FUNDAMENTALS OF EARTH SCIENCE I

FALL SEMESTER 2018

PLATE TECTONICS
Revolution in Earth Science The Earth System

Understanding Earth 6th edition Earth’s layers
CRUST (solid)
MANTLE (solid)

OUTER CORE (liquid)

INNER CORE (solid)
Understanding Earth 6th editionTectonic plate~70 km**Oceanic crust: av. 7 km
Understanding Earth 6th edition

Continental crust: av. 40 km

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

SUPPORTING EVIDENCE• Differentiated meteorites• Seismic wavesCoreMantle• Field observations (ophiolites, xenoliths ofmantle 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 todensity differences resulting from changes in composition ofcrust and mantle)• High-pressure experiments(diamond anvil cell)• Computer modelsMantle

http://www.jamstec.go.jp/chikyu/e/about/data/ Continental drift (Alfred Wegener, 1880-1930)

Continent are moving and they once formed
one single supercontinent called Pangaea.
A. Snider-Pellegrini (1802-1885), source: USGSSUPPORTING EVIDENCE
1. Jigsaw-puzzle fit of continents
around the Atlantic Ocean

2. Similarities in rock types and ages
on both sides of the Atlantic
3. Similarities in geological structures(orientation of mountain chains)
Appalachian mountain belt
(eastern USA)
Caledonian mountain belt
(NW Europe)“Appalachian-Caledonian orogeny”(~500-400 Ma)
Orogeny = episode of mountain formation

UnderstandingEarth (modified)4. Geographic distribution of fossils of plants and animals

Distribution of some fossils (Gondwana, 300 10 6 yr ago)

USGS

5. Paleoclimate data

Glacial deposits (~300-250 Ma, Permian)found in South America, Africa, India,Antartica, and Australia→ Suggests high-lat. location
Tillite = rock composedof unsorted material
deposited directly byglacial ice and showing
no stratification

Hamilton and Krinsley (1967)

Glacial erratic

Glacial striation

Wikipedia

Robert Siegel (Stanford Uni.)

Glacial till (moraine) – coarse unsorted sediment
Glacial valleyin fine-grained (clay) matrix

Wikipedia (Mick Knapton)

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

http://www.snowballearth.org

 What is the driving force of continental drift?
Arthur Holmes (1890-1965) suggested in 1928 that convection currents resulting
from the heat generated by radioactivity within the Earth’s interior could push andpull continents apart (note: radioactivity discovered in 1896 by Henri Becquerel)

BUT LACK OF EVIDENCE… UNTIL…
Figure from Holmes’ article published in 1928, source: Gohau (1990)

Understanding Earth 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 atMid-Ocean Ridges (MORs).

SUPPORTING EVIDENCE

1.

Mapping of the Mid-Atlantic Ridge* (MAR) revealed a rift along its axis
Note that oceanographic surveys conducted after WWII benefitted from newtechnologies developed during the war, particularly the SONAR (Sound NAvigationand Ranging) which is used to map the seafloor with great accuracy.

2.

The seafloor is made of young basaltic rock (getting older away from the ridge)* MAR is ~1000-km wide, ~2-km high, discovered in 1872 during the installation of
the transatlantic telegraphic cable

RIFT

Buck and Poliakov (1998)

3. Seismic data: almost all earthquakes in the Atlantic occur along the ridge
Black dots =Locations ofearthquakes

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

Le Pichon (1968)

Understanding Earth Plate tectonics: the unifying theory
The Earth’s surface is divided into rigid plates that are moving relativeto one another.
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 amountain chain (include Subduction Zones)
3. Transform faults where plates slide horizontally past each other.

