Slow alteration of gene pool by changing landscape over
In the oceans, species don't have to deal with
whole changes in the physical characteristics of their
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:
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
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
Summary of Evidence for Plate Tectonics:
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
- 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.
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 R-process 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:
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:
Some Other Internet Resources on Plate Tectonics:
overview of all of this
the Ocean Floor