

 


 

 

What Do We Mean by Plate Tectonics?

  • The paleomagnetic proof of continental drift and the discovery of seafloor spreading set off a revolution in geology in the 1960s and 1970s.
  • geologists realized many of their existing interpretations of global history based on fixed positions of continents were wrong!
  • Researchers focused their attention  to studying the broader implications of continental drift and seafloor spreading
  • it became clear that these phenomena required that the outer shell of the Earth be divided into rigid plates that move relative to each other
  • New studies:
    • clarified the meaning of a plate
    • defined the types of plate boundaries
    • related plate motions to earthquakes and volcanoes
    • showed how plate motions can explain mountain belts and seamount chains
    • outlined the history of past plate motions
  • from these, the modern theory of plate tectonics evolved

 

 

 

 

The Concept of a Lithosphere Plate

  • the outer part of the Earth has two layers:
    • the lithosphere consists of the crust plus the top (cooler) part of the upper mantle
    • it behaves relatively rigidly, meaning when a force pulls or pushes on it, it does not flow but rater bends or breaks:

 

 

 

  • the lithosphere floats on a relatively soft , or "plastic", layer called the asthenosphere
  • the asthenosphere is composed of warmer (>1,250oC) mantle that can flow very slowly when acted on by a force
  • as a result, the asthenosphere convects, like water in a pot, although much more slowly

 

  • continental and oceanic lithosphere differ markedly in their thicknesses
    • continental lithosphere: has a thickness of 150 km
    • oceanic lithosphere: 100 km thick

 

 

 

 

 

The Basic Principles of Plate Tectonic Theory

  • the Earth's lithosphere is divided into plates that move relative to each other
  • as a plate moves, its internal area remains mostly, but not perfectly, rigid and intact.
  • rock along plate boundaries undergoes intense deformation:
    • cracking
    • sliding bending
    • stretching
    • squashing
  • the continents that form part of the plates move too
  • because of plate tectonics, the map of Earth's surface constantly changes

 

 

 

 

 

Identifying Plate Boundaries

  • earthquake locations allow us to identify plate boundaries

 

 

 

  • earthquakes are vibrations caused shock waves that are generated where rock breaks and suddenly slips along a fault
  • epicenter: marks the point on the Earth's surface directly above the:
  • focus: the location where the rocks broke or slipped

 

 

Plate Boundary Types

  • divergent
  • convergent
  • transform

 

 

Earth: The Power Of The Planet (BBC Documentary, 1:13 min segment on plate motions)

 

 

 

 

 

 

 

 

 

 

 

 

Divergent Plate Boundaries & Seafloor Spreading

  • seafloor spreading takes place
  • new lithosphere develops
  • the process takes place at mid-ocean ridges
    • ridges rise 2 km above abyssal plain

 

 

 

 

 

 

 

Mid-Ocean Ridge In Detail

 

 

 

 

How Does Oceanic Crust Form at a Mid-Ocean Ridge?

 

 

 

  • some of the liquid magmas rises higher to fill vertical cracks, where it solidifies and forms wall-like sheets called dikes, of basalt
  • some magma makes it all of the way to the surface of the seafloor at the ridge axis and spills out of small submarine volcanoes as lava
  • this lava cools in a layer of blob-like shapes call pillow basalt:

 

 

 

Pillow Basalt Video (50 seconds)

 

 

 

 

  • observers in research submarines have found places along ridge axes where hot, mineralized water spews out from small chimneys called black smokers or hydrothermal vents

 

 

 

 

Giant Tube Worms: Up to 4 meters long

 

 

 

 

 

 

 

 

 

 

  • the tension (stretching force) applied to newly formed solid crust as spreading takes place breaks the crust, resulting in the formation of normal faults
  • slip on the faults cause divergent-boundary earthquakes and produces numerous cliffs, or scarps, that trend parallel to the ridge axis

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Convergent Plate Boundaries & Subduction

  • when two plates (at least one oceanic plate) move toward one another
  • one oceanic plate bends and sinks
  • sinking process is subduction
  • subduction forms deep ocean trenches
  • amount of subduction equals amount of seafloor spreading

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Earthquakes & Subducted Plates

 

 

 

 

 

 

 

 

 

Geologic Features of Convergent Boundaries

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Transform Plate Boundaries

 

 

 

 

 

  • slip only occurs along the segment of the fracture zone between the two ridge segments
  • transform boundaries are defined by a vertical fault

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Special Plate Locations

 

Triple Junctions

  • where three plate boundaries intersect
  • where the Southwest Indian Ocean Ridge intersects two arms of the Mid-Indian Ocean Ridge is a ridge-ridge-ridge triple junction:

 

 

  • the triple junction north of San Francisco, where the Cascadia trench, the San Andreas fault, and the Mendocino fracture zone intersect, is a trench-transform-transform triple junction

 

 

 

 

Hot Spots

  • most volcanoes are plate-boundary volcanoes formed along mid-ocean ridges or volcanic arcs
  • some volcanoes are not formed on plate boundaries
  • they exist as isolated points call hot spots

 

 

 

 

 

 

 

 

  • the Hawaiian Islands serve an example of a hot-spot track
  • about 47 mya the Pacific plate changed direction

 

 

 

 

 

 

 

 

 

 

  • some hot spots are positions on mid-ocean ridges
  • Iceland is a product of hot-spot volcanism on the axis of the Mid-Atlantic Ridge
  • we recognize Iceland as a hot-spot because its volcanic activity yields vastly more volcanic rock than do typical localities along the Mid-Atlantic Ridge
  • the additional magma production built Iceland into a plateau that has risen almost 3 km above normal ridge-axis depths

 

Iceland Geology (add PowerPoint)

 

 

 

 

 

How Do Plate Boundaries Form & Die?

 

 

Continental Rifting

  • continental lithosphere pulls apart

 

 

 

 

 

 

 

  • East African Rift, extends in a north-south direction for over 3500 km.

 

 

 

 

 

 

 

 

 

  • there's a deep trough bordered on both sides by high cliffs formed by faulting

 

 

 

Basin & Range Rift of the Western US

 

 

 

 

 

Collision

  • India: continental-continental collision that started about 40 to 50 mya
  • created Himalayan Mountains

 

  • other continental-continental collisions:
    • Alps
    • Appalachians

     

 

 

 

What Drives Plate Motion?

  • some convection
  • gravity flow along ridge axes
  • slab-pull force

 

 

 

 

 

 

Plate Motion Velocity

 

 

  • at a rate of 10 cm/year a plate can move 100 km in a million years!

 

 

 

 

400 Million Years of Plate Tectonics