physical structure of the earth Flashcards by Rosie Pritchard

layers of the earth in order

Crust
(Moho)
Upper mantle
lower mantle
(Guttenberg)
outer core
(Lehmann)
inner core

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Crust properties

up to 40km

rich in granite and basalt
continental =thicker than O
o= denser than Continental

mostly silicate rocks

solid-floats on top of plastic like mantle

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Moho

discontinuity between crust and U mantle

distinct boundary- abrupt change
plastic like

change:
less dense to more dense
change in rock type
S to UM

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Upper mantle properties

35-700km

50% olivine
35%pyroxene
5-10%calcium, Al oxide

lots of peridotite

very solid

plastic like, solid nut flows slowly

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Lower mantle properties

700-2900km

extremely high pressure and temp
(around 7000 degrees F)

solid more rigid

solid, plastic like

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Guttenberg discontinuity

2900km

sep outer core from mantle

powerful forces here

distinct boundary

changes:
solid to liquid
change in rock type- peridotite to Fe and Ni

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outer core properties

2900-5100km

mostly liquid Fe and Ni

very hot 4500-5500 degrees C

liquid

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Lehmann discontinuity

Separates outer and inner core

5100km

Phase boundary- gradient change

changes:
abrupt increase in P wave
liquid to solid but comp similar

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inner core properties

5100-6371km

iron and nickel

solid

most dense

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evidence for structure of the Earth

ophiolites

volc eruptions

boreholes

direct observation

density

study of meteorites

gravity

seismic waves

isostacy

presence of magnetosphere

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what is a seismometer and how does it work?

SEE BOOKLET FOR DIAGRAM

seismogram-graph produced

seismograph- equipment + graph

seismometer- piece of equipment

Equipment- graph paper, cylinder, pendulum, stylus , spring

device which is sensitive to vibrations-
whole stand moves with ground
pendulum tries to remain centre and the relative movement creates graph

higher the amplitude = greater the movement

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P wave properties

Primary wave- arrive first

longitudinal waves

2* speed of S waves

vibrate rock back and forth- compression and rarefaction (5000 m/s in granite, 1450m/s in water)

travels slower through liquid
denser + colder= faster

smallest amplitude

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S waves properties

secondary waves- arrive second

transverse

only through solids

60% speed of P waves

travel perpendicular to movement of rock

larger amp than p waves

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L wave properties

Love waves- arrive last

surface waves

most destructive- oscillate in circular motion, lose e v quickly so most destructive

largest amplitude

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why do some seismograms only show P and L waves?

recorded in S wave shadow zones

103-103

created as S waves cant travel through liquid outer core

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Why do some seismograms only show L waves?

in P and S wave shadow zone

103-142 degrees on each side of globe

SWSZ- cant travel through liquid O core

PWSZ- as they are rarefracted as they travel through diff mediums (liquid outer core , slow down, rarefracted)

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changes in p waves through earth

speed increases through the crust and upper mantle- more dense (rate of increase lower in lower mantle still increased speed tho)

slight decrease at Moho as plastic solid

speed decreases at Gutenberg as liquid outer core

increases through outer core

speed increases quickly at lehmann as l to S but then remains constant

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changes in S wave through earth

speed increases through the crust and upper mantle- more dense (rate of increase lower in lower mantle still increased speed tho)

cease at Gutenberg as liquid outer core

then reappear in inner core as can be generated from P waves

density of the Earth as evidence for structure

Density = mass/vol

5.9710^27g / 1,0810^27=
5.53gcm-3

density of continental-2.7gcm-3
oceanic-2.9gcm-3

so density of rest of earth must be much higher- greater than average

gravity def

force of attraction that exists between any 2 masses

greater mass + shorter the distance = shorter

gals

ways to measure gravity (old)

pendulum- release mass - wants to return to centre as closest to earths centre, stronger attraction faster it returns
wouldn’t return if not for gravity

