Chapter 5 Flashcards
Sir Francis Bacon
noted that shorelines of the Americas “fit” those of Africa and Europe (late 1600s, early 1700s)
Antonio Snider-Pellegrini
- proposed that all continents were once connected but catastrophically separated in the past
- thought the mechanism by which this occurred was Noah’s flood
- published book in 1859
Alfred Wegener
- meteorologist and geophysicist
- studied glaciers
- continental drift
- Pangaea
Continental drift
- 1915
- slow and gradual movement of continent over eons and eons of time
scientific revolution
profound reversal of ideas
Pangaea
“all land” -for a supercontinent whose breakup results in all current continents
Wegener’s Lines of Evidence
- “Fit” of continents
- matching geology of continents
- fossil distribution
- living organisms distribution
- ancient climate
fit of continents evidence
especially South America and Africa
matching geology of continents evidence
especially with mountain chains
fossil distribution
-similar fossils found on continents now separated by oceans
good candidates for fossils
-terrestrian-meosaurus (reptile) and glossopteris (plant)
living organisms distribution
-marsupials in South America and Australia
ancient climate
evidence of glaciers in rock record at places that today could not make/sustain glaciers
Sir Edward Bullard and 2 associates constructed a map that what?
pieced together the edges of the continental shelves of South America and Africa at depths of 900 meters
glossopteris fossils were found where?
-Africa, Australia, India, South America, and Antarctica
when did Wegener’s proposal attract criticism?
1924 when it was translated into English, French, Spanish and Russian
main problem with continental drift
no mechanism to move the continents-doesn’t explain Rocky Mountains. in all only explains 1/3 of geology
1960s
declassified Navy data seafloor
big discovery
mid-ocean ridge
how was mid-ocean ridge discovered?
sonar
dredging of ocean floor revealed 2 patterns
1) sediments were thickest near continent and thiner near mid-ocean ridge
2) sediments near continents were young and old types, but those near the mid-ocean ridge were only young
seafloor spreading
occurs at the mid-ocean ridge (divergent boundary)
lithosphere
uppermost mantle and crust behave as a strong rigid layer
plates
segments of lithosphere
where is lithosphere thinnest?
oceans
asthenosphere
weak region in mantle underneath lithosphere
tectonic plates
- aka lithospheric plates
- 20 segments
- in constant motion with respect to one another
largest plate
Pacific plate
plates that account for 94% of Earth’s surface area
North American, South American, Pacific, Eurasian, Australian-Indian, Antarctic
as plates move the distance between 2 locations on different plates ____ _____ whereas the distance between sites on the same plate remains ____ _____.
gradually changes; relatively constant
most major interactions among plates occurs on their
boundaries
plate tectonics=
continental drift + seafloor spreading
The earth’s surface is composed of
mobile plates (lithosphere)
divergent boundaries
- aka constructive plate margins
- aka spreading centers
- where new crust is made
2 locations divergent boundaries are prominent
1) Mid-ocean ridges
2) Continental Rift Valley
example of continental rift valley
east african rift
oceanic ridges
elevated areas of the seafloor that are characterized by high heat flow and volcanism
how much of earth’s surface do oceanic ridges account for?
