Download Exam Block #5

Exam Block #5
• Chapter 12 – Earth’s Interior
• Chapter 13 – Divergent Boundaries
• Chapter 14 – Convergent Boundaries
How to study for this class:
1. Read the chapter and answer the Review
Questions. As you read, follow along with
the Chapter Outline (download from our
class site below) and these PowerPoint
Notes.
2. Review each chapter using the GEODe:
Earth CD-ROM in your book.
3. Go online and take the Concept Quizzes
and Chapter Test for each chapter at:
www.prenhall.com/tarbuck
4. Check your grades online at:
http://pages.sbcglobal.net/solanogeo/index.htm
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Page 1 of 20
Chapter 12 – Earth’s Interior
1
2
ƒ At their birth, planets form from an
accumulation of nebular debris (Nebular
Hypothesis) but quickly begin to form layers.
ƒ The planet’s gravity causes the densest
material (iron) to settle to the center and form
the core. Less dense rock rises to form the
mantle and even less dense rock forms the crust.
ƒ The best way to learn about the Earth’s interior
is to dill a hole, but the deepest well in only 12.3
km – not even through the crust! (Earth’s
diameter is 6371 km).
ƒ Most of our knowledge of Earth’s interior
comes from the study of earthquake waves.
Mantle convection.
Gravity and Layered Planets
Gravity and Layered Planets
3
4
Probing Earth’s Interior
ƒ The seismic waves from large earthquakes
(> 6 magnitude) are well-recorded by
seismographs all around the globe and
provide the means to “see” into our planet.
ƒ Energy travels
as wave fronts
but often is
shown as rays
perpendicular to
wave fronts.
• Transition Zone: bottom of
the asthenosphere (upper
mantle).
• D” (D double prime): bottom
of the mesosphere (lower
mantle).
5
Seismic Body Waves
ƒ P waves –
compressional,
travel through
solids and liquids.
ƒ S waves –
shear, cannot
travel through
liquids because
they have no
shear strength.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Nature of Seismic Waves
6
ƒ Travel times of P (compressional) and S (shear)
waves through the Earth vary depending on the
properties of the materials. Variations in the travel
times correspond to changes in the materials
encountered.
ƒ Velocity depends on the density (g/cm3) and
elasticity of the intervening material. Within a given
layer the velocity generally increases with depth due
to pressure forming a more compact elastic material.
ƒ In all cases, P waves travel faster than S waves.
ƒ When seismic waves encounter a sudden change in
physical properties (discontinuity) they may refract
(bend) or reflect (bounce) or diffract (go around).
Page 2 of 20
7
Reflection vs. Refraction
8
Search For Oil and Gas
Q: Which are
reflected and
which are
refracted?
Seismic waves can locate the location of oil
and gas reserves within the Earth.
9
Refracting Ray Paths in the Earth
Refracted
seismic
rays have a
bent path,
indicating
that seismic
velocity
increases
with depth.
Earth’s Crust
11
ƒ Crust – the thinnest of all the Earth’s layers.
Eggshell-thin layer compared to the Earth’s size.
ƒ Oceanic crust - about 7 km thick, basaltic
composition, average density of 3.0 g/cm3, and up
to 160 million years old.
ƒ Continental Crust: average thickness is 40 km,
but more than 70 km thick in mountain belts,
granitic composition (typical Sierra Nevada
batholiths), average density of 2.7 g/cm3, and up
to 4.0 billion years old.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Compositional/Mechanical Layers
Early in
Earth’s
history,
heavier
elements
sank and
lighter ones
floated
upward,
forming a
layered
Earth.
10
Pressure and
temperature
greatly affect
the mechanical
strength of
materials.
Earth’s Major Boundaries – Moho
12
ƒ Moho – boundary that separates the crust
and mantle (Andrija Mohorovičič in 1909).
Identified by a change in the velocity of P
waves with distance from the source.
P wave velocity 6 km/sec
Crust
P wave velocity 8 km/sec
Mantle
Page 3 of 20
The Mantle
13
ƒ Mantle – over 82% of Earth’s volume; it is solid
because S waves travel through it.
