Download chapter 15B - plate tectonics 2

Plate Tectonics II
Modern discoveries supporting Plate
Tectonic Theory
• Symmetry of magnetic polarity across mid-ocean
ridges
• Mid-ocean ridges – underwater mountain chains
that circle the globe and often mimic the shape of
the coastline
• Distribution of earthquakes and volcanoes
• Relatively young age of the oceanic crust (less
than 180 million years) and lack of deep-ocean
sediment
• Deep mantle plumes (aka hot spots).
The Earth’s Magnetic Field
The earth has a
magnetic field,
caused by a
complex
interaction
between the
molten outer
core and the
solid inner core.
The Earth’s Magnetic Field
The earth acts like a
giant (but pretty weak)
bar magnet.
Refrigerator magnets
won’t stick, but a
compass needle will
be deflected.
The lines are the
standard way to depict
the (invisible) magnetic
field. The arrows point
in the same direction
as a compass needle.
Paleomagnetism: study of ancient magnetic fields
• Lava is rich in iron-bearing
minerals.
• As lava cools and solidifies on
the earth’s surface, those
minerals orient themselves
towards the north pole like little
compass needles.
• Once the lava becomes solid,
the iron minerals are “stuck” in
place.
• Ancient lavas tell us the
strength and direction of the
earth’s magnetic field during
geologic history.
Magnetic polarity reversals
•At irregular time intervals, the “magnet turns around”,
due to changes in the inner and outer core.
•Lava that solidified during these reversals allows us to
determine the date of these reversals.
•Volcanic rocks dated to 760,000 years ago in several
locations, including the ski area up at Mammoth Lakes,
California, show evidence of reversed magnetic polarity.
Reversed
Normal
Recent time scale of Earth’s magnetic polarity
Magnetic polarity reversals on the ocean floor
•Development of oceangoing magnetometers
allowed remote mapping
of the magnetic field of
the ocean crust.
•Symmetrical pattern at
mid-ocean ridges could
best be explained by the
sea-floor spreading
hypothesis, which was
still being debated at the
time.
polarity reversals recorded by basaltic flows at
mid-ocean ridges
Seafloor Spreading and Seafloor Magnetization
Oceanic Ridge System
•The Mid-Atlantic Ridge was first discovered in 1872.
•By the 1950s, extensive mapping showed a system of
underwater mountains that circled the globe
• The existence of the ocean ridge system led Harry Hess
and others to formulate the sea-floor spreading hypothesis,
which led to our present-day theory of plate tectonics.
Distribution of earthquakes and volcanoes
The next 2 slides show the world distribution
of volcanoes and earthquakes. In both
cases, scientists observed that they occur
in the same places.
This supported the idea of separate plates,
with interacting boundaries.
Volcano distribution
Earthquake distribution
Evidence from ocean drilling
Lack of sediments at mid ocean ridges supports
idea of seafloor spreading.
Evidence from ocean drilling
• Next slide shows youngest ocean crust
(red) at oceanic ridge system, and
oldest ocean crust (blue) farthest from
the ridge. This supports sea-floor
spreading (note symmetry).
• Oldest ocean crust is about 180 million
years old (myo), while oldest continental
crust is close to 4000 myo (that’s 4
billion!). Where could the old stuff have
gone?
Age of the sea floor
Hot spots and mantle plumes
• Caused by rising stationary plumes of
mantle material
• Volcanoes can form over them
(Hawaiian Island chain)
• Originate at great depth, perhaps at the
mantle-core boundary
• Since plate moves, but plume does not,
it allows us to measure rate of plate
motion.
Figure 15.18
What drives plate motion?
Plate Mantle Convection
• Convective flow in the mantle is the underlying driving
force for plate movement.
• Mantle convection and plate tectonics are part of the
same system. Warm, buoyant rock rises and “cold”
dense rock, e.g. subducting plates, sink.
• Plate tectonic movements are ultimately due to
unequal heat distribution in the earth’s interior.
Slab-pull, ridge-push, slab suction
• Descending oceanic crust pulls the plate in direction of
subduction. This results in suction which pulls the two sides
closer. Slab-pull is likely the main mechanism of plate motion.
• Elevated ridge system pushes the plate away from ridge.
Convection and Tectonics
Two possible
hypotheses of
plate-mantle
convection
– 2-layer model
– whole mantle
convection model