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GC1: Lecture 5
Our Shifting Earth – Plate Tectonics
Differentiation of the Earth – Segregation by Density
Int.-C.01
W. W. Norton
Depths from Surface of Boundaries
Crust
10-70km
U. Mantle
~700 km
L. Mantle
~3000km
Outer Core
~5000km
Inner Core
6371km
Interior Structure
Layering by Density: Interiors of terrestrial planets are
layered by density
• Core: Highest density material, mainly nickel and iron
• Mantle: Intermediate density material
• Crust: Lowest density rock
Disproportionate cores
Earth Structure
Crust
•
Continental crust
- Density of 2.7 g/cm3
- Avg. thickness of 35-40 km
- Composition of granite
•
Oceanic crust
- Density of 3.0 g/cm3
- Avg. thickness of 7-10 km
- Composition of basalt
Earth Structure
Mantle
• Solid, though able to flow
below 100-150 km
• Composition of ultramafic
rock peridotite
• Density of 3.5-5.0 g/cm3
• Upper mantle and lower
mantle (based on velocity of
earthquake waves)
• Convection takes place in the
mantle
Earth Structure
Inner & Outer Core
• Outer
- Liquid iron-nickel-sulfur
- Density of 10-12 g/cm3
• Inner
- Solid iron-nickel alloy
- Density of 13 g/cm3
- Inner core is hotter than
outer, but is solid
because of immense
pressure
• Flow of outer core generates
Earth’s magnetic field
HOW IS THE MASS OF THE EARTH DETERMINED?
G= gravitational constant
R= distance between objects
Also, F=ma where a=acceleration due to gravity
Therefore,
M=ar2/G
M= 6.0 x 1024 kg
Fig. 1.05
W. W. Norton
Radius of the Earth 6.3713 × 103 k
Composition of the Whole Earth and Earth’s Crust
Density of the Earth
Whole Earth
Crust
Mantle
Core
5.519 g/cc
2.2-2.5 g/cc
3.5-5.5 g/cc
10-13 g/cc
Validation of Liquid and Solid Core
-- S-Waves do not pass through liquids
-- P-Waves pass through liquids and are refracted by
the Earth’s solid inner core
Earth’s Magnetic Field
Earth’s magnetic field is nearly a dipole with one pole
near geographic north pole and the other near the
geographic south pole
• Magnetic fields are produced by the
motion of electrical charges
-
N
Magnetic field is due to motion of
electrons in the magnet
• Electrical charge on Earth requires:
-
Metallic (conducting) core
-
Convection of core
-
Heat for convection derived from
crystallization of molten core
S
Earth Structure
Mantle
• Solid, though able to flow
below 100-150 km
• Composition of ultramafic
rock peridotite
• Density of 3.5-5.0 g/cm3
• Upper mantle and lower
mantle (based on velocity of
earthquake waves)
• Convection takes place in the
mantle
Seafloor Spreading – Continental Drift – Plate Tectonics
Convection – Transport of Heat and Material
OVERVIEW
SEAFLOOR SPREADING AND
PLATE TECTONICS
1. The Observations
Continental Shape
2. Continents were once together as Super-Continent
3. The Proof
Ocean Floor Magnetics
Geological Structures, Fossils, History
Seismic Tomography – Imaging the Earth’s Interior
4. Understanding the Mechanism for Seafloor Spreading
Seafloor Spreading – Continental Drift – Plate Tectonics
Continental Drift
Continental drift: hypothesis that the continents are
mobile
Poor, ridiculed Alfred
• Proposed by German meteorologist
Alfred Wegener in his 1915 book,
“The Origins of Oceans and
Continents”
• Asserted that there had once been a
supercontinent, Pangaea
• Evidence included:
- Fit of the continents
- Location of glaciations
- Paleoclimate evidence
- Fossil distribution
- Rock type and structural similarities
Continental Drift
Fit of Continents
Glaciation
Glaciations: past times of continental ice
• Evidence of Permian (~300
Ma) glaciation was found in
South America, Africa, India,
Australia, and Antarctica
• Today India and Africa are
near the equator
Paleoclimate
Climate zones– From the Tropics to the Poles
• If southern Pangea was at the
South Pole in the Permian,
Southern North America and
Europe and North Africa should
have been equatorial.
• This matches the Permian
distribution of:
- Tropical coals
- Tropical reefs
- Subtropical deserts
- Subtropical evaporites
Fossil Distributions
Identical fossils on widely separated land
• Mesosaurus – an estuarine reptile.
- From South America & South Africa
only
• Glossopteris – extinct seed fern
- Found in India, Australia, South America,
Africa
- Heavy seeds, subpolar climate
• Lystrosaurs – therapsid, reptilelike mammal
- Africa, India, Australia
• Cynognathus – mammal-like
reptile
- Africa, South America
Geological Affinities
Similar geological features on
separate continents
•
•
•
•
•
Geological structures
Mountain belts
Mineral deposits
Rock types
Rock ages
Caledonides
Appalachians
Yes!!! The Geology is Compelling
BUT WHAT IS THE MECHANISM
Criticisms of continental drift: primary criticism was that
there was no mechanism
•
Wegner’s ideas
- Continents plowed through the ocean crust, much like ice
breakers cut through ice.
- Invoked rotational (centrifugal) forces.
- Neither of these ideas were tenable.
