Download Class 9 - Earth and Atmospheric Sciences

EAS 109, Dinosaurs
Class 9, Text Page 1 of 2
THE DINOSAURS’ WORLD
PLATE TECTONICS
Tectonic Plates and Their Boundaries — The lithosphere is divided into about a dozen major
tectonic plates defined by boundaries of three types: spreading ridges, subduction zones, and
transform faults, as shown in accompanying diagrams.
The Earth’s Internal (Endogenic) Heat Engine
What’s A Heat Engine? — A mechanism that conveys energy from a heat source (here mainly
rocks of the Earth’s mantle, which contain dispersed radioactive uranium, thorium, and
potassium) to a heat sink (here the surface environment and, ultimately, outer space) by way of a
working fluid (here mantle rock that is hot enough to flow as a very visous fluid, such as window
glass, in response to long-acting stresses) and in the process does mechanical work (here
rearrangement of continents and oceans, mountain building)
The Lithospheric Plates as the Heat Engine’s “Cooling Fins” — Perhaps the pithiest
generalization to come out of the Plate-Tectonic Revolution of the late 1960s and early ’70s is
that the lithosphere is a chemically (crust vs. mantle) and mechanically differentiated
(lithosphere vs. asthenosphere) thermal boundary layer developed in conjunction with
thermal convection in the Earth’s mantle. The lithosphere includes the crust and that part of the
mantle which is cool enough to have the strength to resist permanent deformation in response to
long-acting stresses; rock in the underlying mantle, in contrast, yields to these stresses and flows
as a very viscous fluid.
The Endogenic Cycle — The makings of oceanic lithosphere are cycled out of the mantle at
spreading ridges, where partial melting yields the magma that makes oceanic crust, and back in
again at subduction zones, where continental lithosphere forms through partial melting and
fractionation of subducting oceanic lithosphere and the mantle above it; the mantle portion of the
lithosphere is gravitationally unstable; only the crust, the upper portion, is light enough to float in
equilbrium on the underlying mantle, but oceanic crust is thin enough that is generally subducted
along with the rest of oceanic lithosphere
Mountain Building, Volcanoes, Magma, and the Earth’s Crust — Tectonic activity, seismic
activity (e.g. earthquakes), and volcanoes are concentrated at plates’ boundaries; oceanic crust
(e.g. basalt) forms from magma generated at spreading ridges; continental crust (e.g. granite),
from cooler, more iron- and magnesium-depleted, silica-enriched magma generated at
subduction zones
Sea-Floor Spreading and Continental Drift —see accompanying diagrams
WORLD GEOGRAPHY THROUGH TIME
Charting Sea-Floor Spreading and Continental Drift in Four Dimensions — Paleolatitude,
paleolongitude, paleodepth or paleoelevation, and geological age
The History of Pangaea — See accompanying diagrams
Tectonics and Life’s Environment
Biodiversity-Area Effect —For any given taxonomic group, the number of species or genera or
higher taxa represented on a biogeographic island (i.e. a geographic island, a continent, a lake) is
proportional to the fourth root of the island’s area. Since species typically last only a few million
years and (unlike humans) tend to be limited to a single continent, continents’ collision promotes
lowered biodiversity (fewer islands); a continent’s breakup, higher diversity (more islands).
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EAS 109, Dinosaurs
Class 9, Text Page 2 of 2
INFLUENCES OF GEOGRAPHY ON SEA LEVEL AND CLIMATE
The Age of the Ocean Floor — The sea-floor subsides as it cools, the amount depending on the
time the lithosphere has had to cool and thicken; the overall mean age of the ocean floor is
determined by the heat supplied from beneath by mantle convection; when the midocean ridge
system lengthens and sea-floor spreading speeds up (as when a supercontinent breaks up), ocean
basins’ depth decreases relative to midocean ridges and sea level rises relative to continents,
flooding them with water that tends to buffer seasonal temperature fluctuations; when the
midocean ridge system shortens and sea-floor spreading slows (as when oceans close and
continents collide), ocean basins’ depth increases relative to midocean ridges and sea level falls
relative to continents, decreasing the area of sea and thus water’s buffering of seasonal
fluctuations.
Climate and Geography — The amount of water stored in continental ice sheets as opposed to
ocean basins; a kilometers-thick ice sheet can grow only on a continent, generally a continent at
or near a pole; this is because ice flows under its own weight and because ; where ice is
grounded on bedrock over a continent-sized area, a pile can grow kilometers thick at its center,
but where ice floats in water, it can grow no more than a few meters thick before it begins to
collapse under its own weight.
Influences of Geography on Climate, Ocean Circulation, Burial of Organic Matter, and
Atmospheric Oxygen Level
The Carbon and Oxygen Cycles — buildup of atmospheric oxygen from oxygenic
photosynthesis requires burial of organic matter:
CO2 + H2O → CH2O + O2
The Earth’s Glacial (“Icehouse”) and Non-Glacial (“Hothouse”) Modes — Ice sheets can
form only on a continent at or near a pole
Deep Oceanic Circulation, Burial of Organic Matter, and Atmospheric Oxygen —
Thermohaline (glacial) vs haline (non-glacial); haline deep oceanic circulation during the
equable (ice-free?) Mesozoic may have promoted increased burial of organic matter and thus
elevated atmospheric O2 levels, and may perhaps have made possible dinosaurs’ large size; if so,
a drop in atmospheric O2 to modern levels near the end of the Cretaceous could have promoted
dinosaurs’ extinction (an interesting, plausible, but quite inconclusively supported 1993 theory).
STUDY QUESTIONS
1. About how many tectonic plates are there today? About how fast do they move? What
are the three kinds of plate boundary? What distinguishes these three? How old is the
oldest oceanic lithosphere? The oldest continental lithosphere?
2. What were the major continents when dinosaurs first appeared? When ornithischian
dinosaurs became extinct? What was Pangaea? What was Gondwanaland? When did
they form? When did they break up?
3. How and why might Pangaea’s breakup have contributed to dinosaurs’ (and mammals’)
diversification? How did Pangaea’s breakup at first make for high sea levels and warm,
equable climates? How and why might subsequent collisions among Pangaea’s
fragments (e.g. North and South America) have contributed to the later Cenozoic drop in
mammals’ diversity? How did Pangaea’s breakup end up promoting climatic cooling and
the ongoing Late Cenozoic ice age?
4. How is deep oceanic circulation driven today? How was it evidently driven during nonglacial times such as the mid-Cretaceous? How does the style of oceanic circulation tend
to affect burial of organic matter and the atmosphere’s oxygen content?
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