EAS-100-51: Earth Science
A. Earliest theories of moving continents
1. During the 17th century, some European map-makers speculated that some continents seemed to "fit" together like a jigsaw puzzle (such as South America and Africa), and must have once been joined together.
iii. Similar rock types were found in mountain ranges on either side of the Atlantic Ocean (Appalachians and Swiss Alps)
Wegener spent much of his life trying to convince the academic world that the continents drifted over geologic time, but he was unable to describe a convincing mechanism or force that was powerful enough to move land masses. Most geologists of that time believed that continents were immobile. However, geologists in South Africa supported many of Wegener's ideas because local geological conditions matched many of his predictions. He died in 1930 while on a field expedition in Greenland.
It had been known for centuries that he Earth actually has
properties of a huge magnet, with a North and a South pole close (but
not coinciding with) the geographic poles. During the 1930s, rocks
and stone tools containing magnetite from several archeological digs
were analyzed with new techniques that showed the earth's magnetic
field of the past was reversed relative to the present. When these
objects were heated in a fire (or by some other process such as
volcanism), the magnetite grains align themselves to the Earth's
magnetic field; when cooled, the objects became "records" of the
geomagnetic field for that particular time. Further investigation
revealed that the Earth's magnetism changed in polarity and intensity
repeatedly over geologic time.
From modern evidence provided by the study of seismic waves and other geophysical evidence, the earth's magnetic field is now believed to be generated by slow convection currents in the liquid iron outer core (which spins at a different speed and direction relative to the solid iron inner core).
A. Oceanographic and geophysical evidence establishes plate tectonics
During the 1960's, the results of ocean floor geophysical surveys
revealed symmetrical patterns of magnetic reversals in the mid-ocean
ridge rock, indicating that the youngest ocean floor rock was at the
ridge, and older rock was further away. This conclusion was further
corroborated by drill cores of deep ocean sediments, which showed the
same age pattern relative to the mid-ocean ridge. Furthermore, no
ocean floor rock was found to be older than 200 million years,
although the age of the Earth was known to be 4.6 billion years old.
This evidence led to the wide acceptance of plate tectonics as a
theory. Some examples of the many unifying features of plate
tectonics include: an explanation of why volcanism and seismic
tremors are associated with plate boundaries, why mountain ranges and
geologic uplift occur, why the development of past life was affected
by plate movement, etc.
B. The layered Earth
Geophysical evidence depicts the Earth's interior as being
constructed of layers, somewhat like an onion. The thin outermost
layer, the crust, consists of two type of rock, broadly generalized
as continental and oceanic crust. The two rock
types differ in mineral composition, and therefore, in density.
Continental crust, which approaches the composition of granite, is
less dense than oceanic crust, which approaches the composition of
basalt. The crust is broken up into about a dozen blocks, or
plates , averaging about 70 km in thickness. The word
"tectonics" comes from the study of regional changes in the Earth's
structure; plate tectonics incorporates the motions
of these plates over geologic time.
C. Spreading Centers and Subduction Zones
The older models of seafloor spreading and continental drift may
be integrated with plate tectonics: continental crust, being less
dense, "rides" on top of oceanic crust. As the seafloor grows larger
at mid-ocean ridges, ocean floor is being destroyed
elsewhere; these areas are called subduction zones - a good
example is the perimeter of the Pacific Ocean. The margin of the
continental crust subducts , or forces the oceanic crust and
sediments downwards into the Earth's interior, where they are