ƒ Kimberlite Pipes – are volcanic features
associated with diamonds and have brought up
direct samples of the upper mantle from 200 km in
depth to the surface and they are peridotites.
ƒ Mantle – divided into the:
ƒ Upper Mantle (asthenosphere) – base of
crust to 660 km in depth. Transition Zone at
bottom of the upper mantle.
ƒ Lower Mantle (mesosphere) – 660 km to
top of core. D” at the bottom of the lower
mantle.
Mineral Physics – Diamond Anvil
15
ƒ Seismology alone cannot determine what
the Earth is made of. Mineral samples are
squeezed at great pressures by diamonds and
heated by lasers to simulate conditions in the
mantle.
14
Transition Zone
ƒ At 410 km Depth – The mineral olivine
(main component of peridotite) collapses into
a more compact mineral β-spinel. Density
increases and seismic velocities increase.
ƒ At 660 km Depth –
The mineral β-spinel
collapses into a more
compact mineral
ringwoodite. Density
increases and
seismic velocities
increase again.
16
✓REVIEW QUESTIONS
1. What role does gravity play in the layering of planets?
2. What are two major reasons for the increase in density with
depth within Earth’s mantle?
3. Why is seismology responsible for gasoline prices being so
affordable?
4. List three differences between oceanic and continental crust.
Lower Mantle
17
ƒ In the lower mantle,
olivine and pyroxene
take the form of the
mineral perovskite.
ƒ D” Layer – lowermost
200 km of mantle;
partially molten layer;
seismic velocities
decrease; area may
create mantle plumes
that erupt as hotspots
on the surface.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Core-Mantle Discovery
18
ƒ Richard Oldham
discovered the P
wave shadow
zone in 1906. Beno
Gutenberg
calculated the size
of the core in 1914.
The dense core
refracts rays and
creates a zone
where P waves are
largely absent.
Page 4 of 20
The Outer Core is Liquid
19
ƒ Since S waves
cannot travel
through liquids, the
S wave shadow
zone is much
larger.
ƒ A few weak S
waves do diffract
around the core into
this region, but are
largely absent.
20
The Inner Core is Solid
ƒ Inge Lehmann
discovered the
inner core was a
solid in 1936, by
noting the
refraction of P
waves in this
region.
ƒ The inner core is rotating slightly
faster than the rest of the Earth.
✓REVIEW QUESTIONS
21
6. How do S waves allow us to determine the mantle is solid?
8. What mineral phases occur at the top & bottom of the
transition zone?
9. What layer of the Earth has the greatest volume?
11. True or False: No seismic waves arrive in the shadow
zones? Explain.
The Core
23
22
The Core
ƒ The average density of the core is 11 g/cm3.
Even under extreme pressure, silicate minerals
would not compact enough to account for this
density.
ƒ Since the Earth accreted form meteorites, they
provide important clues to the Earth’s internal
composition.
ƒ Metallic meteorites contain large amounts of iron
compared to the composition of the Earth’s crust
and mantle. Therefore, the core must be enriched
in iron.
ƒ Composition: Core is mostly iron, with 5% 10% nickel, and perhaps some sulfur and oxygen.
Variations in P and S Wave Velocities
24
ƒ Origin – early in Earth’s history, the
heavier iron sank to the core and the lighter
materials floated up to form mantle and
crust.
ƒ In this early stage, the entire core was
probably liquid. However, as Earth began to
cool, iron in the core began to crystallize and
the inner core began to form.
ƒ The inner core continues to grow at the
expense of the other core.
ƒ The Earth’s magnetic field is produced in
the liquid outer core by convection.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Page 5 of 20
Earth’s Temperature
25
ƒ Heat travels by 3 different
mechanisms:
ƒ How did the Earth get so hot?
ƒ (1) by colliding particles during the formation of
the Earth and collision of the Mars-sized object with
the Earth that led to the formation of the moon.
ƒ Convection – moving
material in a fluid-like manner.
This is the primary means by
which heat is transferred within
the Earth’s core and mantle.
ƒ Conduction – slowly moving
of heat through a material on an
atomic scale; important in the
lithosphere.
ƒ Radiation – electromagnetic
radiation of heat into space.
