geology chapter 12

is gravity on earth everywhere uniform?

gravitational acceleration is less at the equator than at the poles due to centrifugal force being greater at the equator and because the earth's surface is further from the center of gravity at the equator than at the poles

convection

heat from the core transferred through the mantle to the base of the crust

what is good evidence for a solidified, inner core?

p waves are faster in the inner core than in the outer core

what is the moho and where does it occur?

seismic discontinuity that forms the boundary between the crust and mantle

why do seismic waves bend outward with depth in the mantle?

seismic waves bend up when going low to high, velocity speeds up seismic waves bend down when going high to low, velocity slows down

what does NOT contribute to earth's internal heat?

solar radiation from the early sun

geoid

the geoid is a representation of sea level and is distorted relative to a perfect spheroid formed solely by earth's rotation

what produces the earth's magnetic field?

weak electrical currents associated with fluid motions in the outer core

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what is the composition of the mantle?

-2900km thick -base of the crust (moho) to the liquid outer core -composed of silicate minerals that are rich in iron and magnesium -upper mantle, lower mantle, d" layer

TRUE OR FALSE: differences in the height of earth's geoid is measured relative to a perfect ellipsoid.

TRUE

what causes p and s wave shadow zones? which is wider? why?

bending in the outer core and the refraction of p waves crossing the mantle-core boundary creates a p wave shadow zone because s waves do not travel through the outer core, they have a bigger shadow zone.

seismic refraction

bending of seismic waves while passing through the earth

what are the characteristics of the d" layer?

-compromises the bottom few hundred km of the lower mantle -large horizontal variations in both temperature and composition -possible ducted oceanic lithosphere -birth place of some mantle plume -does NOT occur directly below continental and oceanic crust.

what is the composition of the outer core?

-layer beneath the mantle about 2270km thick -it has the properties of a liquid -it is the responsible for the earth's magnetic field generated by the churning of the iron rich material in the hot fluid outer core -composed of liquid iron

describe mantle convection and how this process transports heat through the mantle

-less dense material rises and more dense material sinks -the outer core, mantle, lithosphere

do the geographic poles coincide with the magnetic poles?

-magnetic poles inclined at angle to earths geographic poles -magnetic lines of force enter the earth perpendicular to the surface at magnetic north pole -magnetic lines of force parallel with earth's surface at equator -magnetic field has weakened by 10% over the last century

what is the composition of the inner core?

-solid innermost layer of the earth -about 1200km in radius -iron-nickel alloy with minor amounts of oxygen, silicon, and sulfur

asthenosphere

-subdivision of the mantle situated below the lithosphere -this zone of weak material -100km-700km -the rock within this zone is easily deformed -composed of hot rocky material that is capable of very slow, weak movement -low velocity zone

describe the processes of conduction. which of the earth's layers transfers mainly by conduction?

-the transfer of heat through matter by molecular activity -occurs in the inner core, outer core, and d" layer

list three processes that have contributed to earth's internal heat

1. heat released by colliding particles during the formation of earth 4.5 billion years ago 2. heat released as iron crystallized to form the solid inner core of early earth 3. heat emitted by radioactive decay of isotopes of uranium, thorium, and potassium

how do the velocities of p and s waves change with depth in passing through the following layers; crust, asthenosphere, transition zones, lower mantle, outer core, and inner core?

CRUST: seismic wave velocity increases abruptly below the crust due to compositional change from crustal rocks to mantle peridotite. ASTHENOSPHERE: a zone of softened. peridotite in the upper mantle seismic wave velocities decrease within the upper asthenosphere because peridotite contains at a few percent partial melt but not enough to completely stop s waves. TRANSITIONAL ZONES: at 400km depth, olivine changes to the spinel crystal structure of higher density, at 600km depth, spinel changes to the higher density perovskite crystal structure. LOWER MANTLE: velocities of s and p waves increase with depth (mesosphere) OUTER CORE: p waves bend downward when entering due to decrease in velocity and bend when they leave again, slows down p waves meaning it's a liquid waves - stops s waves from traveling through indicating it's liquid INNER CORE: p waves speed up again

what are the various factors that affect earth's gravity?

ELEVATION: higher elevation puts earth's surface further from the center of gravity so that gravity decreases DENSITY DIFFERENCES OR ROCKS BELOW THE EARTHS SURFACE: higher density rocks below the surface increase gravitational attraction locally

TRUE OR FALSE: as p waves pass from the mantle into the outer core, velocities increase abruptly

FALSE

TRUE OR FALSE: earths radius is slightly longer at the pole, but shorter at the equator

FALSE

how does oceanic crust differ from continental crust in terms of composition and thickness?

OCEANIC CRUST -is about 3-15km thick -consists of primarily basalt and gabbro CONTINENTAL CRUST -40-65km thick -average composition of granite -less dense (more buoyant) than oceanic crust

what is the geothermal gradient?

a curve showing the increase of temperature with depth within the earth's interior. the average is 30C per km in the upper crust. varies considerably from place to place. rate of increase of heat with depth significantly less in mantle and core.

seismic reflection

a method of viewing the rock structure beneath a blanket of sediment by using strong, low frequency sound waves that penetrate the sediments and reflect off the contacts between rock layers and fault zones. used in search for underground oil and gas reserves.

the two transition zones in earth's mantle, one at 400km and the other at 670km depths, are characterized by...

abrupt increases in p and s waves due to closer atomic packing of crystal structures in mantle minerals