ESS105 final

differentiation

differentiation

the action or process of developing so as to form a system of distinct components

examples of differentiating processes

lava lakes (lava gas becomes rock, degassing process), oil and water (immiscibility), crystalization (supersaturated fluid where crystals and the fluid are two seperate items)

geochemical reservoirs

• caused by differentiation

• Occurs at several scales

• Includes atmosphere, oceans, solid earth

Solid earth

• accounts for most of the earths mass

• Separated into lithosphere, athenosphere, crust, mantle, outer core, inner core

Lithosphere

• Crust and uppermost part of the mantle that form tectonic plates

Athenosphere

• Highly viscous region of the upper mantle on which tectonic plates move

Hypocentre

• where the fault occurs

Epicentre

• the closest thing on earths surface to the centre

Wave propagation

• the closer the seismometer is to the hypocenter the faster it gets to the reader

What does pp mean compared to p wave

• p is the initial wave directs and pp is deflected once

• PPP is deflected twice

Seismic shadows

• some places we can only record p waves

S wave shadows

• They propagate through the core

• Can’t travel through liquids

• Creates big shadow below the core where seismometers dont pick up S waves

P wave shadows

• change diction when they move through the core

• They refract q and change speeds which causes little shadows on the sides where the direct p waves wont reach

PREM

• preliminary reference earth model

• can compare to the depths of the different layers and see how it matches up

• can see when we hit the core and the speed changes due to it now being liquid

Asthenosphere

• corresponds to low velocity zone

• thought to be less stiff because of melting

the crust

• shallowest of the reservoirs everything that we interact with

• small volume of earth

• interacts with hydrosphere, atmosphere and biosphere

• part of the lithosphere

• includes sediments dposited in oceans burried as rocks that can melt to form lavas

• lavas release volcanic gases into the atmpsphere

• has both oceanic and contenential

differences between oceanic and continental crust

• oceanic

  • thin

  • dense

  • basaltic

  • generally younger

• continental

  • thick

  • less dense

  • granitic

  • older

plate tectonics

• recognized that the seafloor is created at mid ocean ridges

  • concept of seafloor spreading (if its spreading that the edges will be younger

• the idea that continents used to be connected and spread over time

• earth has a ridged outermost layer (lithosphere) that overlies less ridged layer (asthenosphere)

• lithosphere splits into tectonic plates that move relative to one another

evidence for seafloor spreading

• fossils assembled found at edges on different sides of the atlantic

• similarities in the rock formations facing the continental margins

Wilson cycle

• from u of t grad

• cycle of the ocean opening and closing was proposed by him

• this cemented the idea of plate tectonics in the broader scientific community

plate movements

• constructive = moving apart

• destructive = moving together

• transform = sliding by eachother

constructive margins

• Atlantic plate boundary

  • area with lots of volcanic activity at constructive zones

  • occurs underwater on ocean floor

  • creates new crust

• can occur at oceanic or continental boundaries

• as the new is formed, it pushes out what was previously there and it cools to create the new crust

• this thickens the lithosphere

• the constructive contenential crust is less dense so it will float on top of the oceanic causing subduction of oceanic crust

Subduction zones

• granitic crust is less dense so it goes on top of the basaltic crust creating a destructive margin that causes the formation of continental crusts

destructive margins

• new crust is forming at destructive margins

• subductions cause a granitic crust which is less dense

crust composition outside building

• high in Si and O cations and anions

  • form SiO4 2-

• the composition of earth changes depending on where you are and where it is grown

• toronto has shales and limestones

bulk crust contains

• mostly oxygen, Si, Al

• less Mg and K

why are different samples of earth containing different minerals

• it depends where the sediment is coming from

• in toronto there is a river catchment that contained different things than the bulk earth

where is the best place to look for dirt that best represents the bulk earth

• its best to take it from a large river basin that has to travel far to get there

• this allows for the best estimate of bulk crust because it collects sediment from the most area

what are seafloor sediments

• limestone (calcareous sediments)

• clays (deep sea mud)

• oceanic sediments are close to lit value but still to high in Ca

minerals are

• naturally occuring

• defined in composition

• have long range order

• minerals are solid

increased calcium means

• more easily weathered at earths surface conditions

minerals in order of decreasing stability at earths surface

• quarts

• muscovite

• feldspar

• botite

• albite

• amphiboles

• pyroxene

• anorthite

• olivine

• calcite

  • as we move down we increase the amount of calcium

why is there an increase in calcium in the oceanic sediments

• because rocks that include calcium are most easily weathered which accounts for the sampling bias in the river run offs

more stable at earths surface includes

• granites

• contain SiO in higher quantities

less stable at earths surface contains

• Fe

• Mg

• Ca

minerals that are stable vs unstable in the deep earth

• stable

  • Fe, Mg, Ca,

• unstable

  • SiO

  • Al

  • Na

  • K

how can we see samples of the deep earth composition

• mid ocean rift valleys

• ophiolites

• mantle xenoliths

Mid ocean rift valleys

• where plates are moving apart (constructive) and creating new crust at plate valleys

