PLATE TECTONICS (Mostly) Chapter 2
A. Introduction
1.
Plate Tectonics is a theory that developed very
rapidly over a very brief time span in the 1960s and early 1970s that
represents a revolutionary new way of looking at the earth. It is a major benchmark in the geological
sciences.
2.
Plate Tectonics envisions the earth as similar
to a hard-boiled egg that has been cracked but not yet peeled. The shell of the egg is analogous to the
plates (= lithosphere) except the plates move relative to the earth’s interior
whereas the shell fragments on the egg do not.
3.
There are 8-12 major plates (depending on what
is considered “major”) and a dozen or so “minor” plates. The larger ones are shown in Fig. 1.13, p. 17.
4.
Major Plates to be considered include: Fig. 1.13, p. 17
& Fig 2-14, p.38.
i.
North American Plate
ii.
Pacific Plate
iii.
Eurasian Plate
iv.
African Plate
v.
Indian-Australian Plate
vi.
South American Plate
Note:
a plate surface may consist of continental crust, oceanic crust or both
5.
The thickness of the plate (=lithosphere)
is generally 50-150 miles, which is in the upper mantle. At this depth the T/P conditions are such
that ductile (plastic) flow is possible and the plates can slowly move over the
material below (=asthenosphere) See Fig. 2.24,
p. 47
6.
Plates may move up to several inches per
year. The major forces are concentrated
at the plate boundaries, not the plate interiors.
1.
Continental Drift and Alfred Wegener pp.29-30 (note:
I cover this a bit more extensively than the text)
a.
People have been intrigued by the apparent “map
fit” of the continents since the first accurate world maps began to emerge in
the 1500s and 1600s---but this was just a curiosity and nothing more until 1912
b.
In 1912, Alfred Wegener, a meteorologist by
training, gave a paper at the German Geological Society Meeting in Frankfurt
that, based on geologic and fossil evidence, proposed all of the earth’s
current landmasses (continents) were once joined together in a single
supercontinent which he called Pangaea.
Pangaea subsequently broke up and the continents have been drifting
apart since then. Wagener’s idea envisioned
the continents moving over the ocean basins and was thus different from Plate
Tectonics where the entire surface (both continents and oceans) is divided into
plates.
c.
Wegener continued to develop his theory and
wrote several books on it. Except for a
few geologists in the Southern Hemisphere, most thought his ideas were ludicrous.
d.
In 1926 Wegener was invited to a symposium on
continental drift where the geologists attacked his idea based on his inability
to explain:
i.
The driving force for the plates—Wegener
invoke the centrifugal force of the earth’s rotation but that is clearly insufficient
ii.
Why the brittle continents do not shatter when
dragging over the ocean floor
2.
Differences in the
state of knowledge in 1960 from that in 1926 (why was Plate Tectonics so rapidly
accepted in the late 1960s-early 1970s when continental drift was so strongly
rejected in 1926?
a.
Accurate maps
of the sea floor did not emerge until the mid-1950s---See Fig 2.12, p. 30 these
maps showed strange features on the ocean floor—among other things, long
underwater volcanic mountain chains (mid-oceanic ridges) frequently cut by
perpendicular faults
b.
Unexplained
deep earthquakes in the western Pacific---strain energy necessary for the
earthquake can only accumulate in rocks showing elastic behavior. At a depth of 300-500 miles within the earth,
the T/P conditions are such that this mantle material should show plastic
(ductile) behavior and therefore EQ should not occur here.
c.
Paleomagnetism See pp. 33-37
i.
Iron bearing
minerals in a cooling magma tend to orient with the earth’s magnetic field—much
like a compass needle
ii.
Magnetic
reversals in the earth magnetic polarity See
Fig. 2.8, p. 34
iii.
“Polar
Wandering”: According to the text,
Wegener did some polar wandering constructions based on paleoclimates,
however, the polar wandering curves developed in the 1950’s & 1960’s based
on paleomagnetic studies merited more attention See
Fig. 2.9, p.35—we now know that it is the continents (and ocean basins)
that are “wandering”, NOT the magnetic poles
3.
Sea Floor Spreading p35-37
a.
Harry Hess 1962—general idea
b.
Mid-Oceanic Ridge---symmetry and age
implications
c.
Vine-Matthews magnetic data on Mid-Atlantic
Ridge: confirmed Hess’ theory see Fig. 2.11, p. 36; Fig
2.13. p. 37.
d.
Subduction---because new plate material is being
formed at spreading centers (the Mid-Oceanic Ridges), plate material must also
be destroyed (recycled) somewhere because the surface area of the earth is not
increasing
e.
