GEO EXAM 2 Flashcards
Absolute time
Putting numbers on rocks, estimates of the earth’s age
Ideas of earth’s age
Salinity of the oceans, heat loss, thickness of sedimentary rocks, radioactive decay
Half-Life
The amount it takes for 1/2 of the orginial parent atoms to decay to the daughter.
The Geologic Time Scale
Originally based on fossils - now based on radioactive decay
Precambrian (time)
4.6 billion years to 570 million years. Comprises of 80% of earth’s time.
Paleozoic (time)
540 million years to 250 million years. (explosion of life)
Mesozoic (time)
250 million years to 65 million years. (dinosaurs)
Ceozoic (time)
Age of mammals. 65 million years to present.
declination
the angle between magnetic north ( the direction the north end of the compass needle points) and true north
Sea Floor Spreading
The formation of new areas of oceanic crust, which occurs through the upwelling of magma at midocean ridges and its subsequent outward movement on either side.
Features of sea floor
Long continuous mountain chains, trenches (deep valleys), continental shelves, abyssal plains.
Age of sea floor
200 million years
Sea Floor Spreading theory
- The sea floor moves laterally away from the ocean ridges (spreading centers)
- Magma rises up to the sea floor ridges
- The magma hardens to form a new seafloor
- The ridges crack in half and move away from each other
Evidence that supports seafloor spreading
- The age of the seafloor
- Seafloor sediment thickens away from the ridge
- Valleys run down the center of the ridges
- Paleomagnetism
Paleomagnetism
magnetic polarity stripes on the seafloor
How do polarity stripes form?
as basalt cools at the ridges, it preserves the polarity of earth’s magnetic field.
Three compositional layers
crust, core, mantle
Crust features
- Made up of oceanic and continental crust
- 6 to 25 miles thick
- covered by sediment and sedimentary rock
Mantle features
- makes up about 80% of the earth
- made mostly of olivine
Outer core
liquid Fe
Inner core
solid Fe
What results in the Earth’s magnetic field
circulation of the liquid Fe outer core
Lithosphere features
100 km thick, rigid, strong, cold, brittle, less dense
Lithosphere
Contains the crust and a small amount of the upper mantle
Asthenosphere features
100 to 300km, plastic, pliable, weak, hot, flows, more dense
Theory of plate tectonics
- The surface of the earth is covered by 7 large lithospheric plates
- These lithospheric plates travel in directions that differ from different adjacent plates
How are lithosphere plates created?
at ocean ridges and slide over the asthenosphere
Seismology
Study of earthquakes
Where do faults occur?
crust and upper mantle
Elastic rebound theory
explains origin of earthquakes
elastic rebound theory features
- If a rock is stressed it can store up its energy elastically
- When the strength of the rock is exceeded the rock breaks (brittle)
- The rocks on either side of the fault snap back to their orginial shape and release the stored energy
What determines the size of the earthquake?
the amount of energy stored up
What do seismographs rely on?
inertia
geophone
a device that converts ground movement into voltage which can be recorded
seismic waves
waves that travel through rock
properities of waves
- wavelength
- amplitude
- period
- frequency
high energy wave
high amplitude
low energy wave
low amplitude
classification of seismic waves
body waves and surface waves
body waves
pass through the earth and travel outward in all directions from the focus
surface waves
- travel along the surface of the earth
- travel more slowly than P and S waves
- last wave to be detected
types of body waves
p wave (primary, compression) and s wave (secondary, shear)
p-wave
- consist of alternating pulses of compression and expansion
- produce changes in volume and density
- travel through solids, liquids, and gases
- motion of the wave is parallel to its direction
s-wave
- shear waves
- motion is perpendicular to direction
determining the location of the epicenter
- measuring the time interval between P and S wave arrival
- three seismic stations are needed
modified mercalli scale
- qualitative measures based on earthquake effects in populated areas
- intensity ratings between 1 - 12
shake map
shows the intensity of shaking over an area that has experieced an earthquake
richter scale
- quantitatively measures the amount of energy released from an earthquake
- needs instrument measurement
- only one magnitude for an earthquake
How is magnitude calculated?
