Transcript Lecture 3 - Introduction to Plate Tectonics

Introduction to Plate Tectonics
A Revolution in the Earth Sciences Peter Wyllie
GLY 2010 – Summer 2015
Lecture 3
1
Alfred Wegener, 1880-1930
German meteorologist
Published a book in 1915 whose
(translated) title is
The Origin of Continents and Oceans
Wegener proposed the idea of
Continental Drift
2
Continental Drift
• Pangaea – supercontinent containing all
land
• Based on evidence available in early
1900’s
3
Wegener’s Evidence
• Fit of continents
when map cut
apart and
rearranged
• Climate
similarities in
adjacent areas
4
Lithologic (Rock) Evidence
• Unusual rocks found only where
continents fit together
• Cratons – cores of continents, strongly
showed this pattern
• Karoo (South Africa) and Santa
Catarina (Brazil) formations appear
identical
5
Fossil
Evidence
• Glossopteris had left leaf remains in large
areas of Southern Hemisphere
• Wegener concluded that southern
continents must have been joined
6
Fate of Continental Drift Hypothesis
• Biggest objection: How to move a continent?
• Wegener died in Greenland in 1930 – before most
people accepted his ideas
• Wegener’s ideas languished until the end of WWII
• Use of submarines during the war spurred research
after the war
• This lead to oceanographic exploration
7
Oceanographic Exploration
• Ocean floors were mapped to add
submarine navigation
• Knowledge gained revitalized Wegener’s
ideas
8
What Does “Plate Tectonics”
Mean?
• Plate = Large, Rigid slab of rock
• Tectonics comes from Greek root meaning
“to build”
9
Plates
• The earth’s surface is divided into
about a dozen major plates
• Composed of lithosphere - crust plus
the extreme outer mantle
• Lithosphere comes from lithos,
meaning stony, and sphere - hard and
rigid
• Lithosphere – extends from the surface
to the top of the mantle
10
Map of Major Tectonic Plates
11
Mid-ocean Ridge Map
12
MOR Video
13
Alvin
• Jan Morton entering
Alvin
14
East Pacific Rise Segment
Computer Generated Image
• Yellow to red shows
high elevation
• Green to blue shows
lower elevation
• Latitude 9° north
15
Asthenosphere
• Behaves as a plastic - a solid that may
deform slowly
• Plastic because it is hot and under
pressure
• Extends a few hundred kilometers
below the lithosphere
• It is entirely in the upper mantle
16
What Supports the Plates?
• Lithospheric plates float on the
asthenosphere, which is denser
than the lithosphere
17
Sea-floor Spreading
• Concept came from oceanographic
investigations
• Uses convection cells, an idea
Wegener would have been familiar
with
18
Convection Cell
• Heat beaker
• Water expands and rises
• It spreads and cools at the
top
• Cool water sinks
19
Harry Hess, 1906-1969
• In Navy during WWII
• Rear Admiral in Naval
intelligence
• Commented that
geologists make good
intelligence officers
because they can work
with incomplete data
sets
20
Hess in WWII
• Keenly interested in geology of
ocean basins
• Used time between battles to
collect data
• Collected echo-sounding surveys
of ocean depths
21
Hess at Princeton
• After WWII, Hess became Professor of
Geology at Princeton University
• Used WWII data to publish a paper called
“History of the Ocean Basins” in 1962
• Paper outlined idea of sea-floor spreading
• Robert Dietz, working independently,
proposed a very similar concept
22
Hess-Dietz Hypothesis
• Asthenosphere contains numerous
convection cells
• Cells cause molten rock (magma) to
rise
• Some magma erupts on surface
• Most magma stays beneath the surface
and spreads, carrying lithospheric
plates with it, and slowly cooling
23
Hess-Dietz Hypothesis, Cont.
• Cooling magma sinks, completing
convection cell
• Mobile sea-floor helped to answer
several puzzles
24
Mid-Ocean Ridge
• Click to start
25
Spreading Center
• Click to start
26
Mantle Convection Cells
• New crust created by
magma hardening at
the Mid-Ocean Ridge
(MOR)
• From other data, we
know the earth is not
expanding
• Crust must be
destroyed somewhere
27
Puzzles Solved
• Why is there so little sediment on
ocean floor?
• What are the rock ages so young?
28
Seismic Evidence
• In 1935, K. Wadati showed earthquakes
occur at greater depths toward the interior
of the Asian continent
• Earthquakes further toward the Pacific
Ocean occurred at shallower depths
• H. Benioff later observed the same
distribution in other regions
29
Age of Ocean Fossils
• Continental fossils are at least 3.5
billion years old
• Oldest marine fossils are about 180
million years
• Since life is though to originate in the
oceans, why aren’t ocean fossils older?
30
Subduction Zones
The key to subduction is the density
of the rock types involved
Density = mass/unit volume
31
Rock Densities
• Continental lithosphere is about
3.00 grams/cubic centimeter
• Oceanic lithosphere gradually
increases in density as it ages,
reaching a maximum value of
about 3.28 grams/cubic centimeter
32
Converging Plates
• When two plates collide, the
denser plate will sink (subside)
beneath the less dense plate
• Density differences as small as 1%
are enough to cause subduction
33
Subduction
34
Plate Movement
• Plates move slowly (up to 15
cm/yr)
• Plates may collide, move apart, or
slide past each other
• Friction during plate movement
often generates earthquakes
35
Asthenosphere Density
• The density of the asthenosphere is
about 3.3 g/cm3
• Density increases with depth below
the surface
36