Geography 140
Introduction to Physical Geography
Lecture: Introduction to Tectonic Uplift
IV. Processes of uplift in the earth's crust are the subject of this lecture,
now that we've covered the structure of the planet and the materials
making up the earth's crust.
A. We can conceptualize the levels of landform relief at different
scales.
1. First order relief would be global scale contrasts between
continents and ocean basins, between, say, Africa and the Indian
Ocean or North America and the Pacific Basin.
2. Second order relief would be regional scale contrasts, such as
between great regional mountain systems (such as the Rockies or the
Himalayas) and lower, flatter features, such as plains (e.g., the
Great Plains) and plateaux (e.g., the Tibetan Plateau).
3. Third order relief would be local scale contrasts, such as between
local mountains and hills or mountain ranges and local valleys and
plains. Locally, think of Signal Hill versus the Long Beach
portion of the Los Angeles Coastal Plain. Similarly, you could
think of the Santa Monica Mountains and the San Fernando Valley.
B. The uplift of the earth's crust is very evident at all orders of
relief: As you look at the map of the world, you first notice it's
divided into oceans (71%) and continents (29%). You then notice other
features, such as the great mountain arcs, lesser mountains, plains,
islands. If you had a map of ocean basin topography, you'd also notice
abyssal plains, continental shelves, and, most conspicuous, the mid-
Atlantic ridges and western Pacific trenches. If you had a more
thematic global map, you'd also be struck by the distribution of
volcanoes and earthquakes. In this lecture, I'll summarize a theory
which accounts for all these features of our planet in one fell swoop.
1. This is the plate tectonics theory, also sometimes known by its
older name: continental drift theory. This was first put forward as
a serious theory by Alfred Wegener (1880-1930). Some geotrivia for
you: He was Wladimir Köppen's son-in-law, Köppen being
the guy who came up with the descriptive climate classification you
learned about in the lecture on climate types (which I used to
situate the tree-dominated, shrub-dominated, and grass-dominated
biomes). THAT Köppen.
2. Wegener's life story is rather sad -- and informative. He earned
his doctorate in the field of planetary astronomy back in 1905. He
somehow became more interested in meteorology (the study of weather
and climate, which is probably how he wound up dating Köppen's
daughter?) and left the field of his degree. He got sent off to
fight in the German army in WWI and was seriously injured in 1914.
While recuperating, he started thinking about how weird it was that
South America's east coast seems to fit so tidily into the west
coast of Africa, and the Arabian Peninsula seems to fit into the
notched northeast coast of Africa.
a. Unlike everyone else who'd noticed it before, he started putting
together a theory that maybe, in fact, these different land
masses DID once upon a time fit right into one another and
drifted away subsequently.
b. By 1915, he'd put together more evidence that this could have
happened:
i. The agreement in thicknesses and compositions of
sedimentary rock beds in Africa and South America older
than about 135 million years ago
ii. Biogeographical evidence involving animals whose closest
relatives were on the other continent, and absolutely
identical plant and animal fossils on the two sides in the
135 million b.p. time frame
![[ fossil biogeography of Gondwana, USGS ]](http://pubs.usgs.gov/gip/dynamic/graphics/Fig4.gif)
iii. The finding of tropical and subtropical fossils in
Antarctican coal deposits
iv. The finding of glacial deposits in South Africa.
v. In other words, a lot more than just the oddball line-up of
the coasts.
c. His book, The Origin of Continents and Oceans, came out
in 1915, but geologists and physical geographers thought he was
a meddler untrained and unqualified in their area of interest,
and they dismissed his theory and evidence as a crackpot idea.
The poor schmo never lived to see his theory vindicated, and he
never got a university professorship until a couple years before
he died (and pretty heroically, too) in 1930. Ironically, by
the early 1970s, it was pretty evident that Wegener was right
and the discovery of the mechanisms behind continental drift
gave rise to the plate tectonic theory, which today dominates
geology and physical geography. You can read a short and rather
sad summary of this unfortunate fellow's life by clicking here.
