Page 55 - Laboratory Manual in Physical Geology

Introduction Seafloor Spreading Hypothesis
During the 1960s more data emerged in favor of
Looking at a world map, you may have noticed how
Wegener’s Continental Drift Hypothesis. For example,
edges of the continents seem like they could fit together
geologists found that it was not only the shapes
like pieces of a jigsaw puzzle. You are not alone. Francis
(outlines) of the continents that matched up like pieces
Bacon wrote about the fit of the continents in 1620, and
of a Pangea jig-saw puzzle. Similar bodies of rock and
geographer Antonio Snider-Pellegrini made maps in 1858
the patterns they make at Earth’s surface also matched
showing how the American and African continents may
up like a picture on the puzzle pieces. Abundant
have fit together in Earth’s past.
studies also revealed that ocean basins were generally
younger than the continents. An American geologist,
Continental Drift Hypothesis
Harry Hess, even developed a Seafloor-Spreading
In 1915, Alfred Wegener matched up all of the continents Hypothesis to explain this. He hypothesized that
and published a Continental Drift Hypothesis —that all seafloor crust is created along mid-ocean ridges above
continents were once part of a single supercontinent (Pan- regions of upwelling magma from Earth’s mantle. As
gea), parts of which drifted apart to form the smaller mod- old seafloor crust moves from the elevated mid-ocean
ern continents. A hypothesis is a tentative idea that must ridges to the trenches, new magma rises and fills
be tested repeatedly to verify or falsify its validity. Little fractures along the mid-ocean ridge. This creates new
did Wegener know, but his hypothesis would start a 50- crust while old crust at the trenches begins descending
year scientific investigation! Testing the hypothesis would back into the mantle.
require years of verification (finding data that supports the Harry Hess’ hypothesis was supported by studies
hypothesis), falsification (finding data that suggests the showing that although Earth’s rocky body (geosphere)
hypothesis must be false), and comparison with competing has distinct compositional layers (inner core, outer
hypotheses. But the scientific process would yield a unify- core, mantle, and crust), it can also be divided
ing view of Earth’s rocky body (geosphere).
into layers that have distinct physical behaviors.
Two of these physical layers are the lithosphere and
Shrinking Earth Hypothesis asthenosphere. The lithosphere (Greek lithos = rock)
When Wegener proposed that the continents had is a physical layer of rock that is composed of Earth’s
drifted apart, most scientists were skeptical. Wegener brittle crust and brittle uppermost mantle, called
presented no evidence of a natural process that would lithospheric mantle . It normally has a thickness of
force continents apart, and “anti-drift” scientists argued 70–150 km but has an average thickness of about
that it was impossible for continents to drift or plow 100 km. The lithosphere rests on the asthenosphere
through solid oceanic rocks. They favored a hypothesis (Greek asthenos = weak), a physical layer of the mantle
of Earth involving stationary landforms that could rise about 100–250 km thick that has plastic (ductile)
and fall but not drift sideways. They also reasoned that behavior. It tends to flow rather than fracture.
Earth was cooling from an older semi-molten state, so The lithosphere is broken into fragments called
it must be shrinking. The Shrinking Earth Hypothesis lithospheric plates ( FIGURE 2.1 ) , which rest and move
suggested that continents must be moving together, not upon the weak asthenosphere. Zones of abundant
drifting apart. If so, then Earth’s crust must be shrink- earthquake and volcanic activity are concentrated
ing into less space, and flat rock layers in ocean basins along the unstable boundaries ( plate boundaries )
are being squeezed and folded between the continents between the lithospheric plates, and many of the plate
(as observed in the Alps, Himalayan Mountains, and boundaries are visible as linear features on Earth’s
Appalachian Mountains). surface. By the end of the 1960s, this new view of
Earth had emerged, and it has become the unifying
Expanding Earth Hypothesis theory of geology. Unlike a hypothesis, which is only
Two other German scientists, Bernard Lindemann a tentative idea to be tested and evaluated, a theory is
(in 1927) and Otto Hilgenberg (in 1933), an idea that is widely accepted because it has been well
independently evaluated the Continental Drift and tested, evaluated, and verified. Theories evolve from the
Shrinking Earth Hypotheses. Both men agreed with testing and evaluation of one or more hypotheses.
Wegener’s notion that the continents had split apart
from a supercontinent (Pangea), but they proposed Plate Tectonics Theory
a new Expanding Earth Hypothesis (which they According to the Plate Tectonics Theory , Earth’s
developed and published separately) to explain how it lithosphere is broken into dozens of plates (flat pieces)
was possible. According to this hypothesis, Earth was that move about and interact in ways that cause
once much smaller (about 60% of its modern size) and earthquakes and create major features of the conti-
covered entirely by granitic crust. As Earth expanded, nents and ocean basins (like volcanoes, mountain
the granitic crust split apart into the shapes of the belts, ocean ridges, and trenches). Plates are created
modern continents and basaltic ocean crust was exposed and spread apart along divergent boundaries such as
between them (and covered by ocean). mid-ocean ridges and continental rifts ( FIGURE 2.2 ),

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