ƒ (2) decay of short-lived radioactive isotopes
(Al-26 & Ca-41).
ƒ Why is the Earth still so hot?
ƒ by decay of long-lived isotopes (uranium,
thorium, and potassium) in the Earth’s crust & mantle.
Mantle Convection
26
Heat Flow
27
28
Seismic Tomography
3-D images of the Earth can be
generated from earthquake waves.
Modeling Mantle Convection
29
Earth’s Temperature Profile
*
*
*
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
30
Geothermal
Gradient or
Geotherm –
change in
temperature with
depth.
Remember: most
of the Earth is
solid due to
pressure –
except for a few
locations (*).
Page 6 of 20
Earth’s Magnetic Field
31
Dipole Field– the
Earth’s magnetic field
has a north and south
magnetic pole (dipole),
just like an
electromagnet and a bar
magnet. But remember
the Curie Point (585° C):
the Earth’s interior is
much too hot to form a
magnetic field from a
solid bar magnet.
Earth’s Dipole Magnetic Field
33
32
Earth’s Geodynamo
Geodynamo – as the
liquid iron in the Earth’s
outer core convects, its
path becomes twisted due
to the Earth’s rotation. This
produces a dipole
magnetic field similar to an
electromagnet. The
dynamo is self-generating
and self-reversing, due to
changing patterns in the
convection currents.
34
✓REVIEW QUESTIONS
13. Why is Earth’s inner core growing in size?
15. What sources of heat caused Earth to get so hot?
16. What keeps the Earth so hot?
17. Explain the difference between conduction and convection.
19. Why is conduction more important than convection within
Earth’s crust?
23. Why is the lithosphere stiffer than the asthenosphere?
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Page 7 of 20
Chapter 13 – Divergent Boundaries
1
2
Mapping the Seafloor
ƒ If all water were drained from the oceans,
the scenery would be nearly as varied as that
on land: great linear mountains, volcanoes,
deep canyons, and large plateaus.
ƒ The first measurements of the depths of the
oceans were made by the H.M.S. Challenger
from 1872 to 1876 using a weighted line.
Q: What is
the old
measurement
of depth used
at sea?
Ocean floor: the last piece of the Plate Tectonic Puzzle.
Mapping the Ocean Floor
3
Seismic Reflection Profile
4
Seafloor Topography from Satellites
6
ƒ A) In the 1920’s, the first echo sounders began to
show topographic features of the ocean floor.
ƒ B) Modern sonar can map narrow segments in
much more detail using multiple sources and receivers.
Seismic Reflection Profile
5
ƒ A satellite altimeter
measures the
variation in sea
surface elevation,
which mimics the
shape of the
seafloor, due to the
gravitational
attraction of the
features on the
ocean floor.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Page 8 of 20
7
Three Major Provinces of the Ocean
(A) Continental Margins, (B) deep-ocean
basins, and (C) mid-ocean ridges.
✓REVIEW QUESTIONS
8
1. Assuming that the average speed of sound waves in water
is 1500 meters per second, determine the water depth if
the signal sent out by an echo sounder requires 6 seconds
to strike bottom and return to the recorder.
2. How can satellites orbiting Earth determine features on the
seafloor?
3. What are the 3 major topographic provinces of the ocean
floor?
Q: Why does the water get deeper away from a ridge?
9
(A) Continental Margins
ƒ Two main types of continental margins have been
identified – passive and active.
ƒ 1) Passive Margins – are found along most of
the coastal areas that surround the Atlantic Ocean.
They are NOT associated with plate boundaries;
therefore, little volcanism and few earthquakes.
Features of passive continental margins include:
ƒ Continental Shelf – submerged part of the
continent; however, it was above sea level during
the last Ice Age when sea level dropped.
ƒ Continental Slope – marks the boundary
between continental crust and oceanic crust.
(A) Continental Margins
11
ƒ 2) Active Margins – occur where oceanic
lithosphere is being subducted beneath the edge
of a continent. The continental shelf is very
narrow, if it exists at all. They ARE associated
with plate boundaries and there is much
volcanism and many earthquakes, like the
Pacific Rim.
