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PLATE TECTONICS Revolution in Earth Science The Earth System - - PowerPoint PPT Presentation

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FUNDAMENTALS OF EARTH SCIENCE I FALL SEMESTER 2018 PLATE TECTONICS Revolution in Earth Science The Earth System Understanding Earth 6th edition Earths layers CRUST (solid) MANTLE (solid) OUTER CORE (liquid) INNER CORE (solid)

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PLATE TECTONICS

Revolution in Earth Science

FUNDAMENTALS OF EARTH SCIENCE I FALL SEMESTER 2018

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 The Earth System

Understanding Earth 6th edition

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CRUST (solid) MANTLE (solid) OUTER CORE (liquid) INNER CORE (solid)

 Earth’s layers

Understanding Earth 6th edition

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Oceanic crust: av. 7 km Continental crust: av. 40 km

Tectonic plate Boundary between crust and mantle = Mohorovicic Discontinuity(or Moho)

~70 km

* *

Understanding Earth 6th edition

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SLIDE 5

SUPPORTING EVIDENCE

  • Computer models

Mantle

Core Mantle

  • Differentiated meteorites
  • Seismic waves
  • Field observations (ophiolites, xenoliths of

mantle rocks)

  • Deep drilling (in 2012, the Japanese drilling vessel Chikyu

retrieved rock samples from around 2200 m below seafloor but has not yet reached the mantle; longest borehole in continental crust was drilled in Russia and is about 12 km long)

  • Gravimetry (examines variations in gravitational field due to

density differences resulting from changes in composition of crust and mantle)

  • High-pressure experiments

(diamond anvil cell)

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SLIDE 6

http://www.jamstec.go.jp/chikyu/e/about/data/

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 Continental drift (Alfred Wegener, 1880-1930)

  • A. Snider-Pellegrini (1802-1885), source: USGS

Continent are moving and they once formed

  • ne single supercontinent called Pangaea.
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SLIDE 8
  • 1. Jigsaw-puzzle fit of continents

around the Atlantic Ocean

Appalachian mountain belt (eastern USA) Caledonian mountain belt (NW Europe) “Appalachian-Caledonian orogeny” (~500-400 Ma) Orogeny = episode of mountain formation

Understanding Earth (modified)

  • 2. Similarities in rock types and ages
  • n both sides of the Atlantic
  • 3. Similarities in geological structures

(orientation of mountain chains)

SUPPORTING EVIDENCE

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  • 4. Geographic distribution of fossils of plants and animals

USGS

Distribution of some fossils (Gondwana, 300 106 yr ago)

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  • 5. Paleoclimate data

Glacial deposits (~300-250 Ma, Permian) found in South America, Africa, India, Antartica, and Australia

Hamilton and Krinsley (1967)

Tillite = rock composed

  • f unsorted material

deposited directly by glacial ice and showing no stratification → Suggests high-lat. location

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Glacial erratic

Robert Siegel (Stanford Uni.) Wikipedia USGS

Glacial striation Glacial till (moraine) – coarse unsorted sediment

Wikipedia (Mick Knapton)

Glacial valley

in fine-grained (clay) matrix

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Glacial tillite (Smalfjord Fm, 600 106 yr) and pavement, northern Norway

http://www.snowballearth.org

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Arthur Holmes (1890-1965) suggested in 1928 that convection currents resulting from the heat generated by radioactivity within the Earth’s interior could push and pull continents apart (note: radioactivitydiscoveredin 1896 by Henri Becquerel)

Figure from Holmes’ article published in 1928, source: Gohau (1990)

 What is the driving force of continental drift?

BUT LACK OF EVIDENCE… UNTIL…

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Understanding Earth

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1. Mapping of the Mid-Atlantic Ridge* (MAR) revealed a rift along its axis 2. The seafloor is made of young basaltic rock (getting older away from the ridge)

 Seafloor spreading hypothesis: a mechanism explaining continental drift

Harry Hess (1906-1969) and Robert Dietz (1914-1995) suggested in the early 1960s that continents move apart through the creation of new lithosphere at Mid-Ocean Ridges (MORs). MAR is ~1000-km wide, ~2-km high, discovered in 1872 during the installation of the transatlantic telegraphic cable *

SUPPORTING EVIDENCE

Note that oceanographic surveys conducted after WWII benefitted from new technologies developed during the war, particularly the SONAR (Sound NAvigation and Ranging) which is used to map the seafloor with great accuracy.

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Buck and Poliakov (1998)

RIFT

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  • 3. Seismic data: almost all earthquakes in the Atlantic occur along the ridge

Understanding Earth

Black dots = Locations of earthquakes

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  • 4. Other Mid-Ocean Ridges discovered in the Pacific and Indian Oceans

Le Pichon (1968)

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Understanding Earth

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Three types of plate boundaries can be distinguished:

  • 1. Divergent boundaries where plates move apart and new oceanic

lithosphere is produced (include Mid-Ocean Ridges)

  • 2. Convergent boundaries where plates come together and form a

mountain chain (include Subduction Zones)

  • 3. Transform faults where plates slide horizontally past each other.

 Plate tectonics: the unifying theory

The Earth’s surface is divided into rigid plates that are moving relative to one another.