3. Plate tectonics is important not just because it explains the
observations above: It also has other implications.
a. By severing land masses and altering their climates, it has
allowed new species of living beings to evolve, separate from
their relatives (vicariance biogeography, produced by allopatric
speciation). An example is provided by the 25 species of
eucalyptus in one genus, Monocalyptus, that
differentiated in southern Australia because of climate changes
that isolated various populations from one another so that they
couldn't exchange genes with one another anymore).
b. By reconnecting land masses with land bridges, plate tectonics
has allowed the dispersal of various life forms. Horses provide
an example of this. They first evolved in North America and
then crossed the Bering Land Bridge between Alaska and Siberia
(which had been exposed as dry land when the Pleistocene ice
ages caused ocean levels to drop). Horses dispersed into Asia,
Africa, and Europe but they went extinct in North America.
Horses survived in the Old World until the Spanish brought them
back to the New World as a rather temporary military advantage
against the Indians. Some Native American nations on the
periphery of the Spanish Empire quickly took to this new
technology of horseback riding (and guns) and utterly
transformed their traditional livelihoods and military
activities -- as a result, they were able to challenge Spanish
and US hegemony for a long while.
c. Plate tectonic activity has changed the earth's climates through
moving landmasses around, through mountain building (orogeny,
which leads to orographic precipitation, rainshadow deserts, and
higher, cooler climates on mountains and plateaux), and through
diverting warm ocean currents.
d. It's determined the distribution of mineral resources (gold,
diamonds, uranium, coal, oil...).
e. It now produces events of great significance to human societies:
volcanic eruptions, earthquakes, tsunamis.
C. Plate tectonics holds that the lithosphere is broken into several
rafts or "plates" of various sizes:
![[ plates, This Dynamic Planet, USGS ]](http://www.eas.purdue.edu/~braile/edumod/platepuzz/platepuzz_files/image004.jpg)
1. There are seven huge plates:
a. North American, including much of the North Atlantic and Arctic
sea floors
b. South American, including much of the South Atlantic sea floor
c. Eurasian, including much of the North Atlantic and Arctic sea
floors
d. African, including much of the South Atlantic and some of the
Indian Ocean sea floors.
e. Indo-Australian, comprising the Indian Subcontinent and
Australia and including much of the Indian Ocean and a part of
the southwesternmost Pacific Ocean sea floor
i. It should be noted that credible evidence of a geologically
new split between the Indian section and the Australian
section has been presented by scientists at Columbia
University in 1995, which suggests that the Indo-Australian
plate began a process of splitting into two distinct plates
with slightly different relative motions about 8 million
years ago (that would be geologically new)!
ii. The notion is not universally accepted yet, but support is
growing for differentiating the Australian major plate from
the Indian minor plate, so this is a new idea that hasn't
yet had time to percolate into the textbooks!
f. Antarctican, including a wide swath of the southern ocean sea
floors
g. Pacific, comprising most of the Pacific Ocean and not including
any continental-scale landmasses.
2. There are also six minor plates:
a. Nazca, a part of the southeastern Pacific Ocean immediately west
of South America
b. Cocos, in the eastern Pacific just north of the Nazca Plate and
west of Central America
c. Caribbean, most of the Caribbian Sea
d. Arabian, including most of Saudi Arabia, Jordan, Syria, Iraq,
and part of Iran.
e. Philippines, consisting of a diamond-shaped area in the western
Pacific northeast of the Philippines and south of Japan.
f. Scotia, in the South Atlantic just north of Antarctica and east
of the southern tip of South America.