• causes sea volcanoes

• magma rises at the center of the ridge leading to the faulting of blocks of rocks and accommodating strain seen in the photo

• the different faults are what cause the exposure of mantle rocks

• this allows us to measure the composition of deeper earth minerals

abyssal perodotites

• mineral samples aquired by the mid ocean rift valleys

• collected by research vessels

ophiolites

• come from the closing of the ocean basins

• are often formed at subduction zones

• when overriding oceanic crust is obducted and end up overriding the oceanic lithosphere and placed on the edge of the continential crust

• often causes flat topped hills

mantle vs crustal rocks

• mantle rocks contain little vegetation, orange brown colour and steep sided edges

• crustal rocks contain vegetation, grey in colour, more rounded morphology

Mantle xenoliths

• pre existing rock in an igneous rock (old rock in a newly formed magma rock)

• kimberlites are what bring the mantle rocks to the surface

Kimberlites

• where we can mine diamonds

• dig down in a v shape

• type of igneous intrusion that originates from the mantle

• bring the mantle rocks to the surface

difference between abyssal peridotites and ophiolites

• kimberlites sample the mantle beneath the continents rather than the oceans so we get a different type of crust

peridotites

• rocks we find in the mantle

• primary minerals are olivine and pyroxene

  • contain low Si and Al but are high in Mg, Ca and

• also aluminous phase that is also feldspar, spinel or garnet depending on depth

how can we look at rock compositions lower than perrdotites

• look at the PREM

• we can see high up where periodites are and can see that there are several jumps in wave velocities as we go deeper indicating different rock types

what happens with depth

• compression

  • increases stiffness of material and increases density

• promote faster wave speeds

• pressure increases gradually so simple compression would change the speeds gradually

changes in wave speeds through earths crust requires

• changes in chemistry

  • changes in elements making up the material

• changes in mineral structure

  • how the atoms of elements are arranged in the molecule

what happens to peridotites as we move down into the earth

• lots of olivine

• then we get wadsleyite

  • transforms at 14 GPa

• this happens from change in pressure

• eventually we get to Ringwoodite which splits the minerals into two

1GPa =

30 km depth

ringwoodite

• goes from Mg2SiO4 → MgSiO3 + MgO

Phase transitions

• when pressure increases changing the type of mineral we find without changing the elemental composition

• can split the mantle into different regions what can be observed with the wave velocities

Most abundant mineral on earth

• bridgmanite

• we dont see if often because it is so low in the mantle that it doesnt come to the surface but it takes up majority of the manle

Bulk Silicate earth

• High in Si, O, Mg

p wave velocities of the core

• match up to olivine, Be, Al, Li

• not unique to one material

why is the core not just melted periodite

• because of the density distribution

• the earth flattening shows that there has to be an inner core that is heavier than the outer layers and large enough to match with the amount of flattening occurring

equitorial bulge

• flattening occuring in the earth when the radius deviates from that of a sphere of the same volume

• more bulge in the middle than on the poles

• only about 3% flattening

why is the bulge at the equator

• this is the region moving the fastest so it has the highest angular velocity

what controls flattening

• speed of spinning

• inward pull of gravity

  • governed by mass

    • g=GM/r^2

    • g= accelleration due to gravity

    • G = gravitational constant

    • r= radius

    • m = mass

centripetal force

• the inward force of gravity that keeps the bulge small

• if mass is spread then the force will be weaker

• if the mass is concentrated it will have a greater force

concentrated central mass

• all mass in small area in centre

• more effective to resist the flattening effects of the rotation of the planet\

rotational inertia

• how much torque is needed to get something spinning at the desired rate

  • larger number is harder to start rotation smaller is easier to start rotation

  • when weight is closer to the centre its easier to start

• can be linked to flattening

• for earth has been found to be 0.331MR^2

  • m is the mass of earth

  • r is the radius of earth

• earth is like the third

How can we tell core size and mass

• size from the s wave shadows

• weight from the rotational inertia and centripitial force

what density should the core have

• 9800-13500 kg/m^3

most likely element making up the core

• iron

  • but there has to be other elements too cause the iron is just below on the wave velocities

