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Plate Tectonics
The evolution of species on the land is linked to and driven by various climatological and geological
changes that operated on the land surface of the earth. As we will discuss later, the earth currently has
significant climate variations on a timescale of 100,000 years. In addition, over the last 200-250 million
years the earth is experiencing an era go global tectonic motion which makes the land surface a
Dangerous Place to Live:
•Map of recent earthquakes Western USA
•Map of recent earthquakes World
•Map of Current LA Earthquakes
Plate Tectonics means that the crust of the earth is divided into large connected units, all of which are
moving relative to one another and colliding with one another in various ways. The idea of Plate
Tectonics was first published by the German geologist, Alfred Wegener in 1915 but this theory was
largely ridiculed until magnetic mapping of the ocean floor was done in the late 50's. This history of the
acceptance of this theory is summarized here .
Summary of Evidence for Plate Tectonics:
The continents appear that they could fit together like a Jigsaw Puzzle. Just a coincidence?
Similar fossil records are found in Brazil and North Africa --> could just mean that the climates and
hence vegetation and animal life were similar
Similar rock types are seen in Brazil and North Africa --> this is much harder to explain away especially
if the ages of the rock types are similar.
Sea Floor Spreading --> Magnetic mapping of the ocean floor revealed a history of polarity reversals
that formed a mirror image about the Mid-Atlantic Ridge. This indicates that the floor of the Atlantic
Ocean is created at the mid Atlantic ridge due to crustal separation. As magma issues forth and
solidifies, it records the orientation of the earth's magnetic field at the time of solidification. These newly
formed rocks are then carried either East or West of the Mid-Ocean ridge. The floor of the Atlantic
Ocean is thus an Age sequence with newly formed rock appearing at the Mid Atlantic Ridge and the
oldest rocks being at the North American and European Coastlines. The time of transport from the mid
Atlantic ridge to these coastlines is about 210 million years which indicates a total sea floor motion of
about 1.5 inches per year.
Now it is recognized that the surface of the earth can be divided up into roughly 10--12 large scale
plates and perhaps a number of smaller ones as in the case of the Pacific Northwest
Plate Tectonics Continued
Break up of Pangea
The driving mechanism of plate tectonics is a network of
convective heat currents, generated by the hot core of
the earth and which circulate in the mantle. The heat is
provided from the decay of Uranium-238 which is an Rprocess Supernova element. The overall transport of heat
from the core through the mantle is quite inefficient so it
takes a long time for these convective heat currents to
become established. Hence, plate movements are
something which occurs late (i.e. now) in the geological
history of the earth.
The earth's crust is actually a two-component layer. The lithosphere is a thin layer of rock (average density of 2.7 grams
per cc) and "floats" on top of a plastic-like layer called the asthenosphere. Plastic-like materials are weird - they deform
under stress but don't really break. A glacier is a good example of a material that moves and flows plastically. The
convective heat currents in the mantle impinge on the asthenosphere causing deformation and subsequent movement of
the lithospheric plates.
This process can be simulated in your kitchen by putting some jello in a bowl and putting some peebles on top of the jello.
As you shake the bottom of the bowl, the jello deforms but doesn't break and the rocks that float on the jello collide.
(apologies to real geologists for this analogy).
As a result of plate movements, interesting things occur at plate boundaries. In general you don't want to live near a plate
boundary as the earth is active there. About 75% of the world's population does live near these boundaries.
There are three types of plate boundaries:
A divergence zone --> crustal separation --> two plates are moving apart in opposite directions
A convergence zone --> collision of two plates. A collision of a less dense continental plate with a more dense oceanic
plate creates a subduction zone where the denser plate dives (subducts) beneath the less dense plate. A collision
between two continental plates results in general uplift.
A translational zone --> here two plates slide by one another in opposite directions. The San Andreas Fault is the most
well-known (and potentially most deadly) translational interface.
Local Manifestations of Plate Tectonics:
Tectonic Details
Pacific Northwest Details
Find the Fault lines
Mt. Adams
Mt. Rainier
Mt. Lassen
Mt. St. Helens
The Pacific Northewest is an active tectonic zone. One of the prime hazards of active volcanoes is the heavy mudflows
which can result from the sudden melting of their heavily glaciated slopes. Some examples of Northwest Hazards are
shown below:
Volcanoes all in a Line
Recent Lava Intrusions
Basalt Flows in the Pacific Northwest
Mudflows from Mt. Rainier
Break up of Pangea
Climate results from the uneven distribution of heating over the
surface of the earth caused by the earth's tilt. This tilt is the angle
between the earth's rotational axis and its orbital plane around the
sun. Currently this angle is 23.5 degrees.
Long term climate is also affected by the heat balance of the earth
which is driven mostly by the concentration of CO2 in the
atmosphere.
Climate change can result if the pattern or solar radiation is changed
and/or if the amount of CO2 changes. There is abundant evidence
that the earth does undergo climatic change. Climatic change can be
a limiting factor for the evolution of many species.
Climate Change:
Periodic Ice Ages ---> Glacial-Interglacial Periods
•Major Period: ---> 100,000 years; 9 degree F decrease
•Minor Period: ---> 12,000 years; 5 degree F decrease
•Smaller Periods??: ---> 1,000 years; 3 degree F decrease
We currently are nearing the end of a small, minor interglacial period
•Major Ice Ages: Most of the Planet is Covered
•Minor Ice Ages: 40o Latitude and Above. The most recent one of these occurred about 15,000 years
ago and the coverage of the ice sheet is shown here
•Little Ice Age: (1000-1850 AD; Irish Potato Famine; Nisqually Glacier)
Effects of Ice Ages
•Changes sea level by 100 meters exposes continental shelfs
•changes continental shelf composition due to increased deposition during melt
•changes hydrological cycle through less evaporation
•changes landscape Great Lakes; Seattle; Stoss and Lee
•Changes drainage patterns
•changes topsoil characteristics
•massive flooding episodes --> Eastern Washington
Causes of Ice Ages --> needs to be PERIODIC!
•Solar Cycle --> no evidence
•Volcanic Dust in Atmosphere --> why periodic?
•CO2 Cycle --> why periodic? --> factor 2 reduction causes 7 F drop --> Record of CO2 and temperature
variations
•Increased precipitation at poles due to changing orientation of continental land masses
•Changes in mean temperatures of ocean currents --> what mechanism?
The most Probable Cause:
•Orbital Variations of the Earth!
•Eccentricity variation 90,000 years 5 variation around mean value. This is due to Jupiter.
•Change of tilt of axis 24,000 years