3. There are also a number of smaller platelets, which are located
along the boundaries of larger plates in a sort of crush zone
between them. It turns out that some of these may not really be
small plates but the remnants of much larger plates swallowed uder
other plates.
a. Rivera Platelet, off the southern tip of Baja California.
b. Juan de Fuca, off the coast of northernmost California, Oregon,
Washington, and British Columbia in the northeastern Pacific
i. This platelet is particularly interesting, as it's quite
possibly broken into three smaller platelets:
a. The Explorer Platelet on the north by BC
b. The Juan de Fuca Platelet proper in the center, west of
Washington and Oregon.
c. The Gorda Platelet to the south, just west of
northernmost California.
ii. This group of associated platelets is now believed to be
what's left of a major plate, the Farallon Plate, that is
now buried under western North America and sticks out as
the Juan de Fuca system AND the Cocos Minor Plate and the
Rivera Platelet!
c. Adriatic, in the Adriatic Sea east of Italy
d. Hellenic, in Greece
e. Anatolian, much of Turkey
f. Iranian, most of Iran
g. Fijian, islands of Fiji, north of New Zealand and northeast of
Australia
h. Solomonic, west of the Fijian Platelet, northeast of Papua New
Guinea
i. Bismarckian, west of the Solomonic Platelet, north of New Guinea
4. There are even dinkier plates, called terranes. These are small
bits of lithosphere believed to have been shorn off plates as they
subduct (or get buried under other plates, about which more later),
which subsequently are accreted (or stuck) onto other plate edges,
sometimes in great number. As a result, their pattern of rock
types and chemical composition does not resemble the geology of
surrounding countryside (those that apparetly originated far away
are called "suspect terranes," and those that seem to come from
nearby are called "native terranes"). There are many of them in the
American West, including many in Southern California in the
Transverse Ranges (e.g., the San Gabriels).
D. These move around over the æsthenosphere.
1. The motion is not straightforward in one direction, though, because
plates on a sphere don't move as a mass in a straight line.
Rather, they rotate in a kind of windshield wiper-like motion,
around a rotational axis that can be imagined as a line extending
from the center of the earth to its surface (the intersection of
the rotational axis with the surface could be thought of as the
rotational pole). There is a small amount of absolute motion in a
tight small circle at the plate's rotational pole and larger
amounts of motion in wider circles farther away from the rotational
pole. The theory describing this rotational kind of motion on a
spheroidal surface is called the "Euler Principle" (pronounced
"oiler"). Like I said, it creates a windshield wiper effect, the
upshot of which is it's hard to characterize an entire plate's
direction of drift.
2. That said, here's an attempt to describe several plates' motion,
anyway:
a. The North American plate is moving west-northwest (northwest off
the east coast and actually southeast by the time you look at
California!).
b. The South American plate is moving generally to the northwest.
c. The Nazca plate just offshore is moving east-northeast against
the Andes Mountains).
d. The Cocos plate is moving northeast against Central America.
e. The Pacific plate is moving northwest.
f. The Eurasian plate is moving eastbound (to the northeast in
Europe and to the southeast in China).
g. The African plate is moving northeast against the Mediterranean.
h. The Indo-Australian plate is moving to the northeast, smacking
into Eurasia and the Pacific. The Australian section (or
possibly separate plate) may be moving counterclockwise to the
Indian section, a little bit north of northeast).
i. The Antarctican plate is moving eastbound, rotating around a
point to the east-southeast of Southern Africa.
j. The Philippines plate is moving westbound, right into Eurasia.
k. The Caribbean plate is moving northeast.
l. The Juan de Fuca system of platelets is moving southeast against
the northwest coast of North America.
![[ relative plate motions determined by space geodesy,
NASA ]](http://cddisa.gsfc.nasa.gov/926/world_plate_color.gif)
In the next lecture, I'll talk about how the RELATIVE motions of these plates
create very different geomorphological circumstances. For now, remember
orders of relief; Wegener's argument and evidence he used; the importance of
plate tectonics; the four size classifications of plates; the names,
locations, and general motions of the major plates, minor plates, and the Juan
de Fuca platelet system; how the Juan de Fuca platelet system and the Cocos
plate may relate to the buried Farallon plate; and how the Euler Principle
describes the motion of plates at the surface of the earth.
Document and © maintained by Dr.
Rodrigue
First placed on web: 11/23/00
Last revised: 07/02/07