German meteorologist and polar explorer; wrote The Origins of the Continents and Oceans
Alfred Wegener
Wegener hypothesized this former supercontinent
Pangaea
Wegener suggested that land masses slowly move apart in a process known as
continental drift
5 pieces of evidence that Wegener's claims of continental drift were based on
fit of the continents, glacial deposits far from polar regions, paleoclimatic belts, distribution of fossils, matching geologic units
2 ways the earth continually changes due to continental drift
continents move, split apart, and recombine; ocean basins open and close
Proposed sea-floor spreading
Harry Hess
2 things that occur due to sea-floor spreading
as continents drift apart, new ocean floor forms between; continents converge when ocean floor sinks into the interior
From what era were glaciers found on five continents
Paleozoic
4 Pangea climate belts
tropical coals, tropical reefs, subtropical deserts, subtropical evaporites
freshwater reptile
mesosaurus
a subpolar plant with heavy seeds
glossopteris
nonswimming land-dwelling reptile
lystrosaurus
nonswimming land-dwelling mammal-like reptile
cynognathus
Pangea explains the distribution of these
fossils
distinctive rock assemblages and mountain belts match across the Atlantic
matching geologic units
Why Wegener's ideas were criticized
He couldn't explain how or why the continents moved
Where the flow occurs that creates the magnetic field
liquid outer core
geographic and magnetic poles are not...
Parallel
the difference between geographic N and magnetic N is called...
declination
declination depends on these 2 factors
Absolute position of the two poles (geographic north, magnetic north); longitude
a compass points to this, not geographic N
magnetic N
these cause a magnetic needle to tilt
Curved field lines
The angle between magnetic field line and surface of the Earth is known as
inclination
inclination depends on this
latitude
the study of fossil magnetism
paleomagnetism
these preserve information about the magnetic field at the time the rocks formed
iron minerals
4 characteristics of hot magma relating to its magnetism
high temperature, no magnetization, thermal energy of atoms is very high, magnetic dipoles are randomly oriented
5 characteristics of cooled magma relating to its magnetism
low temperature, permanent magnetization, thermal energy of atoms slows, dipoles align with Earth's magnetic field, magnetic dipoles become frozen in alignment with field
layered basalts record magnetic changes over time, inclination and declination indicate change in position
polar wandering
not wandering pole, fixed continent but...
fixed pole, wandering continent
before WWII, this allowed rapid sea-floor mapping (with assistance from ships)
sonar
these are now produced using satellite data
bathymetric maps
these run through every ocean
mid-ocean mountain ranges
these occur near volcanic island chains
deep ocean trenches
these poke up from the ocean floor
submarine volcanoes
these segment the mid-ocean ridge
fracture zones
bathymetric features (5) delineated by sonar mapping
Mid-ocean ridges, deep-ocean trenches, volcanic islands, seamounts, fracture zones
covered by sediment; thickest near the continents; thinnest or absent at the mid-ocean ridge
oceanic crust
What the oceanic crust is made up of (3)
basalt, few continental rock types, no metamorphic rocks
this is much greater at the mid-ocean ridges
heat flow
earthquakes are limited to... (3)
oceanic fracture zones, mid-ocean ridge axes, deep ocean trenches
earthquakes define...
zones of movement
Harry Hess published his "Essay in Geopoetry" which stated that... (5)
sediment thickens away from ridges, earthquakes at mid-ocean ridges indicate cracking, cracked crust splits apart, high heat flow from molten rock rises into the cracked crust, new ocean floor forms at mid-ocean ridges
theorized by Hess
sea-floor spreading
process of sea-floor spreading
upwelling magma erupts at the mid-ocean ridges, new crust moves away from ridges, gathering sediment, at trenches the sea-floor sinks back into the mantle
process of continental drift
continents move apart as sea-floor spreading occurs, continents move together as sea-floor sinks into mantle
evidence of sea-floor spreading (6)
magnetism in sea-floor rocks varies farther from MOR, stripes of positive and negative magnetic intensity, recorded in sea-floor basalts, magnetic anomalies map as stripes of positive and negative intensity, magnetic stripes form a pattern, the pattern is symmetric on either side of the MOR
these reveal reversals in magnetic polarity
layered lava flows
the magnetic field flips; a reversed N magnetic pole is near the S geographic pole; can be used as time markers
magnetic reversals
Gives the timing of polarity reversals
Isotopic dating
reversals occur at...
uneven intervals
longer intervals (500 to 700+ Ka)
chrons
shorter intervals (~200 Ka)
subchrons
polarity reversals explain...
magnetic anomaly stripes
sea-floor rock normal polarity
positive anomaly
sea-floor rock reversed polarity
negative anomaly
magnetic anomalies are _ across the MOR
symmetric
width of magnetic anomaly stripes is related to the...
spreading rate
wide magnetic anomaly stripes indicate...
faster spreading rate of sea floor
narrow magnetic anomaly stripes indicate...
slower spreading rate of sea floor
plate tectonics explain... (4)
the distribution of earthquakes and volcanoes, changes in past positions of continents and ocean basins, the origins of mountain belts and seamount chains, the origin and ages of ocean basins
tectonic plates are fragments of...
lithosphere
the lithosphere is in motion over the...
asthenosphere
the asthenosphere flows ______ when loaded
plastically
3 characteristics of continental lithosphere
felsic to intermediate crustal rocks, lighter (less dense), more buoyant
3 characteristics of oceanic lithosphere
mafic crust (basalt and gabbro), heavier (more dense), less buoyant
plates more continuously at a rate of...