•
Prevailing thought
- Ocean and continents were permanently fixed
•
Continental drift wasn’t widely accepted until the 1960s
- Paleomagnetic evidence of continental movement
- Seafloor spreading
- Plate tectonic theory
Earth’s magnetic field
Magnetic field: Earth’s magnetic field results
from convection of the liquid outer core
around the solid inner core
• The field acts like a giant bar magnet.
– It has “north” and a “south” ends.
– The bar is tilted ~11° from the rotation axis.
– The position of the bar gradually shifts.
• Earth’s magnetic field is strong.
– It extends out into space.
• North Magnetic Pole: point on Earth’s surface at
which magnetic field points vertically downward
• Geographic North Pole: point where Earth’s axis of
rotation meets Earth’s surface
Earth’s magnetic field
Magnetic field
• Declination – Difference between
magnetic north and geographic (true)
north. Depends upon…
- Longitude
- Absolute position of the 2 poles.
- Changes with time
• Inclination – Tilt of a compass needle
from the horizontal. Depends upon…
- Latitude
- Normal or reverse polarity.
Paleomagnetism
Paleomagnetism: study of fossil magnetism
• Below 350-550°C, iron-bearing
minerals in rock will record magnetism
- Thermal energy slows atoms.
- Dipoles align with Earth’s field.
- Material becomes a permanent magnet.
• Sediments can also preserve magnetic
information.
- Iron bearing sediments will align with
the magnetic field.
Polar Wandering
Polar wandering: migration of magnetic pole over surface
of Earth through geologic time
• Paleomagnetic directions
- Different than modern
- Changed through time
• Originally thought to indicate path of pole through time (I.e., polar
wandering)
Apparent Polar Wandering
Apparent polar wandering: signature of a fixed pole on a
wandering continent
• Polar wandering paths from North America and Europe are not the
same!
- Indicates that the lava flows themselves must move-- evidence of
continental drift
• Polar wandering paths align when continents assembled
Ocean Floor
Mapping of the ocean floor in the 1950s led to new
ideas
• Bathymetry: depth of ocean
– Deepest parts of the ocean (trenches)
were not in the middle, but near land.
– Mountain range (the mid-ocean ridge)
runs through every ocean basin.
– Ocean deepens toward the coasts.
– Chains of submarine volcanoes run in
straight lines across ocean floors.
Ocean Floor
More observations from mapping
• Oceanic crust is covered by
sediment.
- Thickest near the continents.
- Thinnest (or absent) at the mid-ocean
ridge.
• Oceanic crust is mafic (basalt and
gabbro).
- No granitic rocks.
- No metamorphic rocks.
• High heat flow characterizes the mid-
ocean ridge.
• Earthquakes
- Found at mid-ocean ridges, trenches
Sea Floor Spreading
Seafloor spreading: explanation for ocean floor
observations by Henry Hess in 1960
• Theory of “sea-floor spreading”.
– Upwelling mantle erupts at the mid-ocean ridges.
– This new sea-floor pushes away from the ridge.
– The sea-floor moves across an ocean gathering sediment.
– At trenches, the sea-floor dives back into the mantle.
• Provided a mechanism for continental drift.
Magnetic Reversals
Magnetic reversals: change in the magnetic polarity of
Earth’s magnetic field
• Magnetic polarity: direction of north-end of
magnet
• Reverses abruptly through time with the time
interval variable
• Cause not known
• Geologists have dated these reversals and use
them as a tool to further subdivide geologic time
Magnetic Anomalies
Proof of seafloor spreading
• Magnetometers towed behind ships sensed magnetic signatures from the
oceanic crust.
• Perpendicular to the mid-ocean ridge, the magnetic field exhibit
systematic patterns magnetic field intensities related to the reversed and
normal polarities of the underlying rock
• These are called positive and negative magnetic anomalies.
• Anomalies form linear belts that parallel mid-ocean ridges.
MAGNETIC REVERSAL STRIPES ARE SYMMETRICAL
Fig. 3.24b
W. W. Norton. Modified from
Mason, 1955.
OBSERVATIONS: DISTRIBUTION OF HIGH FLOW OF HEAT
Fig. 3.22
W. W. Norton. Modified from
Rothé, 1954.
OBSERVATIONS: VOLCANIC ERUPTIONS ON SEAFLOOR
AGE OF THE SEDIMENT ON SEAFLOOR – YOUNGEST IN THE MIDDLE
The age of oceanic crust varies with location. The youngest crust lies
along a mid-ocean ridge, and the oldest along the coasts of continents.
Here, the different color stripes correspond to different ages of
oceanic crust. Red is youngest, purple is oldest.
p.68-69d
original artwork by Gary Hincks
Marine magnetic anomalies are stripes
representing alternating bands of oceanic
crust that differ in the measured strength
of the magnetic field above them. Stronger
fields are measured over crust with normal
polarity, while weaker fields are measured
over crust with reversed polarity.
Normal Reversed Mid-ocean ridge
polarity polarity
(normal polarity)
SEAFLOOR SPREADING – YES!!
BUT IF OCEANS GET BIGGER, DOES THE
DIAMETER OF THE EARTH INCREASE?
OBSERVATION: EARTH’S DIAMETER IS CONSTANT
THEN: SOMEWHERE THE EARTH MUST BE DESTROYING
OLDER CRUST.
Plate Tectonics
Plate tectonics: Unifying theory that explains many
geological observations
• Central concepts
- Earth’s outer shell broken into rigid plates
- Plates move relative to one another
- Plates are created at mid-ocean ridges
- Plates are destroyed at subduction zones
- Plate movement changes face of earth
• Plate tectonics has revolutionized earth sciences