Download oceanic ridges

Earth ~200 million years ago
The
Geologic
Time Scale
Based on
*Age of Fossils
determined by
radiometric dating
Look at
timeline:
Breakup of
Pangaea was
200-190 mya:
time of
dinosaurs!
The Continental Drift
Hypothesis
Proposed by Alfred Wegener in 1915.
Supercontinent Pangaea started to break up
about 200 million years ago.
Continents "drifted" to their present positions.
Continents "plowed" through the ocean crust.
Continental Drift:
Evidence
Geographic fit of South America and Africa
Fossils match across oceans
Rock types and structures match across
oceans
Ancient glacial features
Continental
Drift:
Evidence
Tight fit of
the continents,
especially
using
continental
shelves.
Continental Drift:
Evidence
Animal and plant fossils of
same age and type
Continental
Drift:
Evidence
Correlation of
mountains
with nearly
identical
rocks and
structures
Continental
Drift:
Evidence
Glacial features
of the same
age on different
land masses
show these
used to be one
landmass
Continental Drift: Reactions
Received well in Europe and southern
hemisphere. BUT rejected in U.S.
Lack of a suitable explanation prevented
continental drift’s widespread acceptance
Theory provided evidence for WHETHER it
happened and HOW IT USED TO BE but no
answer to WHAT CAUSED IT.
Conflict remained unresolved
The Rise of Plate Tectonics
During WW II and the Cold War
U.S. Navy needed to map the seafloor to find and
hide submarines. Used sonar to create maps of:
oceanic ridges—submerged mountain ranges
fracture zones—cracks perpendicular to ridges
trenches—narrow, deep gashes
abyssal plains—vast flat areas
seamounts—drowned undersea islands
They dredged up rocks of the seafloor:
included only basalt, gabbro, and serpentinite—
no continental materials.
Other interesting
things they learned
about the seafloor
Marine geologists found
that seafloor magnetism
has a striped pattern
completely unlike
patterns
on land.
Mason & Raff,
1961
Black: normal polarity
White: reversed polarity
Both: very magnetic
Their observations led to:
Hypothesis: Stripes indicate periodic
reversal of the direction of Earth’s magnetic
field.
To test this hypothesis, scientists determined the
ages of the older stripes of basalt AND the
polarity of young basalts using the newly
developed technique
of K-Ar radiometric dating.
The study supported the reversal hypothesis...
Yay!
The Rise of
Plate Tectonics
And then (1962-1963)
geologists realized
that the patterns are
SYMMETRICAL
across oceanic ridges.
The K-Ar dates
show the youngest
rocks at the ridge.
The Rise of Plate Tectonics
Meanwhile, U.S. military developed new, advanced
seismometers to monitor Soviet nuclear tests.
By the late 1950s, seismometers had been
deployed in over 40 allied countries and was
recording 24 hrs/day, 365 days/year.
Besides the occasional nuclear test, it recorded
every moderate to large earthquake on the planet.
With these high-precision data, seismologists found
that activity happens in narrow bands.
How magnetic reversals form at a spreading center
Bands of seismicity—chiefly at trenches and oceanic
ridges
The Theory of Plate Tectonics
“group authorship” in 1965-1970
Earth’s outer shell is broken into thin, curved
plates that move laterally atop a weaker
underlying layer.
Most earthquakes and volcanic eruptions happen
at plate boundaries.
Three types of relative motions between plates:
divergent convergent transform
Tectonic Plates on Modern Earth
Divergent boundaries: mostly in oceans
spreading center forming oceanic ridges
How magnetic reversals form at a spreading center
Divergent
boundaries
also can rip
apart (“rift”)
continents
How rifting of a
continent could lead
to formation of
oceanic crust.
ex: East Africa Rift
ex: Red Sea
ex: Atlantic Ocean
Our current
theory says that
Pangaea was
ripped apart by
such continental
rifting & drifting.
Subduction zones form at convergent boundaries
if at least one side has oceanic (denser) material.
Modern examples of O-C convergent: Andes, Cascades
Major features: trench,
biggest EQs, explosive
volcanic mtns.
Another subduction zone—this one with
oceanic material on both sides. O-O
Modern example of volcanic islands: Japan
Earthquake depth indicates subduction zones
Collision zones form where both sides of a
convergent boundary consist of continental
(buoyant) material.
Modern example of C- C: Alps, Rockies, Himalayas
This probably used to be a subduction zone,
but all the oceanic material was subducted.
Most transform
boundaries
are in the oceans.
Some, like the one
in California, cut
continents.
The PACNAmerican plate
boundary is MUCH
more complex than
this diagram
shows.
Hotspots, such as the one under Hawaii,
have validated plate tectonic theory.
Why do the plates move?
Two related ideas are widely accepted:
Slab pull:
Denser, colder plate sinks at subduction
zone, pulls rest of plate behind it.
Mantle convection:
Hotter mantle material rises
beneath divergent boundaries, cooler material sinks at
subduction zones.
All of this caused by earth’s radioactive,
hot, hot core making the asthenosphere
move in convection currents.
How does convection
work? No one
knows—but they
aren’t afraid to
propose models!
Whole-mantle convection
Two mantle convection cells
Complex convection
Let’s talk about the ages of continental
and oceanic crust…
• Based on all the knowledge we have
gained so far, does continental crust ever
get subducted and recycled into the
earth’s mantle?
• Does oceanic crust get subducted and
recycled into the earth’s mantle?
• Review the slides on convergent
boundaries…
Oceanic crust was first formed 4.5 billion years
ago and continental crust came later but…
•
•
Scientists have found that continental crust is
sooooo much older (up to 4 billion years old)
than even the oldest oceanic crust (about 170
million years old) found so far on the earth
today
Scientists have not found really really old
oceanic crust yet because the old stuff usually
gets recycled
Most oceanic crust is recycled by
subduction at convergent boundaries
•
•
•
•
Oceanic crust subducts because it’s more
dense
New oceanic crust is formed at divergent
boundaries.
Continental crust never subducts because it’s
less dense…the really old continental crust is
still on the surface billions of years later
Review the slides on convergent and divergent
boundaries!!
Field Trip Briefing
The California subduction zone (9’ seismic
line)
Subduction to transform (Atwater animation)
Faults of the Bay Area (SF-SJ maps)
Rock types we’ll encounter
Landslide north of Mussel Rock
Occurred between 2 am and 8 am, 2/21/05