1-15 cm/year
locations on Earth where tectonic plates meet
plate boundaries
almost earthquake-free areas
plate interiors
where land meets the ocean
continental margins
margins near plate boundaries are...
active
margins far from plate boundaries are...
passive
3 characteristics of passive-margin continental crust
thins seaward, traps eroded sediment, develops into the continental shelf
three types of plate boundaries
divergent, convergent, transform
divergent boundary
tectonic plates move apart, lithosphere thickens away from the ridge axis, new lithosphere created at boundary; (also called: mid-ocean ridge)
convergent boundary
tectonic plates move together, the process of plate consumption is called subduction; (also called: convergent margin, subduction zone, trench)
transform boundary
tectonic plates slide sideways, plate material is neither created nor destroyed; (also called: transform fault)
early stage of sea-floor spreading
rifting has progressed to mid-ocean ridge formation, before substantial widening of the ocean, forms a long thin ocean basin with young oceanic crust
example of early stage divergent boundary
Red Sea
mid-stage of sea-floor spreading
ocean begins to widen, new sea-floor is added at the mid-ocean ridge, continents move farther apart
example of mid-stage sea-floor spreading
Greenland and the North Atlantic
late stage of sea-floor spreading
mature, wide ocean basin, linear increase in age with distance from central ridge; edge of ocean basin-oldest, ridge proximal-youngest
example of late stage sea-floor spreading
Atlantic Ocean
mid-ocean ridges
linear mountain ranges in Earth's ocean basins
example of mid-ocean ridge
The Mid-Atlantic Ridge
4 characteristics of Mid-Atlantic Ridge
Snakes N-S through the entire Atlantic Ocean, elevated ridge (1500 km wide) 2 km above abyssal plains, new sea floor created only along axis of the ridge, symmetrical
formation of mid-ocean ridges
sea-floor spreading opens the axial rift valley, rising asthenosphere melts, forming mafic magma, pooled magma solidifies into oceanic crustal rock
pillow basalt
magma quenched at the sea floor
dikes
preserved magma conduits
gabbro
deeper magma
formation of "black smokers" (found in some mid-ocean ridges)
water entering fractured rock is heated by magma, hot water dissolves minerals and cycles back out of rock, water reaches the sea and minerals precipitate quickly
oceanic crust spreads away from...
the ridge axis
new oceanic crust is...
closer to the ridge
oldest oceanic crust is found at...
the far edge of the basin
this is at the base of the mid ocean ridge
the hot asthenosphere
in convergent boundaries, lithospheric plates move ... one another
toward
in convergent boundaries, one plates sinks back into the mantle. the subducting plate is always...
oceanic lithosphere
continental crust cannot be...
subducted (too buoyant)
subduction ... ocean lithosphere
recycles
subduction is balanced by ...
sea-floor spreading
Earth maintains a constant...
circumference
convergent boundaries are also called...
subduction zones
old oceanic lithosphere is more ... than mantle
dense
a flat-lying oceanic plate doesn't ... easily
subduct
process of subduction
plate edge bends down and slips into mantle, then the leading edge sinks downward like an anchor rope
the subducting plate descends at an average of __ degrees
45
plate descent is revealed by...
Wadati-Benioff earthquakes
this layer of the earth may be a "plate graveyard
lower mantle
4 unique features of subduction
deep-ocean trenches, accretionary prisms, volcanic arcs, back-arc basins
volcanic arc
chain of volcanoes on overriding plate (magmas rise and melt through overriding plate)
continental arc
the overriding plate is made of continental crust
island arc
overriding plate is made of oceanic crust
back-arc basins
a marginal sea behind an arc
formation of back-arc basins
forms between an island arc and a continent; offshore subduction traps a piece of oceanic crust or stretching lithosphere creates a new spreading ridge
transform boundaries
lithosphere fractures and slides laterally; no new plate forms, none consumed, many transforms offset spreading ridge segments, some transforms cut through continental crust
transform boundaries are characterized by...
earthquakes, absence of volcanism
the mid-ocean ridge axis is offset by...
transform faults
these lie at right angles to ridge segments
fracture zones
this occurs between ridge segments
active slip (earthquakes)
portions of fracture zones extending beyond ridges are not...
seismically active
continental transforms
cut across continental crust
example of transform boundaries
The San Andreas Fault, California
triple junctions
point where three plate boundaries intersect
hot spots
plumes of deep mantle material independent of plates; not linked to plate boundaries, originates as a deep mantle plume, plume partially melts lithosphere and magma rises to surface
hot spots ___ overriding plates
perforate
volcanoes build above...
sea level
plate motion pulls volcano off...
plume
hot-spot track
chain of extinct volcanoes
hot spots reinforce...
sea-floor spreading
process of continental rifting
continental lithosphere stretches and thins, brittle upper crust faults, ductile lower crust flows, asthenosphere rises and melts, magma erupts
continuation of continental rifting can create a new...
mid-ocean ridge
Western US Basin and Range Province is a...
rift
East African Rift
the Arabian plate is rifting from the African plate
rifting has progressed to sea-floor spreading in...
the Red Sea, the Gulf of Aden
In the East African Rift, the rift and two spreading ridges form a...
triple junction
subduction consumes...
ocean basins
ocean closure ends in...
continental collision
process of continental collision
subduction stops and the subducting plate detaches and sinks, the continental crust is too buoyant to subduct, the collision deforms crust, mountains are uplifted
ridge-push force
elevated mid-ocean ridge pushes lithosphere away
slab-pull force
denser subducting plate is pulled downward
_________ in the asthenosphere speeds or slows motion
convection
absolute plate velocities may be mapped by... (2)
plotting plate motion relative to a fixed spot in the mantle, measuring volcano ages/distance along a hot spot track
plate motion is now determined by...
GPS measurements