measuring the height of the largest S wave
effects of an earthquake
1) ground motion (destroys buildings)
2) faults (breaks the ground surface)
3) fire - gas lines break
4) landslides, avalanches
5) liquefaction
6) tsunami
7) flooding
8) differential ground settlement
seismic gap
- location where a earthquake hasn’t occured in a long time
- areas where built up stress has not been released
stress
push or pull on a rock
strain
deformation - the measure of change in shape to a stressed material
types of deformation
- elastic
- ductile
- brittle
elastic
reversible or nonpermanent change in shape - the rock stores energy
ductile deformation
stretching or flowing of material without breaking
brittle deformation
breaking or fracturing
continental drift theory
Alfred Wegener’s theory stating the world used to all be connected but drifted
Evidence for continental drift
Identical: continents, fossils, mountains, age of rocks, glaciated areas, ice flow
divergent plate boundaries
a line along which two adjacent plates move apart from each other
divergent margins
places where new oceanic lithosphere is created
origin and evolution of divergent boundaries
1) begin as rift zones on the continents - pulling apart of continental crust
2) as the continent is pulled apart, basaltic magma is extruded into the rift valley (oceanic crust is being formed)
3) the valley floor sinks and is filled with sea water
4) divergence continues and results in a large ocean basin
examples of divergent margins
- mid-atlantic ridge
- iceland
physical aspects of divergent margins
- state of stress (tensional or pulling)
- earthquake activity ( abundant, low magnitude, shallow depth)
- volcanism (intense volcanism, basaltic)
convergent margins
places where two lithosphereic plates move towards each other
oceanic - oceanic crustal convergence
one lithospheric plate turns down into the earth and is recycled
trench
arcuate shaped valleys on the seafloor
subduction zone
the trench is the location of a downturning lithosphere
physical aspect of oceanic-oceanic crust convergence zones
- state of stress (compression)
-earthquake activity (abundant, follows the down going slab - shallow and deep, high magnitude earthquakes)
-volcanism (abundant)
examples of oceanic-oceanic crust convergence
- Japan
- aleutian islands
oceanic - continental crust convergence
oceanic crust dives beneath the continental crust
features of oceanic - continental
- trench - location of a subduction zone
- continental volcanic chains
physical aspects of oceanic-continental crustal convergence
1) stress (compression)
2) earthquakes (shallow to deep, lots of them, high magnitude)
3) volcanic activity (abundant)
examples of oceanic to continental
- West coast of south america - andes
- Cascades
continental - continetal
continents are buoyant and don’t subduct ( continental collisions)
features of continental-continental crustal convergence
- double thickness of continental crust
- huge mountain chains
physical aspects on continental-continental crustal convergence
compression, earthquakes (high magnitude, shallow), mountains
examples of continetal crust collisions
- himalyas
- alps
- appalachians
convergent margins
crust is a passive passenger on the lithosphere
hot spots
- heat anonomly in the asthenosphere
- magma makes its way to the surface from deep depths
Examples of hot spots
yellowstone, hawaiian islands
mechanisms of plate tectonics
- convection of the mantle
- slab pull
shear
a stress which is applied horizontal or tangential to the face of a material
mantle convection
motion caused by density changes due to heating
remanant magnetism
rocks can preserve a “fossil” record of the earth’s magnetic field
earth’s magnetic field
the earth behaves as if there is a giant magnetic bar inside
Types of magnetic information
polarity, declination
polarity
- the location of the north and south poles
normal polarity
todays magnetic field
reversed polarity
180 degrees from today’s field
paths of apparent polar wandering
ancient rocks, layered rocks and diff continent rocks
does the earth’s magnetic pole wander?
NO, the continents wander, not the magnetic poles
dipping bed
a layer of rock that is not horizontal
strike
is a line representing the intersection of that feature with a horizontal plane
dip
The dip gives the steepest angle of descent of a tilted bed or feature relative to a horizontal plane
anticline
a type of fold that is an arch like shape and has its oldest beds at its core. Older towards center of the fold.
syncline
a type of fold that has its younger layers closer to the center, lower than a anticline
normal fault
drops rock on one side of the fault DOWN relative to the other side (ex, san andreas)
reverse fault
one rocky block is pushed up relative to the rock on the other side
strike-slip
scrape along side by side
dip slip
inclined fractures where the blocks have shifted vertically. (normal or reverse)
how to read a seismogram
first p waves, then s waves, then surface waves
distance between p and s waves
can be used to determine the distance between the station and the earthquake
passive margin
transition between oceanic and continential lithosphere that is not an active plate margin.
mechanism for plate tectonics
convection currents beneath the plates move the crustal plates in different directions. - thermal convection
transform features
shallow earthquakes, shear stress, no volcanism
1989 Loma Prieta
6.9 magnitude, 63 deaths, 3757 injured
2011 Sendai Earthquake
9 magnitude, around 20k deaths
Seismograph
Instrument that detects vibrations and shaking of earth
Where do Texas earthquakes occur?
near El Paso and in the Panhandle
