CHAPTER 22 - STRUCTURAL GEOLOGY
Continental Drift and Seafloor Spreading
Continental drift is the idea that the large land masses
on the surface of the Earth are slowly moving. In some
cases they are moving apart and in others they are
moving together.
The idea of continental drift is not new. Until the early
1900's, there was no solid argument supporting the
idea. Alfred Wegner, a German geophysicist, brought
together several types of evidence to support the idea.
Wegner assumed that all of the continents were once
part of a single giant continent he called Pangea(all
lands).He proposed that Pangea broke apart into the
continents we know today and that they drifted apart to
their current locations.
Figure 22.1a
Sequence of Events in the Breakup of Panagea
Figure 22.1b
Sequence of Events in the Breakup of Panagea
(continued)
Figure 22.1c
Sequence of Events in the Breakup of Panagea
(continued)
There are three main types of evidence to support his
idea.
1. Biological evidence - Fossils of certain land dwelling
animals and plants have been found in the western part
of Europe and the eastern part of North America.
Similar fossils have been found in South America and
Africa. Other fossils have been found in South America,
Africa, India, Australia, and Antarctica.
A certain species of garden snail is found only in the
western part of Europe and the eastern part of North
America.
All of these organisms would have great difficulty in
crossing the Atlantic Ocean as it is today. The living
organisms may have crossed the ocean on trading boats
as fire ants have. However, the fossilized organisms did
not have boats.
2. Continuity of geologic features - If we examine the
shape of the eastern South American coastline and the
western African coastline, they seem to fit together
fairly well. Furthermore, mountain ranges(Sierra and
Cape) line up when we do this. Other geological
structures in Europe and North America align in the
same manner.
3. Glacial evidence - There is solid evidence that a
glacial sheet covered the southern parts of South
America, Africa, India, and Australia about 300 million
years ago. The current locations of these land masses
would not preclude this from happening with the
exception of India. Since India is now located north of
the equator, a glacial sheet covering India as well as the
other three land masses would have covered the entire
world. Since there is no evidence that this happened,
India must have once been much closer to Antarctica.
Although Wegner's theory offered explanations for all
of these phenomena, it was not generally accepted
because he could not explain how continental crust
could move through much denser oceanic crust.
In 1960, Harry H. Hess, an American geologist
proposed the idea of seafloor spreading as the
mechanism that caused continental drift.
A mid-ocean ridge system stretches through the major
oceans of the world. At these ridges, hot magma rises to
the surface and is cooled, forming new surface rock.
The existing rock surrounding the ridge is then pushed
away from the ridge causing the entire ocean floor to
move away from the ridge.
Evidence that supports the idea of seafloor spreading is
called remanent magnetism. When igneous rock that
contains magnetite(iron oxide) is formed, the magnetite
becomes magnetized in the direction of the Earth's
magnetic field. Over many years, the Earth's magnetic
field has changed direction many times. This has been
determined by dating rocks and determining the
direction of the magnetic fields associated with them.
On either side of the Mid-Atlantic Ridge, bands of
remanent magnetism exist that correspond to the shifts
in direction of the Earth's magnetic field. Furthermore,
the farther the igneous rock is from the Mid-Atlantic
ridge, the older it is.
Figure 22.5
Magnetic Anomalies Distributed in Symmetric,
Parallel Bands on Both Sides of Mid-Atlantic
Ridge
The idea of continental drift has now been accepted.
The modification is that the continents are pushed along
by the moving oceanic crust.
Plate Tectonics
The surface of the Earth(lithosphere) is now considered
to be composed of a series of solid sections called plates
rather than one solid rock.
These plates move very slowly and interact with each
other to produce earthquakes, volcanoes, trenches, and
oceanic ridges. There are about 20 major plates.
Figure 22.6
Map of Tectonic Plates of the World
Movement of the parts of the lithosphere occur as the
float on the asthenosphere. The asthenosphere is solid
rock that is very close to its melting point which makes
it more easily deformable and in fact contains pockets
of lava.
The continents float higher than the ocean basins
because continental rock is less dense. The balance
between buoyancy and weight is called isostasy.
It is thought that convection cells within the
asthenosphere causes motion of the plates. Welling up
of the higher temperature rock at mid oceanic ridges
and sinking of cooler, more dense rock at trenches.