Subduction occurs along ocean trenches where
plates slowly descend deep into the mantle to be recycled see Fig. 12.17, 2.18, p42.
f.
Cold
lithosphere plates remain brittle to some depth, thus explain how deep seated
EQ can occur at great depths (where material is normally to ductile or plastic
to support EQ & elastic rebound)
4.
Plate Movement
See page
46-47
` a. Convection Cells
b.
Plumes
c.
Ductility at depth allows lithosphere to
move over asthenosphere without crumbling or shattering
5.
Epilogue:
The timing was such that all of these apparently disparate bits of
knowledge came together at just the right time to ensure that the Plate
Tectonic Theory was almost immediately accepted by an overwhelming majority of
geologists in a remarkably short time.
C. Plate History Throughout Geologic Time
1. History of the current continents See Figures on pages 426-427 &436-437
a.
In the early Mesozoic all of the present
continents were together as one super-continent, called Pangaea, the name
Wegener originally proposed.
b.
Between about 200-140 mya
ago Pangaea broke apart into a northern supercontinent (Laurasia)
and a southern supercontinent (Gonwanda)
c.
This was followed by a breakup of these 2 supercontinents; such that the
present day continents were more or less established by early Cenozoic (65 mya)
2.
There is nothing significant about having one or
two super-continents rather than more.
Prior to Pangaea Europe and Asia were separate lands masses whose fusion
yielded the Ural Mountains
3.
Very early history of plates
a.
Obviously, the further back in time one goes,
the less evidence available to reconstruct the past and therefore, the less
reliable the reconstruction.
b.
There are reconstructions that show the
continents and ocean of the earth billions of years ago (none of the current
oceans and continents exist or can be recognized on these maps)
c.
In the earliest days of the solid earth, the
earth was much hotter because of both the residual heat and the heat generated
by radioactive decay (only about half of the earth’s original U-238
remains). Therefore, it follows that the
plates must have been thinner and there were likely a larger number of (smaller)
plates than we see today.
D. Evidence Supporting Plate Tectonics Theory
1. The
theory explains the sea-floor features
2. The
theory explains the “polar wandering” curves (it is the plates “wandering”, NOT
the poles)
3. The
theory explains the deep seated earthquakes
(Subduction of “cold” slab of lithosphere that maintain elastic
behavior)
4. Map
fit of the continents: best fit occurs
at the continental slope, which makes sense p. 30; Fig.
2.4
5. Geologic
similarities across the continents---S. America & Africa (see p. 30; Fig. 2.5 (p. 31) Appalachians extending
from N. Am to
6. Late
Paleozoic (Permian) glaciation in the Southern Hemisphere continents (see Fig. 2.6, p. 31)
7. Distribution
of land dwelling plants and animals in late Paleozoic and early Mesozoic among
the Southern Hemisphere continents. (See Fig. 2.7, p. 32)
8. Increase
of Biodiversity in Cenozoic land dwelling plants and animals
E.
Types of Plate Boundaries (= Margins) (see
pp. 39-43) Note: Table 2.2, p.39
Divergent
Plate Boundaries (plates moving apart; spreading centers) (see pp. 39-41)
a.
Early Stages:
RIFTING----may be continental or oceanic plates; major EQ and volcanic
activity
Example:
EAST AFRICAN RIFT ZONE
(The African Plate is being torn apart---if this continues the
current rift zone will ultimately become a new plate boundary)
b.
1.
Neutral = Transform Plate Boundary (plates
sliding laterally past one another) See pp. 43;
“Transform Boundaries”—may be continental or oceanic crust. Major earthquakes, NO volcanic activity. Example:
2.
Convergent Plate---Both leading edges are
continental crust: No subduction
(continental material can NOT be subducted). Major collision that causes
crustal shortening, which leads to a major mountain building event. Example: the
3.
Convergent Plate---Both leading edges are
oceanic crust: On plate will be
subducted; forms volcanic island arc with trench; results in major volcanic and
major earthquake activity. Example:
4.
Convergent Plate---One leading edge continental
crust; the other oceanic crust: Oceanic
crust is subducted; form volcanic island arc with trench on the ocean side
similar to #4 above. However, unlike #4,
here there is a sediment source, the continent, which is being eroded. With time a very thick sequence of sediment
accumulates and when subduction ends, the continuing compression leads to
formation of fold-belt mountain systems.
Example: WESTERN PACIFIC
MARGIN---Pacific Plate being subducted beneath (among others) the Eurasian
Plate (see fig 19.1, p. 468)