ƒ Accretionary Wedge – Sediments from the
ocean floor and pieces of oceanic crust are
scraped from the descending oceanic plate and
plastered against the edge of the continent.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Passive Continental Margins
10
Found around the Atlantic Ocean.
Active Continental Margins
12
Found around the Pacific Rim: Ring of Fire.
Page 9 of 20
(2) Deep-Ocean Basins
13
ƒ Trenches – sites where oceanic lithosphere
plunge back into the mantle. Deepest trench is
11,000 meters below sea level at the Mariana
Trench. Location of volcanoes and earthquakes.
ƒ Abyssal Plains – are the most level places on
Earth in the deep-ocean basins. Flatness is due to
a thick accumulation of sediment that have buried
an otherwise rugged ocean floor.
ƒ Seamounts – These volcanic peaks form near
oceanic ridges and hot spots. As islands move
away from ridges, they cool and sink and are cut
by wave erosion, forming coral atolls and
guyots.
Formation of Coral Atolls & Guyots
Trenches of the World’s Oceans
14
Be able to locate these on a map for the final.
15
Coral Atolls & Guyots
16
17
(3) Mid-Ocean Ridges
18
ƒ Coral reefs are constructed from the
skeletal remains of corals and algae. They are
mainly confined to warm, clear waters of the
Pacific and Indian oceans.
ƒ Darwin sailed the globe in the 1830’s and
correctly proposed a hypothesis for the
formation of coral atolls.
ƒ As a volcanic island slowly cools and sinks,
the coral reefs continue to grow upward, as
long as the depth of less than 45 meters from
the surface.
ƒ If the island sinks more quickly, flat-topped
guyots are formed.
✓REVIEW QUESTIONS
4. List three features that comprise a passive continental
margin. What is the flooded extension of the continent?
Which is the steepest?
5. Compare and contrast active and passive continental
margins and give examples of each.
7. How does a guyot form?
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
ƒ Mid-Ocean Ridges (MOR)– are characterized by
an elevated position, extensive faulting, and
numerous volcanic structures. The MOR system is
the longest feature on Earth at 70,000 km (43,000
miles) in length.
ƒ The ridges are offset by large transform faults.
The central portion is a deep down-faulted
structure called a rift valley. Tectonic plates move
apart at ridges and upwelling magma generates
new slivers of oceanic lithosphere (seafloor
spreading). This process continues in a conveyorbelt fashion.
Page 10 of 20
Mid-Ocean Ridges of the World
19
Be able to locate these on a map for the final.
20
Mid-Ocean Ridge
Why does the lithosphere gradually become
thicker away from an oceanic ridge?
Seafloor Spreading
21
Age of Oceanic Crust
22
(3) Mid-Ocean Ridges
23
Structure of Oceanic Crust
24
ƒ Mid-Ocean Ridges (MOR)– are passive features.
As plates are pulled apart, space is created,
pressure is reduced, causing the upper mantle
rocks to partially melt, producing new oceanic
lithosphere.
ƒ MOR are elevated because the newly created
seafloor is hot, and occupies more volume, and
therefore is less dense. As the ocean floor moves
away from a ridge, it cools and contracts and
becomes more dense.
ƒ The oceanic lithosphere increases in thickness for
about 80 million years and then remains relatively
constant thickness until subducted.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
ƒ The ocean crust consists of four distinct layers,
called an ophiolite complex. They are:
(1) Sediments – settle on top of the seafloor.
(2) Pillow lavas – ejection of magma directly
into water creates these pillow-like features.
(3) Sheeted dikes – vertical fractures that
carried the magma to the surface.
(4) Gabbro – is the coarse-grained equivalent
of basalt and cooled at depth.
ƒ This sequence was discovered by old pieces of
oceanic crust thrust upon the land.
Page 11 of 20
25
Ophiolite Sequence
✓REVIEW QUESTIONS
26
8. Describe the oceanic ridge system.
1
2
9. How are MOR mountains different than those found on
continents?
3
4
10. What is the source of magma for seafloor spreading?
11. Why are MOR topographically elevated?
Continental Rifting
27
ƒ New ocean basins begin with the formation of a
continental rift – breakup of a landmass in
four stages:
1. Thinning of lithosphere, normal faults, grabens:
Basin and Range, Nevada.