Plate boundaries can be divergent, convergent or
transform.
Figure 22.7
Block Diagrams Illustrating the
Relationship Between Moving Plates
Copyright © Houghton Mifflin Company
22-7
At divergent boundaries, magma wells up from below
and either passes through the rift in the ridge to form
new igneous rock or is deflected horizontally to
contribute to the horizontal motion of the plates. The
new oceanic crust is composed primarily of basalt.
At convergent boundaries, there are three possibilities.
1. Oceanic- oceanic convergence occurs when two
oceanic plates collide. One plate begins to move under
the other and its crustal material is returned to the
asthenosphere. This process is called subduction and
the plate sliding under the other is the subducted plate.
The region where this happens is called the subduction
zone. A deep sea trench like the Marianas Trench which
is 11 km deep and volcanic islands will form in the zone.
Figure 22.10
Diagram Illustrating the Effects Produced by the
Convergence of Two Oceanic Plates
2. Oceanic - continental convergence occurs when an oceanic
plate collides with a continental plate. The oceanic plate is
always subducted beneath the continental plate. This results in
an off shore trench and inland volcanoes. The Andes of South
America and the Cascades in Oregon and Washington state
are the results of this type of convergence.
Figure 22.11
Diagram Illustrating Results Produced by
Oceanic-Continental Convergence
3. Continental-continental convergence occurs when two low
density plates collide. Subduction does occur, but some of the
material is pushed upward to form huge areas of folded rock.
In this manner two continental plates can fuse together to form
one continent. Geologists believe the Himalayas were formed in
this way when the Indian plate collided with the Eurasian
plate.
Figure 22.12
Diagram Illustrating a ContinentalContinental Convergence
At transform boundaries, the plates are sliding past each other
instead of over and under each other. Earthquakes can occur
at any type of plate boundary, but in the U.S. the San Andreas
Fault is a transform boundary famous for causing earthquakes
in California. Crust on the western side of the boundary is
moving roughly north and crust on the eastern side is moving
roughly south. Occasional jerks in that motion along with the
release of energy causes earthquakes.
Earthquakes and the Earth's Interior
The study of earthquakes is called seismology. An earthquake
is a vibrating and sometimes violent movement of the Earth's
surface.
Earthquakes can be caused by volcanic eruptions or artificially
produced explosions. Most are caused by movements of the
Earth's lithosphere along plate boundaries.
As the Earth's plates attempt to move past each other, friction
causes energy to build up in the deformation of rock much like
the deformation of a stick as it bends. When the rocks break
loose, the energy is released in a sudden movement of the
lithosphere. As the rocks adjust to their new position, more
smaller energy releases may occur as aftershocks.
When an earthquake occurs, the point of initial energy release
is called the focus. The location directly above the focus on the
surface of the Earth is called the epicenter. This is the point on
the surface where the effects of the earthquake are the
strongest.
Energy that is released travels in the form of seismic waves.
They can be detected by a device known as a seismograph.
The Richter scale is used to compare intensities of earthquakes.
Most earthquakes fall between 3 and 9 on the Richter scale
with 3 being a barely noticed earthquake and 9 being the
upper limit of the most devastating earthquake known.
Another result of an earthquake is a Tsunami(sometimes called
a tidal wave). It is a large wave in the ocean that is created by
the disturbance of an earthquake. It travels across the ocean
until it nears land. The topology of the ocean floor causes its
water to be pushed up to 30 or more feet high as it reaches and
then floods across land.
http://earthquake.usgs.gov/
Scientists are able to study the Earth's interior by gathering
information from seismic waves. There are two types of waves,
surface waves and body waves. Most of the earthquake damage
is caused by the surface waves which travel through the upper
regions of the lithosphere.
Body waves travel through the earth well below the surface
and consist of two types, p waves and s waves.
P waves are longitudinal waves that cause particles in the earth
to move back and forth in the direction of travel of the wave.
In s waves, particles move at right angles to the direction of
travel of the waves so they are transverse waves. s waves can
only travel through solids. P waves can travel through solids,
liquids, and gasses.
P waves are called primary waves since they travel faster than
s waves which are called secondary waves. By knowing the
speed of the two types of waves and the time difference
between their arrival, scientists can determine the distance to
the focus of an earthquake. Three seismographs can be used to
triangulate the distance to an event and determine exactly
where it happened.
