physical structure of the earth Flashcards
layers of the earth in order
Crust
(Moho)
Upper mantle
lower mantle
(Guttenberg)
outer core
(Lehmann)
inner core
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
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
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
Lower mantle properties
700-2900km
extremely high pressure and temp
(around 7000 degrees F)
solid more rigid
solid, plastic like
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
outer core properties
2900-5100km
mostly liquid Fe and Ni
very hot 4500-5500 degrees C
liquid
Lehmann discontinuity
Separates outer and inner core
5100km
Phase boundary- gradient change
changes:
abrupt increase in P wave
liquid to solid but comp similar
inner core properties
5100-6371km
iron and nickel
solid
most dense
evidence for structure of the Earth
ophiolites
volc eruptions
boreholes
direct observation
density
study of meteorites
gravity
seismic waves
isostacy
presence of magnetosphere
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
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
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
L wave properties
Love waves- arrive last
surface waves
most destructive- oscillate in circular motion, lose e v quickly so most destructive
largest amplitude
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
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)
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
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