2. Formation of narrow rift valley: East African
Rift.
3. Formation of linear sea with basaltic oceanic
crust: Red Sea.
4. Formation of mid-ocean ridge: form new ocean
like the Atlantic Ocean.
Mechanisms for Continental Rifting
28
Continental Rifting
Basin & Range, Nevada
East African Rift Valley
Red Sea
Atlantic Ocean
East African Rift Valley
& Red Sea
29
ƒ Supercontinents (like Pangaea) have existed
sporadically during the geologic past. Two
mechanisms have been proposed for rifting:
1. Mantle Plumes & Hotspots (this gets plates
moving apart) – large amounts of heat and
volcanism (flood basalts) may initiate rifting - as
evidenced by the opening of the Atlantic Ocean –
but forces are thought not to be great enough to
disperse continental fragments.
2. Slab Pull & Slab Suction (this keeps plates
moving apart) – a subducting slab creates a suction
force that pulls the overriding plate toward the
trench – this was also the case on the western edge
of Pangaea as it broke apart.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Mechanisms for Continental Rifting
30
ƒ 130 million years ago: the head of the
Tristen Hotspot arrived and started the
rifting of Southern Pangaea.
ƒ Northern Pangaea had already
started rifting apart from the Iceland
Hotspot.
Page 12 of 20
31
Mechanisms for Continental Rifting
At a subduction
zone, the slabsuction forces pull
the continent
westward toward
the trench (trench
retreat), causing
tensional forces
that contribute to
the breakup of the
continent on the
eastern shore.
Formation of San Andreas Fault
ƒ
ƒ
ƒ
Franciscan Formation
33
32
Trench retreat moved North America westward toward the
spreading center (mid-ocean ridge).
The spreading center subducted into the trench, both were
mutually destroyed, and replaced by a newly generated San
Andreas transform fault which continues to grow in length.
Granite from the southern Sierras broke off from Southern
California and has slid north along the coast to Pt. Reyes
National Seashore.
✓REVIEW QUESTIONS
34
15. Describe the 4 layers of the ocean crust.
The local
Pt. Reyes
Franciscan
Formation is an
accretionary
wedge that
contains a
complex variety
of sedimentary,
metamorphic,
and igneous
rocks.
✓REVIEW QUESTIONS
Granite
16. Describe how the sheeted dike complex forms and the
layer below it.
17. Name a place to see a continental rift.
Granite
18. Why role are mantle plumes thought to play in the rifting of
a continent?
35
20. Explain why oceanic lithosphere subducts even though the
oceanic crust is less dense than the underlying
asthenosphere.
21. Why does oceanic lithosphere thicken away from a MOR?
22. What remains of the Farallon plate?
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Page 13 of 20
Chapter 14 – Convergent Boundaries
1
2
Mountain Building
ƒ Orogenesis – mountain building processes.
ƒ Mountain building has occurred during many
different periods of geologic time:
ƒ Recent Period (0-100 Million Years):
American Cordillera, Andes Mts., Himalaya Mts.,
Japan, Philippines, and Sumatra.
ƒ Older Than 100 Million Years: Appalachians,
Caledonian Belt, Urals and Tasman Belt.
Elk Mountains, Colorado Rockies
Mountain Building
3
Major Tectonic Plates & Mountains
4
Know major mountain belts and ages for the final.
Q: What do you notice about the location of the older mts. & plate boundaries?
Convergence and Subducting Plates
5
ƒ Subducting zones are located along convergent
boundaries and are sites of plate destruction –
where oceanic lithosphere bends and plunges
back into the mantle.
ƒ Features include:(1) deep-ocean trench (a
deep linear feature where the oceanic slab starts
to descend into the mantle); (2) forearc region
(nearest the trench, often form an accretionary
wedge); (3) volcanic arc (volcanic activity on
the overlying plate); and (4) backarc region
(often dominated by tensional forces that thin
and stretch the crust).
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Convergence and Subducting Plates
6
Water-rich
fluids reduce
the melting
point of hot
mantle rock
and melting
occurs –
producing a
basaltic
magma.
