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GEO 303 EXAM 2
Terms in this set (38)
Where do geophysicists think the Earth generates its magnetic field?
convection within the core
Random, we can't predict when the next will occur
What is the current location of the magnetic north pole?
in the arctic ocean, north of Canada
the angular difference between a compass reading at a certain location and the direction of the true, or geographical north pole
Can the Earth's inner core act as a permanent magnet?
No, b/c its temperature is far hotter than the curie temperature for any magnetic material
How do you read the magnetic record in rocks?
during cooling, magnetic minerals acquire magnetic lines of force parallel to earth's local field
-can measure how each layer is magnetized, says what magnetic field was like in the past
If a flow of basalt were to cool and crystallize at the magnetic south pole, what would be the orientation of its locked-in magnetic lines of force?
vertical to the earth's surface
What is the relationship between the north magnetic pole and the north geographic (rotational) pole?
although the magnetic pole is constantly moving in random directions, it always stays near to the rotational pole
generates electric currents within the liquid outer core that are responsible for generating most of the earth's magnetic field
-origin of the magnetic field
-like heating a pot of water
-in the outer core, motions of liquid Fe-Ni alloy generate the magnetic field
What does magnetic declination tell us?
that the magnetic field is NOT exactly the same as the geographic pole, its offset from true (geographical) north
In continental settings, mountain ranges are isostatically balanced by
development of large low density roots (Airy)
Earth's magnetic field
-similar to a simple magnet at the center of earth
-magnetic lines of force converge near geographic poles
Magnetic force lines
complicated, form loops with north and south poles, magnets will line up with force lines
because the field forms loops the magnetic field lines point more parallel to the surface at the equator and more perpendicular at the magnetic poles
True polar wander
the magnetic pole changes location over short time scales ( does not really matter for long time scales )
Is the magnetic field changing?
yes, north pole moves about 10km/year
-intensity changes (gets weaker and stronger)
-periodically reverses polarity
-all this is random and unpredictable (very chaotic)
-field is NOT a dipole
How do you make magnets?
certain materials get magnetized if they cool down in magnetic field
-get magnetized in same direction as field
-happens when cool below Curie temp (600)
-magnetization gets locked in when cold
Does center of earth have a magnet?
no, its too hot
-must be electric currents inside earth
-probably core since Fe is a good electric conductor
hot stuff rises, cold stuff sinks
responsible for magnetic field
-by matching pattern you can provide a chronology
-no reversal is unique by itself
-normal and reversed polarity signatures are non-unique: require stratigraphic correlation and tie points
What is the Airy model for isostasy best repped by?
high topography regions have thicker crust than lower relief areas
Compare to the average density of the mantle, the average density of the continental crust is?
The Force of Gravity is proportional to (m1xm20/r); m=mass, r=distance. Suppose that you weighed yourself at the equator, then again at the north pole. Where would you weigh less and why?
you would weigh less at the equator because at the north pole, the distance (r) between your center of gravity and the earth's center of gravity is shorter than at the equator
~shorter distance=weigh more bc impact of gravity closer (heavier)
If the gravitational force between 2 meteorites in outer space is 1 unit when they are 1 km apart, what is the gravitational force if the same meteorites are separated by 3 km?
1/9 unit (further apart = less force)
What is the Pratt model for isostasy best repped by?
high elevation regions have lower density crust than lower elevation areas
a state of gravitational equilibrium maintained by a block of lithosphere that floats in the underlying mantle at a level determined by both its density and its thickness
Glacial Isostatic Rebound
associated with the retreat of large ice sheets since the last glacial maximum
Imagine a mountain range supported by a low density root zone (Airy's hypothesis). What is the isostatic response as rivers erode the mountain away?
the mountains and root zone rise, and the mountains become lower as the root zone also diminishes
gravitational force between two objects determined by their masses and the distance between them
-further away=less attracted
-differences in density of materials (rocks) in earth's interior produce small differences in local gravity field (anomalies)
-measured with gravimeter (attraction of mass to earth stretches spring)
Observation of Gravity
surprising! A, B & C about the same
-same mass in high areas relative to low areas
-means that high topography is underlain by low density rocks and low areas high density rocks
Pratt's Isostasy Proposal
high mountains (ex. himalayas) are made of less dense crust that floats on a denser substratum
-blocks of different densities will float higher/lower depending on density
-more dense float higher
-isostasy accomplished by variations in density
-lowlands are made of more dense crust
-line up at bottom (underwater)
-Raw bread dough is of high density and the mass sits low in a pan, when the dough rises (expands) it becomes less dense
-Similarly, higher-density segments of the earth's crust are more compact, and they sit lower (even possibly below sea level)
-Lower-density segments are expanded; they rise to various elevations above sea level including the Himalayas or other great mountain ranges
-Places where crust is heated (make less dense) from below: Mid ocean ridges, Colorado Plateau
-Western U.S. is high (mountains) but has thin crust (mantle is hotter there and thus lower density than normal mantle) : mid ocean ridges and ocean floor : ocean crust cools as you move away
Airy's Isostasy Proposal
floating crustal blocks have the same density, but different thicknesses
-topography results solely from differences in crustal thickness (all have same density)
-large topographic features effectively float on the asthenosphere
-crust is analogous to larger or smaller logs made of the same wood, floating in a pond, or to an iceberg, of which 90% lies below water level. The base of the crust is a mirror image of the surface of the crust.
-blocks of wood in water are in isostatic equilibrium
-for blocks of the same density - if one block sits higher than another it must have less mass below the surface
-More vertical elevation means a deeper root to offset the mass
-most collisional mountain ranges
-Continental Crust has "roots"
As a result of isostasy, the thicker the surface exposure of rock, the thicker the crustal "roots"
Avg. continental crust is 40 km thick, under large mountain ranges can be 50-70 km, with the majority of the difference in the "roots"
the mass of water displaced by an object will be equal to the total mass of the object
-objects more dense than water will sink
-objects less dense than water will float (less dense will have more above water, more dense will have less)
Which ocean ridge system is spreading faster?
What is the relationship between Isostasy and erosion?
-After formation, the mountain will erode, removing some of the material needed to be isostatically compensated
-In response, the crust will rise, due to the buoyancy force to achieve isostatic balance
-Originally deep rock can end up at surface
earth adjusts to reach isostatic equilibrium
-if weight on surface changes, surface moves vertically
In continental settings mountain ranges are isostatically balanced by?
development of large low density roots (Airy)
Whats surprising about the gravity of A, B & C?
all nearly the same
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