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  • J. T. Wilson (1908-1993) was the first to describe this model in 1965
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Understanding Earth

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NOAA

Challenger deep (Mariana Trench, depth = ~11 km) Everest (elevation = ~8.8 km)

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Weak, ductile asthenosphere(layer of the upper mantle on which tectonic plates slide) Phase change: olivine → spinel (~400 km) Phase change: spinel → perovskite (~650 km) Rigid, brittle lithosphere(crust + uppermost part

  • f mantle) = tectonic plates
  • p. 376 of Understanding Earth
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In the mantle below tectonic plates: hotter material rises, colder material sinks ( CONVECTION )

S.L. Butler simulation (University of Saskatchewan, Canada)

2 main sources of heat: 1. Original heat (meteorite impacts + contraction)

  • 2. Decay of radioactive elements

Think about the “miso soup effect”

 Earth’s internal heat engine

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  • 1. DIVERGENT BOUNDARIES

MID-OCEAN RIDGES (MOR) CONTINENTAL RIFTS

Understanding Earth

 Plate boundaries

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MBARI MBARI R/V Atlantis, WHOI

Juan de Fuca Ridge

Hydrothermal vents Faults Pillow lavas

Mid-Ocean Ridge characteristic features

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DEEP-SEA VOLCANIC ERUPTION ALONG THE AXIS OF JUAN DE FUCA RIDGE Video capture by ROV Jason in 2011 (NOAA) – depth: ~1600 m

The eruption occurred a few months earlier before the video was taken

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Schuessler et al. (2009) Understanding Earth

The Mid-Atlantic Ridge is cutting Iceland in two.

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The East African Rift

Agostini et al. (2011) http://jules.unavco.org

African Plate Somali Sub-plate Arabian Plate

Lakes and volcanoes occupy the depression of the rift.

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East African Rift – geological features LAVA FLOWS

4 km

  • alt. 2280 m
  • alt. 1730 m

→ Volcano height = 550 m

Google Earth

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East African Rift – geological features NORMAL FAULTS

  • alt. 2130 m
  • alt. 1800 m

→ Fault scarp height = 330 m

Google Earth

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http://www.geology.com

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Gregory Dimijian / Science Photo Library http://ethiopiaembassy.eu/country-profiles/tourism/ www.photovolcanica.com Google Earth

Ol Doinyo Lengai (Tanzania) Ethiopian Rift Valley Eastern wall of African Rift (Kenya) Eastern wall of African Rift (Ethiopia)

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  • 2. CONVERGENT BOUNDARIES

OCEAN-OCEAN CONVERGENCE OCEAN-CONTINENT CONVERGENCE

Understanding Earth

Subduction zones

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CONTINENT-CONTINENT CONVERGENCE

Understanding Earth

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Google Earth

Subduction zone: Japan

Eurasian Pl. Philippine Sea Pl. Pacific Pl.

http://usgsprojects.org/fragment/

Okhostk Pl.

Plate boundaries from http://www.earthquakes.bgs.ac.uk/

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Deepest earthquakes at subduction zones (no deeper than 700 km)

Scalera (2007)

South American Pl. Nazca Pl.

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TED-Ed video 2012 “Deep ocean mysteries and wonders” (David Gallo, Woods Hole Oceanographic Institution)

DEEP-SEA VOLCANIC ERUPTION NEAR A SUBDUCTION ZONE (TONGA TRENCH) Video capture by ROV Jason in 2009 (NOAA) – depth: 1200 m

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USGS

Continental collision: Himalaya

Google Earth

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NASA

SOUTH

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  • 3. TRANSFORM FAULTS

OCEANIC TRANSFORM FAULTS CONTINENTAL TRANSFORM FAULTS

Understanding Earth

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http://www.lostcity.washington.edu WHOI (Tim Shank)

Oceanic transform faults

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North Am. Plate Pacific Plate North American Plate Pacific Plate

USGS

  • Cont. transform fault: San Andreas Fault
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 Rates of plate motion

Understanding Earth (p. 389)

The magnetic polarity time scale

Normal Reversed

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Physical Geology

Magnetic minerals forming in lava (molten rock) align with Earth’s magnetic field lines and preserve their orientation once the lava cools and becomes a solid rock. NB: also useful to reconstruct the position of old continents.

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Understanding Earth

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Wikipedia

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Plates moved apart 60 km in 3.3 Ma = 18 mm/year Average spreading rate ~50 mm/year

Understanding Earth

Highest spreading rate ~150 mm/year

30 km

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Figure and caption from http://pubs.usgs.gov/gip/dynamic/magnetic.html

Magnetic striping mapped by

  • ceanographic surveys offshore
  • f the Pacific Northwest.

This is yet another piece of evidence that has lead to the hypothesis of seafloor spreading and the theory

  • f plate tectonics.
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Understanding Earth

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Rates of plate movement probably control primarily by 1 and 3

  • 1. Gravitational pull of plates sinking into the mantle (plates with a large

portion of their margin subducting are moving faster)

  • 2. Sucking effect of the subducting plates (force acting on overriding plate)
  • 3. Gravitational force related to elevated Mid-Ocean Ridges
  • 4. Heat rising from the Earth’s interior (may initiate break-up of plates)

 Driving forces of plate tectonics

3 1 4 2

Understanding Earth

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  • 1. Slow, diffuse rise of magma beneath spreading centers
  • 2. Fast, narrow plumes of magma known as hot spots (e.g. Hawaii)

 Two types of rising magma

1 2

Understanding Earth

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Hand, E. (Science, 2015)

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Hand, E. (Science, 2015)

Hawaii Galapagos Iceland Yellowstone Reunion Isl. Canary isl.