20%
seafloor spreading
the mechanism that operates along the oceanic ridge to create new seafloor
primary reason for elevated position of oceanic ridge
newly created oceanic crust is hot, making it less dense than cooler rocks found away from the ridge axis
cooling ___ hot material directly below oceanic crust, adding to plate’s _____
strengthens, thickness
continental rifting occurs where
opposing tectonic forces act to pull lithosphere apart
convergent boundaries
- aka destructive plate margins
- aka subduction zones
oceanic lithosphere is ____ ____ than the athenosphere
more dense
continental lithosphere is ___ ___ and resists _______
less dense, subduction
deep ocean trenches
surface manifestations produced as oceanic lithosphere descends into the mantle
angle at which oceanic lithosphere descends depends on its
density
as oceanic lithosphere ages, it
thickens and increases in density
oceanic-continental convergence
leading edge of plate caped with continental crust converges with a slab of oceanic lithosphere
sediments and oceanic crust contain a large amount of
water that is carried to great depths by a subducting plate
partial melting
the wedge of mantle rock is sufficiently hot that the introduction of water from the slab below leads to some melting
examples of oceanic-continental convergence
South America (Andes), Pacific NW (Cascade Mountains)
oceanic-oceanic convergence
2 oceanic slabs converge, one descends beneath the other, initiating volcanic activity by the same mechanism that operates at all subduction zones
examples of oceanic-oceanic convergence
Japan, Philippines, Middle Earth
continental-continental convergence
one landmass moves toward the margin of another because of subduction of the intervening seafloor
examples of continental-continental convergence
Himalayan, past Appalachians
subduction
sinking of ocean crust into the mantle
transform boundaries
- aka fault boundaries
- plates slide past one another
- near MOR’s
ex. transform boundaries
San Andreas Fault
fracture zones
include active and inactive transform faults
where do active transform faults lie?
between 2 offset ridge segments, defined by weak, shallow earth quakes
transform faults provide
means by which oceanic crust created at ridge crests can be transported to a site of destruction
hot spots
a stationary plume of magma sourced from the lower mantle. Plates move over top of the hot spot.
examples of hot spots
Hawaii and Yellowstone National Park
new plate boundaries can be created or destroyed in response to
changes in the forces acting on the lithosphere
mantle plume
cylindrically shaped upwelling of hot rock (like beneath island of Hawaii)
hot-spot track
a chain of volcanic structures
paleomagnetism
the record of Earth’s magnetic field as known via rocks
-helps settle out polar wander and magnetic
some naturally occuring minerals are magnetic and hence
are influenced by Earth’s magnetic field (ex. magnetite which is abundant in lava flows of basaltic composition)
Curie point
threshold for magnetism
polar wandering
idea that magnetic poles migrated
If the magnetic poles remain stationary, their apparent movement is produced by
continental drift
normal polarity
when rocks exhibit the smae magnetism as the present magnetic field
reverse polarity
when rocks exhibit the opposite magnetism are said to have reverse polarity
chrons
major divisions of magnetic time scale
magnetometers
used to do magnetic surveys of the ocean floor
high-intensity magnetism
regions where paleomagnetism of the ocean crust exhibits normal polarity that enhance Earth’s magnetic field
low-intensity magnetism
regions where the ocean crust is polarized in the reverse direction and weaken the magnetic field
simplest type of convection
- analogous to heating a pot of water on a stove
- heating base causes material to rise in thin sheets that spread out at the surface and cool
- surface layer thickens and sinks to bottom until it reheats again
mantle convection
- more complex than simple convection
- mantle shape is spherical with a larger upper boundary than lower boundary
- driven by combination of 3 thermal processes
3 thermal processes that drive mantle convection
1-heating at the bottom by heat loss from Earth’s core
2-heating from within by decay of radioactive isotopes
3-cooling from the top that creates thick, cold lithospheric slabs that sink into the mantle
what researchers agree on concerning convection
1-connective flow in the rocky mantle is the underlying driving force for plate movement
2-mantle convection and plate tectonics are part of the same system
3-convective flow in the mantle is the primary mechanism for transporting heat away from Earth’s interior to the surface where it is eventually radiated into space
driving forces sof plate motion
- subduction of cold, dense slabs of oceanic lithosphere (aka slab pull)
- ridge push that results from elevated position of oceanic ridge and causes lithosphere to slide donwn the flanks of the ridge
- mantle drag-when flow in athenosphere is moving at a velocity that exceeds that of the plate
any acceptable model for plate-mantle convection
must explain compositional variations know to exist in the mantle
types of plate-mantle convection
-layering and whole-mantle convection
layering
2 zones, thin dynamic layer in upper mantle and thick sluggish layer underneath
whole-mantle convection
cold oceanic lithosphere sinks to great depths and stirs the entire mantle
-predicted that this type would cause entire mantle to completely mix which would eliminate chemically distinct magma sources
The unequal distribution of heat in Earth’s interior generates
some type of thermal convection that ultimately drives plate-mantle motion
