Transcript 2015 Revised history of Plate Tectonics

 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
• 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.
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
• 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)
• Current: Contracting Earth
• 1912: Continental Drift
• Observations
•
•
•
•
Fit of Continents
Geology
Paleontology
Climate belts
• Pangea (“all lands”) 300 -200 Ma
• Breakup 180 Ma
Alfred Wegener
• Rigid bodies moving through
yielding seafloor
The same land plant and
animal fossils are found
on separate continents!
• 1926
• Based on
glossopteris
fern
No mechanism to make continental drift happen
• 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
But from
where?
• Mapping the seafloor
1947-1959
• Lockney Texas
• Rice University Trained
• UTMB - Division of Earth
and Planetary Sciences of
the Marine Biomedical
Institute
• 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)
• 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
• 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
• 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 … takes 5,000 to
10,000 per reversal. Lasts 10’s of thousands to
millions of years
Symmetric patterns of magnetism on
either side of mid-ocean ridge
Seafloor as a magnetic tape recorder
magnetic iron-bearing minerals align with
Earth’s magnetic field
Original copyrighted image removed; there is
an image available at
that may be copyrighted.
• Transform faults: opposite sense of movement than
expected.
• Proven correct (Sykes)
• Sealed theory of sea-floor spreading and plate tectonics
for most scientists
• 1960s-1970s
• The result of seawater percolating
down through fissures in the ocean
crust in the vicinity of spreading
centers or subduction zones
• The cold seawater is heated by hot
magma and reemerges to form the
vents. Seawater in hydrothermal
vents may reach temperatures of
over 340°C (700°F)
• Discovered in 1977 while exploring
an oceanic spreading ridge near
the Galapagos Islands
• 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
• 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
Emperor
Seamount
Chain
Midway
Hawaiian
Ridge
Hawaii
• Stationary magma chambers under mobile
plates …