Continental Drift and Plate Tectonics

1
Continental Drift and Plate Tectonics
From the time maps of the globe became available,
people wondered about the arrangement of the
continents and oceans. Hundreds of years later,
valid explanations were constructed.
2
Early Observations
Leonardo da Vinci and Francis Bacon wondered
about the possibility of the American and African
continents having broken apart, based on their
shapes. This thinking continued up into the
early 20th century, to a meteorologist named
Alfred Wegener.
3
Pangaea
Wegener revived the early idea of continental
drift, contending that all of the present-day
continents were connected, side-by-side, as long
ago as the Carboniferous (300 Myr). He called
the supercontinental mass Pangaea, Greek for
all lands.
4
Wegeners Evidence
Wegeners summary was based on a number of
careful observations
-- matching rock, fossil, glacier, and structural
relations among different parts of different
continents
5
Continental Drift Fossil Evidence
Mesosaurus purely freshwater reptile Glossopteris
seeds too large to be effectively
wind-transported
6
Continental Drift Glacial Evidence
Large ice masses carve grooves in the rocks over
which flow. Such masses tend to flow outward
(generally downhill) from a central locality.
7
Continental Drift Rock Ages
Even before geochronology, the relative
framework of rock ages showed strong correlation
across the Atlantic, as did mountain ranges of
similar age.
8
Mechanism of Continental Drift?
Wegener never lived to see the general acceptance
of continental drift, largely because of the lack
of a mechanism. Wegener considered the buoyant
continents to be plowing through the mantle,
resulting in mountain belts on continental edges.
9
Mantle Convection

Beginning just after Wegeners end, Arthur Holmes
began to
describe mantle heat flow in terms of convection.
Deep materials, hotter than their surroundings
(and hence
buoyant), would tend to flow upward. In
approaching the cool
surface of the Earth, the material would lose its
thermal
energy, cool and sink, having lost buoyancy.
The motion of mantle material put into action by
convection
thus becomes a plausible mechanism for moving
rigid pieces
of the crust over some more actively flowing
mantle material.
10
Mantle Convection
Materials that can flow tend to lose thermal
energy by the convection process. This explains
circulation in a pot of water that is being
heated from below in the same way it describes
the cooling of the Earth.
11
Harry Hess and Marine Geology
From the 1940s to the 60s, Harry Hess made many
key intellectual contributions to the coming
revolution in geologic thought He also
speculated that the continents did not plow
through ocean crust, but that the two are linked
and move as a unit.
-- echo-sounding of sea floor revealed deep sea
features like guyots and seamounts, and the
topography of mid-ocean ridges -- ridges are
areas of high heat flow and volcanic activity --
young age of ocean floor, based on thickness of
sediment
12
Topography and Age of the Sea Floor
thin sediment cover