Page 14 of 20
7
Aleutian Islands – Volcanic Island Arc
8
Backarc Spreading
Suction Force – think
of being on a lifeboat
as the Titanic sank –
you would be pulled
toward it!
Active backarc basins:
Mariana & Tonga Islands.
Inactive backarc basins:
South China Sea & Sea of
Japan.
✓REVIEW QUESTIONS
9
1. Which type of plate boundary is most directly associated
with mountain building?
2. List the four main structures of a subduction zone.
3. How does a backarc basin form?
4. Describe the process that generates most basaltic magma
at subduction zones?
Andean-Type Margins
11
ƒ As the oceanic lithosphere descends into the
mantle, water is driven from the plate and
partially melts the mantle rocks, which are made
of periodotite. This produces a primary magma
of basaltic composition.
ƒ The newly formed magma is less dense and rises
and ponds at the base of the crust. At this point,
heavier iron-rich minerals settle out, leaving the
remaining melt richer in silica and lighter; this is
called magmatic differentiation.
ƒ New low-density melts continue to rise with a
composition of andesite (intermediate) and with
assimilation form felsic (granitic) batholiths.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Subduction and Mountain Building
10
ƒ There are two types of subduction:
1. Oceanic-Oceanic: When two oceanic
lithospheres converge, the older, colder slab
subducts beneath the younger one, creating
island arc volcanics. Examples include: Aleutian
Islands, Japan, Philippines; Tonga, and Mariana.
2. Oceanic-Continental: Also known as “AndeanType Margin” – oceanic plate subducts beneath
the continental plate; this builds a continental
volcanic arc and well developed accretionary
wedge. Active examples: Andes & Cascade
Range; inactive example: Sierra Nevada & Coast
Range (accretionary wedge).
Andean-Type Mountain Building
12
A) Passive margin: East
Coast of US.
B) Active subduction: Andes.
C) Inactive: Sierra Nevada
Coast
Range
Great
Valley
Sierra Nevada
Plutons (made of granite) are large magma bodies that collect in
the mid-crust, cool, and then are exposed by erosion – like the
granite in the Sierra Nevada. Many plutons form a batholith.
Page 15 of 20
Sierra Nevada Mountains
13
Active Sierra Nevada
ƒ
ƒ
Before the San Andreas
Fault (SAF), there was
active subduction across
California that created the
Sierra Nevada Mountains.
This stopped about 30
million years ago when the
SAF formed.
The Franciscan Formation
of the Coast Range is the
accretionary wedge of
material scraped off the
subducting plate.
Active Sierra Nevada
Coast Ranges
Inactive Sierra Nevada
Continental Collisions
✓REVIEW QUESTIONS
14
5. How are magmas with intermediate to felsic composition
thought the be produced from mantle-derived magmas at
Andean-type plate margins?
6. What is a batholith? In what modern tectonic setting are
batholiths being generated?
7. In what ways are the Sierra Nevada and the Andes similar?
How are they different?
8. What is an accretionary wedge? How does it form?
15
Continental Collisions
16
Himalayas
18
ƒ When a continent collides with another continent,
the subduction of the oceanic lithosphere stops
and forms a suture zone – where the continents
become welded together. Fold-and-thrust
belts are tightly folded belts of sedimentary rock
resulting in considerable crustal shortening and
thickening.
ƒ Himalayas – An active collision that began about
45 million years ago. The Indian block is much
older and has remained intact, while deforming
the younger continental block of southeast Asia.
Himalayas
17
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Page 16 of 20
Continental Collisions
19
Appalachian Mountains
20
Terranes and Mountain Building
22
ƒ Appalachians – These formed from the closing of
the ancestral Atlantic Ocean 250-300 million years
ago, when Africa collided with North America.
ƒ The collision created the tightly folded Valley and
Ridge Province. Shortly after the formation of the
Appalachian Mountains, the newly formed
supercontinent of Pangaea began to break apart.
ƒ The new rifting occurred east of the old suture
between Africa and North America, leaving a
remnant of Africa welded to the east coast of North
America.
ƒ The Alps (active) and Urals (inactive) are also the
result of continental collisions.