  • it is also above the pressure density so has to be less dense

• so we have to add light element

iron meteorites

• formed by planetesimals

• thought to be fragments of the cores of these that were around during planetary formation

• contains Fe and Ni alloy in pattern

  • but we still need a light element

• limited though cause planetismal only have low pressures and earth is higher

  • also temp in earth is higher

  • and oxygen availability may have been different

Widmanstatten pattern

• cross hatched

• in iron meteorites

• occurs cause of slow cooling of Fe- Ni alloy that separates into Ni rich and poor mineral phases

what is likely present with Fe and Ni in earths core to lighten it

• S, P, C, Si, O in the core likely to decrease desity

Fraunhofer lines

• dark absorption lines seen when looking at the suns light wavelengths

• energy is absorbed by elements promoting electrons to higher energy orbitals

sun composition

• sun made of H and He

  • diff to earth

• also O, Fe, Si, Mg

Refractory / non volatile

• Si, Fe, Mg

• have similar abundances to the sun

volatile

• C, O, S

• different abundances to the sun

• easily evaporated at normal temperature

Stony meteorites

• chondrites and achondrites

  • undifferentiated and differentiated

chondrites

• no core

• all one

• have good refractory element matches to the earth

igneous rocks

• formed by crystallization of magma and lava

metamorphic

• formed by recrystallization of pre-existing rock

sedimentary

• formed by deposition and compression of mineral and rock particles

where does magma formation occur

• at mid-ocean ridges and subduction zones

magma formation at mid ocean ridges

• decompression melting happens

• this allows the melting point of peridotite to be at the average geotherm

• hotter deeper rocks in the mantle are brought upwards to shallower levels at the mid ocean ridges

• when the hotter rocks rise it changes the thermal profile giving partial melting

average oceanic geotherm

• plot of the relationship between depth and temperature in the earth

• theres an increase in temperature with depth

• but at average the peridotite wont melt so something else is happening at mid ocean ridges to cause melting

decompression melting

• when the rising of hot rocks from lower in the earth causes a temperature increase in the oceanic geothermal and causes partial melting

• occurs at mid ocean ridges

melting at subduction zones

• caused by depression melting

• as minerals breakdown when moving deeper into the earth they release H2O

  • this hydrates the mantle above the subducted slab

• when something is hydrated it decreases the melting temperature of that rock

melting point depression

• the decreasing in the melting temperature of rocks by the addition of water from hydrous minerals being pushed deeper into the earth

partial melt

• when part of a mixture melts and produces a coexisting solid and liquid

• remember salt and water experiment

  • the more salt there is the less water freezes

liquidus

• boundary above which the system is entirely liquid

• rocks undergoing decompression dont come close tothe liquidus

solidus

• boundary below which the system is entirely solid

eutectic point

• the temp and composition of the very first liquid to form if we were to heat up ice

• composition of the first melt to form in multi-component system

when you melt 10% of paridotite you get

• a basaltic composition which explains the oceanic crust being basaltic

decompression melting gives

• a mix of melt and crystals

• only 10% of peridotite is done

crystallization

• melting of rock leads to formation of melt crystals

• magmatic processes create differentiation when the heavier of the crystallized material sinks to the bottom

bowens Reaction series

• crystallization sequence

• shows rock type, what elements its rich in, and what mineral it is as it moves down and cools to recrystallize

mafic minerals

• Fe and Mg rich minerals

• crystallize first at the highest temperatures

Felsic minerals

• silica rich minerals

• crystallize at lower temps than the mafic

Plagioclase

• type of feldspar mineral

• crystallizes over a wide temp range

  • changes composition from Ca rich to Na rich from high to low temperatures

extrusive rocks

• form on earth surface

• can undergo weathering and erosion

intrusive

• form below earths surface

• cannot undergo weathering and erosion

  • must go through uplift to become outcrop so that they can be weathered

uplift

• the vertical elevation of rocks in response to geological processes

• through

  • orogenic events (mountain building)

  • isostatic rebound

  • plate flexure

denudation

• weathering and erosion which acts to reduce the amount of vertical elevation that uplifted rocks undergo

• helps to bring them back toward sea level

Isostasy

• the rising or settling of a portion of earths surface that occurs when weight is removed or added in order to maintain equilibrium between buoyancy forces that push the surface upwards

• cork in water has to displace water of the same mass

• all columns must add up to the same amount of mass

density of the earths things

• crust is lower

• mantle is higher