thick sediment cover
thick mantle "ballast" pulls the whole plate
down

As ocean crust ages, it cools and is less
buoyant. The cool mantle root on this crust
helps pull it down into the mantle, resulting
in deeper sea floor progressively away from the
ridges.
13
Harry Hess and Sea Floor Spreading
Hess rationalized all of his observations into a
system linked by the old Holmes concept of mantle
convection. He conjectured that hot material
rose at the oceanic ridges, thus explaining the
high heat flow and basaltic volcanic activity,
and why the ocean floor is bulged up at the
ridges. The logical next step is that where
continent and ocean meet, at the trenches, ocean
crust is being returned to the mantle at the same
rate it is being generated at the ridges.
14
Sea Floor Spreading
Hess combined his observations with the earlier
ideas of Wegener and the mechanism of Holmes into
the concept of sea floor spreading, which lead
to plate tectonics.
This hypothesis makes a number of testable
predictions.
15
Earths Magnetic Field
The Earth has an invisible magnetic field, which
has been critical to the earliest nautical
navigation all free-floating magnets at the
Earths surface point to magnetic
north. Iron-rich minerals crystallizing from
molten rock will orient towards magnetic north
when they cool below the Curie point, the
temperature above which permanent magnetism is
impossible (580oC for magnetite). Thus lavas lock
in the record of Earths magnetic field when
they form.
How do we measure the magnetism of a rock?
16
Magnetic Reversals
Interestingly, the polarity of the magnetic field
shifts every 0.5 - 1.0 Myr. That means rocks
formed over time will record either normal
magnetic orientation (like today), or reversed.
Since this is a global phenomenon, these changes
can be used for global stratigraphic correlation.
We are apparently headed into a polarity
reversal, to be complete in 3000 yr.
Taking magnetic stratigraphy back in time is
paleomagnetism.
17
Geomagnetic reversals MECHANISM
How does the field reverse? currents in outer
core slowly change direction new computer
model demonstrates how currents flow and field
reverses field weakens and loses dipolar form
while changing direction
18
Geomagnetic reversals CONSEQUENCES
Effects of a future reversal
solar wind will hit Earth more strongly
increased radiation will cause greater skin
cancer disaster is unlikelyEarth has survived
countless reversals in the past
19
Geomagnetic reversals ANOMALIES
Gothenburg flip
worldwide data shows a reversal around 10,500
B.C. some data from same time shows no
reversal coincides with mass extinction and
end of ice age
20
Paleomagnetism on the Sea Floor
An amazing discovery was made when the magnetic
profile of the sea floor around the Mid-Atlantic
Ridge was mapped.
The maps showed parallel magnetic stripes that
were perfectly symmetrical across the ridge axis.
Colored stripes represent rocks with
present-day magnetic orientations (normal
polarity), grey represents rocks with reversed
polarity.
21
Paleomagnetism and Sea Floor Spreading
Vine and Matthews interpreted the magnetic
stripes as products of steady creation of new
ocean crust over geologic time, supporting the
hypothesis of Hess.
22
Magnetic Field Direction and Inclination
Rock magnetism has two components the direction
of magnetic pointing and the inclination of
this with the Earths surface. Magnetic
inclination goes from nearly horizontal at the
equator to vertical at the magnetic pole.
Magnetic North vs True North
Thus, magnetic records give an indication of
where the rock was on the surface when it was
magnetized.
23
Magnetism and Wandering Continents
Another key contribution to the geology
thought-revolution came from paleomagnetic
studies on the continents. It was noticed that
the magnetic pole positions indicated by rocks of
known age were not constant. If magnetic north
remained in an essentially similar position over
Earth history (despite the periodic polarity
changes), then the different magnetic
orientations meant that the continents had moved.
These results showed that some rocks on
continents currently at equatorial positions had
occupied high latitudes in the past.
24
Apparent Polar Wander Paths
25
The Key Features of Plate Tectonics
(1) The Earths crust is constantly being created
and destroyed (recycled). (2) Ocean crust,
formed at divergent margins, is mafic and dense.
(3) As ocean crust ages and cools, its great
density relative to the continents results in
subduction as plates converge. As a result,
old ocean crust cannot persist, whereas old parts
of the buoyant continents can survive for
eons. (4) The other kind of plate margins,
transforms, are parallel to the current motion of
the plates.
26
Testing Plate Tectonics
Like any theory, plate tectonics has been
rigorously tested, and from a startling array of
disciplines. This model is consistent with the
key tests thus far, including
sea floor spreading paleomagnetic paths
age structure of the sea floor and continents
locations and focal depths of earthquakes
seismic tomography hotspot tracks
27
Mechanisms of Plate Tectonics
Ridge-Push
2
1
Mantle
3
drag
convective flow of mantle
28
Mechanisms of Plate Tectonics
4
Plume-Driven
29
Credits
Some of the images in this presentation come
from Plummer, McGeary and Carlson, Physical
Geology, 8/e Hamblin and Christiansen, Earths
Dynamic Systems, 8/e Press and Siever,
Understanding Earth, 3/e Paul Tomascak
(University of Maryland)