spring- greater extension of spring=greater gravity

why does gravity change in different places on surface

9.81m/s2 is mean acceleration due to gravity

sometimes above or below due to:
altitude and latitude-
earth is squashed sphere- poles closer to centre than equator so stronger gravity

density- e.g. could be over gas field - lower gravity

subduction zone - changes density

positive vs negative gravity anomalies

+, suggest greater mass + density below surface than expected

-, suggest less mass + density below surface than expected

isostacy + evidence

lithosphere sinks slightly into mantle (tells us it is plastic solid/rheid specifically upper) with added mass, rises up as mass removed
e.g. glaciation

over mountains we see - gravity anomaly as lots of lower density cont crust where expect mantle

direct evidence for comp of Earth (only upper 250km)

Volcanoes and magma deep boreholes crust beneath our feet mines and boreholes ophiolite suites

deep boreholes as evidence

project manhole- tried to drill into mantle from O crust got 187m- obtained core samples of basalt kola super deep borehole- 12626 m, deepest in world (1/3 of cont crust) found and chemically analysed metamorphosed granite

ophiolite suites as direct evidence of comp

collisions of lithospheric plates, break O crust + thrust onto edge of continental plate examine to understand ancient ocean floor

volcanoes and magma as direct evidence of comp

magma erupted onto crust from upper mantle carries up rock sample- chemically analysed to see comp of upper mantle occasionally igneous rocks like peridotite brought to surface too

Mines and boreholes as direct evidence of comp

give direct access to crust conditions mean limited to depth of 4km boreholes- samples of rock brought up to surface and remote sensing can occur

crust beneath our feet as direct evidence of comp

areas where older rocks brought to surface so we can see how upper continental crust is varied

upper layers of earth

lithosphere- all the crust and upper upper mantle rigid asthenosphere- upper mantle from below lithosphere to 670km plastic/rheid- allows plates to move

rheid definition

non molten solid that deforms by viscous or plastic flow in response to applied force

continental crust

up to 4000Ma rich in Al and Si rocks lots of granite average thickness of 35km 2.7gcm-3 density

oceanic crust

rich in Fe and Mg up to 200Ma Lots of basalt (pillow lavas) and gabbro 2.9gcm-3 density average 7km deep

electromagnetic surveys

ground conductivity survey pass current through ground and see how easily it conducts measure ground conductivity e.g. metal = high metal detectors use this to detect metal/liquid underground

seismic tomography basic info

subsurface imaging technique of earth using seismic waves produce by earthquakes etc. study velocity of waves to produce 3d image: P waves faster through cold old rigid material, slows in plastic solids/liquid

how can seismic tomography identify cont crust

extension of cont crust into mantle in mountain ranges p waves travel faster in roots as older and colder than mantle usually there produce high velocity zone

how can seismic tomography identify subduction zones

sub plate is older and colder and more rigid than mantle. seismic waves travel faster through produce high velocity zone

how can seismic tomography identify hot spots and mantle plumbs

mantle plumbs- rising fluid areas of mantle which produce hotspots low velocity zones as P waves slow down in liquid and S waves stop

explanation of Earths magnetic field

magnetosphere as if bar magnet inside running north to south protects from solar winds and radiation indicates movement of iron in earth- in core

origin of magnetic field

Fe and Ni core- spinning spins at diff rate to Earth as liquid opposing rotation of inner and outer core causes self exciting/geomagnetic dynamo

magnetic reversals

Earths magnetic field begins to weaken and poles wander eventually flip completely

how do Fe rich minerals indicate palaeomagnetism

Ig rocks containing Fe behave like frozen compasses at formation Fe minerals align to north then rock crystalises leaving permanent record of direction of north at time of formation date rock to determine age/ N and S over geological time

Magnetic inclination

magnetic minerals align but also incline incline- at an angle (dipped) dip of magnetic mineral based on where they are in comparison to north at north- straight down at south- straight up at equator- parallel to surface