Appalachian Mountains
21
ƒ Terranes are accreted crustal blocks along the
continental margins by Andean-type subduction
zones.
ƒ These terranes have many sources, including:
microcontinents (like Madagascar off Africa),
island arcs (like Japan and the Philippines), and
submerged crustal fragments (like submerged
oceanic plateaus – created by massive outpourings
of basaltic lavas associated with hot spot activity).
ƒ All these units are either too thick or too light to
subduct into the mantle at subduction zones. When
they collide with the continental margin, they
accrete and increase the size of the continent.
Accretion of Foreign Terranes
23
Accretion of Foreign Terranes
24
ƒ Mountains may also occur
when smaller continental
fragments merge and
accrete to the continental
margins. Most of western
North America has been
added by this method.
ƒ Paleomagnetic data
indicate these terranes
originated thousands of
kilometers to the south of
their present locations.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Page 17 of 20
Terrane Formation
25
✓REVIEW QUESTIONS
26
9. Give examples of passive and active continental margins.
10. Explain how volcanic island arcs like Japan can be just one
step in the development of a major mountain belt.
11. What tectonic structure is the Coast Ranges of California?
12. Suture zones are often described as being ‘welded
together’ – explain why this may be misleading.
✓REVIEW QUESTIONS
27
13. Why was India not deformed as much as Asia in the
continental collision?
14. Where might magma be generated in a newly formed
continental collision?
28
18. Briefly explain the three types of compressional mountain
building.
15. How could a sliver of oceanic crust be found in the interior
of a continent?
16. How can the Appalachian Mountains be considered a
collision-type range when the nearest continent is 3000
miles away?
Fault Block Mountains
✓REVIEW QUESTIONS
17. How does the plate tectonics theory help explain the
existence of fossil marine life in rocks atop compressional
mountains?
19. Define the term terrane.
20. In addition to microcontinents – what other structures can
terranes consist of?
29
Basin and Range Province
30
ƒ Fault Block Mountains – are created by not by
compressional forces, but by tensional force,
resulting in high-angle normal faults.
ƒ When the crust is pulled apart, it thins and
stretches and produces volcanism
(decompressional melting). The high heat flow
causes the crust to expand and rise and
contributes to the extension in the region.
ƒ Important examples today are the Sierra Nevada of
California, the Grand Tetons of Wyoming, and the
Basin and Range Province of Western USA.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Page 18 of 20
Grand Tetons, Wyoming
31
Vertical Movements of the Crust
32
ƒ Isostasy – concept of a floating crust in
gravitational balance. Think of a floating
wooden block or iceberg.
Isostatic Adjustment
33
A) Young mountains with
thick roots.
B) Erosion lowers
mountains and crust
rises.
C) Erosion and uplift
continue until “normal”
crustal thickness is
reached.
Postglacial Rebound – Hudson Bay
35
ƒ
Q: What happens if you add a block on top?
ƒ
A:
Postglacial Rebound – Isostatic Adjustment
34
After the
continental
glaciers melted,
the crust was
unloaded (like a
ship) and the
crust has been
rebounding ever
since (see Fig.
18.26).
Vertical Movements of the Crust
36
ƒ Mantle Convection – the up-and-down
convective flow in the mantle also affects the
elevation of Earth’s major landforms. The buoyancy
of hot rising material accounts for broad upwarping
in the overlying lithosphere, while downward flow
causes downwarping.
ƒ Southern Africa is one region where large uplift has
produce expansive plateaus of nearly 5000 feet in
elevation – convective upflow – even though this
region has not experienced a plate collision for
nearly 400 million years.
ƒ Large downwarped structures, like the Michigan
Basin are thought to be linked to convective down
flow.
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Page 19 of 20
Vertical Movements of the Crust
37
✓REVIEW QUESTIONS
38
21. Describe the differences between the evolution of the
Appalachians Mountains and the NA Cordillera.
22. Compare fault-block mountains to the other types.
23. Give evidence that supports crustal uplift.
24. Isostatic adjustment - see Slide #32.
25. What elevates portions of southern Africa?
BLOCK EXAM #5 - CHAPTERS 12, 13 & 14
Page 20 of 20