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

Understanding EarthEverest (elevation = ~8.8 km)

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

NOAA

Rigid, brittle lithosphere (crust + uppermost partof mantle) = tectonic plates
Weak, ductile asthenosphere (layer of theupper mantle on which tectonic plates slide)

Phase change: olivine → spinel(~400 km)Phase change: spinel → perovskite(~650 km)p. 376 of Understanding Earth Earth’s internal heat engine
In the mantle below tectonic plates:
hotter material rises, colder material sinks ( CONVECTION )S.L. Butler simulation (University of Saskatchewan, Canada)

Think about the “miso soup effect”

2 main sources of heat: 1. Original heat (meteorite impacts + contraction)
2. Decay of radioactive elements Plate boundaries
1. DIVERGENT BOUNDARIES
MID-OCEAN RIDGES (MOR)

Understanding Earth

CONTINENTAL RIFTS

Hydrothermal vents
Juan de Fuca Ridge

Mid-Ocean Ridge
characteristic features

Faults Pillow lavas
R/V Atlantis, WHOI

MBARIMBARI

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

The Mid-Atlantic Ridge iscutting Iceland in two.
Schuessler et al. (2009)

Understanding EarthArabian Plate

African Plate

Somali Sub-plate

The East African Rift
Lakes and volcanoes occupy
the depression of the rift.

http://jules.unavco.org

Agostini et al. (2011)

East African Rift – geological features
LAVA FLOWS

alt. 2280 m

4 km

alt. 1730 m→ Volcano height = 550 m
Google EarthEast African Rift – geological features
NORMAL FAULTS

alt. 2130 m

alt. 1800 m→ Fault scarp height = 330 m
Google Earthhttp://www.geology.com

Eastern wall of African Rift (Ethiopia)

Eastern wall of African Rift (Kenya)

Google EarthOl Doinyo Lengai (Tanzania)

Gregory Dimijian / Science Photo Library

Ethiopian Rift Valley

www.photovolcanica.com

http://ethiopiaembassy.eu/country-profiles/tourism/2. CONVERGENT BOUNDARIESOCEAN-OCEAN CONVERGENCE

Subduction zones

OCEAN-CONTINENT CONVERGENCE

Understanding EarthCONTINENT-CONTINENT CONVERGENCE

Understanding EarthSubduction zone: Japan

Okhostk Pl.
http://usgsprojects.org/fragment/

Eurasian Pl.

Pacific Pl.

Philippine Sea Pl.
Google Earth
Plate boundaries from http://www.earthquakes.bgs.ac.uk/

Nazca Pl.

South American Pl.

Deepest earthquakes at subductionzones (no deeper than 700 km)Scalera (2007)

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

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

Continental collision: Himalaya

USGS

Google EarthSOUTHNASA

3. TRANSFORM FAULTSOCEANIC TRANSFORM FAULTSCONTINENTAL TRANSFORM FAULTS

Understanding EarthOceanic transform faults
WHOI (Tim Shank)

http://www.lostcity.washington.edu

Cont. transform fault: San Andreas Fault
North Am. Plate

Pacific Plate
North American PlatePacific PlateUSGS Rates of plate motion
The magnetic polarity time scale
NormalReversed

Understanding Earth (p. 389)

Magnetic minerals forming inlava (molten rock) align withEarth’s magnetic field lines andpreserve their orientation once
the lava cools and becomes asolid rock.
NB: also useful to reconstruct
the position of old continents.

Physical Geology

Understanding EarthWikipedia

Average spreading rate ~50 mm/year
Highest spreading rate ~150 mm/year

30 km
Understanding EarthPlates moved apart 60 km in 3.3 Ma= 18 mm/yearThis is yet another piece ofevidence that has lead tothe hypothesis of seafloor
spreading and the theoryof plate tectonics.

Magnetic striping mapped byoceanographic surveys offshoreof the Pacific Northwest.
Figure and caption from http://pubs.usgs.gov/gip/dynamic/magnetic.html

Understanding Earth Driving forces of plate tectonics
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)
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2

4
Understanding Earth

Rates of plate movement probably control primarily by 1 and 3

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

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Understanding EarthHand, E. (Science, 2015)

IcelandYellowstone
Hawaii

Galapagos

Canary isl.

Reunion Isl.

Hand, E. (Science, 2015)