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Geology As + A2 > physical structure of the earth > Flashcards

physical structure of the earth Flashcards

1
Q

layers of the earth in order

A

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

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2
Q

Crust properties

A

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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3
Q

Moho

A

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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4
Q

Upper mantle properties

A

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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5
Q

Lower mantle properties

A

700-2900km

extremely high pressure and temp
(around 7000 degrees F)

solid more rigid

solid, plastic like

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6
Q

Guttenberg discontinuity

A

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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7
Q

outer core properties

A

2900-5100km

mostly liquid Fe and Ni

very hot 4500-5500 degrees C

liquid

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8
Q

Lehmann discontinuity

A

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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9
Q

inner core properties

A

5100-6371km

iron and nickel

solid

most dense

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10
Q

evidence for structure of the Earth

A

ophiolites

volc eruptions

boreholes

direct observation

density

study of meteorites

gravity

seismic waves

isostacy

presence of magnetosphere

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11
Q

what is a seismometer and how does it work?

SEE BOOKLET FOR DIAGRAM

A

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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12
Q

P wave properties

A

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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13
Q

S waves properties

A

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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14
Q

L wave properties

A

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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15
Q

why do some seismograms only show P and L waves?

A

recorded in S wave shadow zones

103-103

created as S waves cant travel through liquid outer core

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16
Q

Why do some seismograms only show L waves?

A

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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17
Q

changes in p waves through earth

A

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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18
Q

changes in S wave through earth

A

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

19
Q

density of the Earth as evidence for structure

A

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

20
Q

gravity def

A

force of attraction that exists between any 2 masses

greater mass + shorter the distance = shorter

gals

21
Q

ways to measure gravity (old)

A

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

22
Q

why does gravity change in different places on surface

A

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

23
Q

positive vs negative gravity anomalies

A

+, suggest greater mass + density below surface than expected

-, suggest less mass + density below surface than expected

24
Q

isostacy + evidence

A

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

25
Q

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

A

Volcanoes and magma

deep boreholes

crust beneath our feet

mines and boreholes

ophiolite suites

26
Q

deep boreholes as evidence

A

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

27
Q

ophiolite suites as direct evidence of comp

A

collisions of lithospheric plates, break O crust + thrust onto edge of continental plate

examine to understand ancient ocean floor

28
Q

volcanoes and magma as direct evidence of comp

A

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

29
Q

Mines and boreholes as direct evidence of comp

A

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

30
Q

crust beneath our feet as direct evidence of comp

A

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

31
Q

upper layers of earth

A

lithosphere- all the crust and upper upper mantle
rigid

asthenosphere- upper mantle from below lithosphere to 670km
plastic/rheid- allows plates to move

32
Q

rheid definition

A

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

33
Q

continental crust

A

up to 4000Ma

rich in Al and Si rocks

lots of granite

average thickness of 35km

2.7gcm-3 density

34
Q

oceanic crust

A

rich in Fe and Mg

up to 200Ma

Lots of basalt (pillow lavas) and gabbro

2.9gcm-3 density

average 7km deep

35
Q

electromagnetic surveys

A

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

36
Q

seismic tomography basic info

A

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

37
Q

how can seismic tomography identify cont crust

A

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

38
Q

how can seismic tomography identify subduction zones

A

sub plate is older and colder and more rigid than mantle.

seismic waves travel faster through

produce high velocity zone

39
Q

how can seismic tomography identify hot spots and mantle plumbs

A

mantle plumbs- rising fluid areas of mantle which produce hotspots

low velocity zones as P waves slow down in liquid and S waves stop

40
Q

explanation of Earths magnetic field

A

magnetosphere

as if bar magnet inside running north to south

protects from solar winds and radiation

indicates movement of iron in earth- in core

41
Q

origin of magnetic field

A

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

42
Q

magnetic reversals

A

Earths magnetic field begins to weaken and poles wander

eventually flip completely

43
Q

how do Fe rich minerals indicate palaeomagnetism

A

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

44
Q

Magnetic inclination

A

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

Geology As + A2 (15 decks)

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