Scientists have used data from seismographs to investigate the
internal structure of the Earth.
The Earth's core is thought to consist of a solid inner core and
a liquid outer core. P waves speed up and slow down as they
pass through the core, s waves can't penetrate the core at all.
Outside of the core lies the mantle. It is made of rocky material
as opposed to the core's metallic composition. There is a
distinct boundary between the core and mantle.
The crust is the thin, rocky layer surrounding the mantle. It is
fron 5 km to 40 km thick.
A sharply defined boundary separates the crust from the
mantle. It is called the Mohorovic discontinuity or Moho for
short.
Figure 22.20
Interior Structure of the Earth
Copyright © Houghton Mifflin Company
22-11
Crustal Deformation and Mountain Building
Folding of the Earth's crust is the formation of a structure
resembling a series of waves due to horizontal or vertical
pressure. If the pressure becomes large enough, layers of rock
will form crests and troughs that show up on the surface as
mountain ridges and valleys although weathering may cause
elevation changes.
The name for a crestlike structure is anticline and the name for
a troughlike structure is syncline. Anticlines open down and
synclines open up. A structure shaped like a syncline could
hold water. A structure shaped like an anticline could not.
A fault is a break in the rock near the surface caused by stress
that moves part of the rock relative to the rest of it. This
motion may be vertical producing uplifts or horizontal
producing compressions or stretching of the Earth's crust.
The fault plane is the surface where the break has occurred.
The hanging wall is the rock on the upper side of the fault
plane and the footwall is the rock on the underside of the fault
plane. Upthrown rock has moved upward and downthrown
rock has moved down.
The three types of faults are normal, reverse and transform.
1. Normal faulting occurs when the hanging wall moves
downward relative to the footwall. In this case, forces are
pulling the crust apart.
2. Reverse faulting occurs when compression forces the
hanging wall to move up and over the footwall. In this case,
forces are pushing the two segments of crust together. A
special type of reverse faulting is called thrust faulting and
occurs when the fault plane is at an angle less than 45 degrees
with the horizontal. This happens in subduction zones.
3. Strike-slip faulting occurs when the forces cause the motion
along the fault boundary to be horizontal with neither section
of rock being significantly uplifted. This type of motion occurs
along a transform boundary and this type of fault is called a
transform fault.
Figure 22.25
Block Diagrams
Illustrating Three
Types of
Faultings
Copyright © Houghton Mifflin Company
22-13
Mountain building occurs primarily at converging plate
boundaries. Mountains are generally classified as volcanic,
fault-block or fold.
1. Volcanic mountains are formed by repeated lava flows that
build up over time near subduction zones. If the plates
involved are both oceanic, then volcanic mountain chains form
on the ocean floor of the overlying plate. These can develop
into islands like Japan and the West Indies.
When the overlying crust is continental, volcanic mountain
ranges develope along the edge of the continental boundary.
The Andes in South America and the Cascades in Washington
and Oregon are examples.
The Hawaiian chain is a group of vocanic islands that
developed near the center of the Pacific plate. It is thought that
there is a "hot spot" that produces new volcanoes and new
islands as the Pacific plate moves over it.
2. Fault-block mountains are believed to have been formed by
normal faulting causing huge pieces of the Earth's crust to be
tilted and uplifted above the surrounding land. Very often
these mountains will have one very steep face along the fault
plane and a gentler slope on the opposite side. Examples of
these are the Sierra Nevada range in California and the Grand
Tetons in Wyoming.
3. Folded mountains are characterized by the presence of
folded rock strata indicating the presence of compression
forces that caused buckling of the rock layers into anticlines
and synclines. Folded mountains can be complex structures
with evidence of faulting and igneous activity.
Many folded mountains contain sedimentary strata which
indicate that they were formed from the bottom of an ocean
basin. Marine fossils have been found at high elevations in the
Himalayas.
The Himalayas were formed by the collision of the Indian plate
with the Eurasian plate. Since the Indian plate continues to
move north, uplifting of the Eurasian plate continues and the
Himalayas continue to increase in height.
P 652 Terms
P 654 Matching, Multiple Choice, Fill in the Blank
P 655 Questions 3, 7, 10, 13, 16, 18, 23, 26