Transcript Historical Geology - Lunar and Planetary Institute

Plate Tectonics
A Brief History
of a Unifying
Theory
Plate Tectonics as the
Unifying Concept of Earth Science
Accumulation of
Observations Evidence
 Patterns of continents
 Paleontology
 Geology
 Patterns of sea floor ages
 Patterns of seafloor depth
 Patterns of seafloor
sediments
 Patterns of magnetism
 Patterns of volcanoes
 Patterns of earthquakes
Earth’s Great Puzzle Pieces
•
1620 – Sir Francis Bacon observed
similarities of coasts of Africa and
South America … “no mere
accidental occurrence.” A few years
later it was suggested that they
were once one, but had been
separated by the Flood.
•
1782 – Benjamin Franklin, based on
observed oyster shells on mountain
tops “The crust of the Earth must be
a shell floating on a fluid interior....
Thus the surface of the globe would
be capable of being broken and
distorted by the violent movements
of the fluids on which it rested.”
•
1799 – Alexander Von Humbolt,
German explorer and naturalist,
observed the similarities in the
geology and features of the west
coast of Africa and east coast of
South America (separated by a
valley filled by the flood)
http://pubs.usgs.gov/gip/dynamic/historical.html
1858 - Geographer Antonio Snider-Pellegrini
made these two maps showing his version of
how the American and African continents may
once have fit together, then later separated
A Man and His Model
• Current: Contracting Earth
• 1912: Continental Drift
Observations
• Fit of Continents
• Geology
• Paleontology
• Climate belts
• Pangaea (“all lands”) 300 to 200 Ma
• Breakup 180 Ma
Alfred Wegener
• Rigid bodies moving through
yielding seafloor
Scientific Community says:
Scientific Community says:
No Mechanism to Make Continental Drift Happen
Mechanism for Plate Movement!
• Arthur Holmes (Late 1920’s)
• Interior of Earth has sluggish
convection (transport of heat
from core); hot stuff rises, cool
stuff sinks
• New ocean crust injected into
ocean floor (where?)
Maurice Ewing
• Mapping the seafloor 19471959
• Lockney Texas
• Rice University Trained
• UTMB - Division of Earth
and Planetary Sciences
of the Marine
Biomedical Institute
Maurice Ewing
• Mapping the seafloor 1947-1959
• Surprises:
– Thin sediment
– Basalt crust – glasses
– Age less than 150 Ma (hadn’t
identified a pattern yet)
– Ridges – later shown to circle
globe
– Valley within ridge (Tharp)
– Earthquakes along ridges
– High heat flow (Bullard)
Harry Hess and Seafloor Spreading
• 1962 – startling new theory
“History of the Oceans”
• New ocean crust at midocean ridges
• Ocean crust dragged down
at trenches; mountains
form here
• Continental crust too light;
remains at surface
• Earthquakes occur where
crust descends
“It explains everything….”
Rocks and Magnetism - Tools
• When magma cools, takes on
signature of Earth’s prevailing
magnetic field
• Three magnetic measurements
can be taken from rocks
– Inclination - ~ latitude
~distance to the pole
– Declination - ~ direction to the
pole
– Positive (normal) or negative
(reversed) - depending on
what Earth’s field is doing
• Add age = powerful tool
You are Here!
Magnetic Reversals
• Earth’s present magnetic field is called normal
– magnetic north near the north geographic pole
– magnetic south near the south geographic pole
• At various times in the past, Earth’s magnetic
field has completely reversed
– magnetic south near the north geographic pole
– magnetic north near the south geographic pole
– 171 times in last 76 million years … take 5,000 to
10,000 per reversal; last 10’s of thousands to millions
of years … review your storage media …
Vine and Matthews
… The Final Push
Symmetric patterns of magnetism on
either side of mid-ocean ridge
Magnetic Stripes on Seafloor
Seafloor as a magnetic tape recorder
Oceanic Crust Is Young
Age of Continents
Original copyrighted image removed; ;there is an image
available at http://www.lithosphere.info/TC1-
2006/TC1_Fig2-ages-062006.jpg
that may be copyrighted.
Tuzo Wilson
A Final Blow …
• Transform faults: opposite sense of movement than
expected.
• Proven correct (Sykes)
• Sealed theory of sea-floor spreading and plate tectonics
for most scientists
Theory of Plate Tectonics
• The upper mechanical layer of Earth (lithosphere) is divided into rigid
plates that move away from, toward, and along each other
• Most deformation of Earth’s crust occurs at plate boundaries
How can you calculate the
rate of plate movement?
• Pick an object and watch it …
• Better on glaciers than on slow moving
plates …
• Use magnetic reversals … long time
periods
• Date rocks across a mid-ocean ridge really
really carefully … tedious
Hot Spots
• Stationary magma chambers under mobile
plates …
Prominent Hot Spots
Plate Movement Rates
using Hot Spots
http://www.classzone.com/books/earth_science/terc
/content/investigations/es0810/es0810page01.cfm?c
hapter_no=investigation
Tectonics on Other Planets?
Do you recognize
either of these
locations?
Mars Topography
Mars Magnetic Field