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- 1Where and Why do magmas form?Abundant volcanism on the modern Earth is linked to
thermal energy stored in the planet. In contrast, volcanism
is almost extinct on the Moon. - 2Where does the energy come from?Isotope creation may occur in the atmosphere due to solar
radiation, inside stars, or during Super-novae events.
Isotopes generated in Super Novae are the primary source
of mantle heat when the decay. - 3A major source of internal heat within the Earth?Is the radioactive decay of the longlived
isotopes of 238U, 235U, 232Th, 40K, 87Rb, 176Lu, and 147Sm, which have half-lives of
billions of years. - 4Contribution of Tidal Energy?The contribution of tidal energy due to deformation of solid Earth by gravitational pull
of the Sun and Moon is estimated to be an order of magnitude less than that of
radioactive decay. - 5Another source of the thermal energy in the earth ?is the ongoing solidification of the
liquid outer core.
Original heat from the accretion planetesimals as their gravitational potential energy
was transformed into kinetic and thermal energy. - 6The liquid mantle ?Heating generated a liquid mantle beneath this outer crust (a “Magma Ocean”) that
might have lasted a few 100 million years maximum. - 7Collision of Earth with Theia?Collision of Earth with Mars-size Theia created the
moon and modified the Earth. - 8Earth’s Internal Heat derives from ?Most of Earth’s internal heat derives from radioactive decay of uranium, thorium and
potassium 40. Rock is a poor heat conductor, so it takes relatively little radioactive
decay to build up significant heat. - 9Whats the temperature at the base of the crust? And mantle core boundry?The temperature at the base of the crust is 800-1200°C. the mantle-outer core boundary it is 3500-5000°C, Earth’s center
estimated to be very near that of the Sun’s surface, 6500°C. - 10Is it earths magma still molten?The Earth’s mantle is not a region of pure molten magma anymore. It is mainly a solid,
although it does convect. - 11Temp of subducted oceanic crust in the upper mantle?The
crust remains a cool thermal
anomaly for 10’s or 100’s of
million years because of the
very poor thermal
conductivity of the mantle. - 12Mantle convection rate?Even though the rate at which the
mantle convects (cm/year) is low, it is
far greater than the rate of heat
transfer by conduction. - 13Where can igneous rocks trace back too ?All igneous rocks (and therefore all rocks) can
ultimately be traced back to the Earth’s mantle - 14What do oceon class consist of ?The continents and ocean floors consist of material extracted from this mantle. The Earth’s
mantle represents a solid (not liquid) shell 2900km thick, which surrounds a partially molten
metallic core. - 15What are layers of earths defined by?The layers of the Earth are defined by the presence of a marked change in the seismic wave
velocity - 16What are mantle subdivisions defined by?Mantle subdivisions are based on seismic wave velocities that depend on the
stability of particular minerals at differing pressures. Below 350 km is the
mesosphere where high pressures keep the hot rock solid. Seismic discontinuities are
present within the mesosphere as a result of differing mineral stabilities with depth - 17How thick is the earths crust?The Earth’s crust ranges from 5-10km thick beneath the ocean basins
to in excess of 70km beneath the major mountain ranges.
• The average crustal thickness in continental areas is approximately 35
km - 18What does the crust ride upon?• The crust rides upon a relatively stiff layer in the upper mantle. The
combination of the crust and the stiff upper mantle layer is called the
lithosphere. - 19What components can the upper mantle be subdivided into?Lithospheric, Asthenospheric and mesospheric.
- 20What is belore 350km?The mesosphere where high preassure keeps the hot rock solid.
- 21How thin is the earths crust?5-10km beneath the oceon and in excess of 70km beneath the major mountain ranges.
- 22What are Archean Cratons ?Unusually Thick Crust
Cratons are relics of a hotter mantle 2.5 billion years ago or more. Unusually hot mantle melts left “refractory” mantle residues that form thick
keels beneath Archean continents. - 23What are the models of mantle convection?Two main models of mantle convection proposed either convection cells that are
restricted to the asthenosphere or convection through the entire mantle. Recent models are more complex but favour whole mantle - 24Definition of Magma ?Magma is molten rock that consists of 3 states of matter: crystals, liquid and gas.
- 25What are the origin and properties of Magma?The transformation from magma to rock and vice versa is controlled by
temperature, pressure and water content.
• As magma becomes more fluid-like, its density is reduced. - 26What is Magma partial melting?Occurs when some
components melt first. The
process is also called
fractional melting.
Fractionation occurs when
the melt is separated from
the residual solid. - 27What is Magma Fractionation?Fractionation refers to the
removal of some part of the
system or the physical
separation of parts of an
original system. - 28What temperature do rocks melt at and whats it called?Rocks melt over a range of temperatures. The point at which melting begins is the
“eutectic”. This is generally lower than the melting temps of individual minerals. - 29How does Mantle material in the asthenosphere rise?Mantle material in the asthenosphere rises almost adiabatically, because the
temperature does not drop much as the pressure is reduced. - 30How does the athenosphere melt ?The asthenosphere melts to progressively greater extents once it crosses the
geotherm but does not melt completely in the modern Earth (The P-T path does not
cross the Liquidus). - 31What was the mantle like 4 billion years ago?Probably, 4 billion years ago the upper mantle was entirely liquid over some depth
range because of greater heat in the mantle at that time (a “magma ocean” in the
mantle). - 32Why does volcanism occour where it does?The information given above provides an explanation for the distribution of
magmatism at: 1) ocean spreading centres, 2) subduction zone volcanoes and
3) with mantle plumes or hotspots. - 33How do mid ocean ridges form ?Mid-ocean ridges form where two
oceanic plates are being pulled away
from each other. As they are pulled
apart Asthenosphere rises toward
the "gap”. This material ascends to
near surface almost adiabatically at
temperatures of ~1200° C. - 34What are normal mid oceon ridgeds and enriched mid oceon ridges?Normal MORB (N-MORB) are basalts
derived from typical upper mantle
that has been “depleted” by melt
extraction over > 4 billions years.
Some parts of the mantle may be
comparatively enriched either by
veining (most likely) or because they avoided prior melting and are un-depleted
(probably unlikely or rare). When this mantle material melts the resulting basalts are
“enriched MORB (E-MORB). - 35What do Mantle Plumes do?Mantle plumes and Hotspots are widely considered to form islands, such as Iceland or
the Hawaiian Chain. In the case of Hawaii, the volcanism does not occur at a spreading
centre but is “intra-plate”. Why do melts form in an intra-plate setting? The adiabatic
rise of mantle material is still important but the reason that magma melts occur where
MORB would not is because the plumes rising from the deep mantle are hotter than
the upper mantle associated with MORB. - 36Why do mantle plumes occur?Plumes from the core – mantle boundary or 670 km are generally considered to be
the products of recycling that is ultimately linked to subduction. - 37Effect of subduction on mantle plumes ?Subduction of oceanic crust causes the
release of aqueous fluids and partial melts
that can modify upper mantle chemistry,
whereas the refractory (dense) remnants of
oceanic slabs probably sink through the
mantle to the 670 km boundary where they
form a chemically-distinct layer that may
ultimately be transient- It can break through
the boundary and descend to the mantle
core boundary. T - 38What happens at the core mantle boundryLow-velocity scattering
structures (S*1, S*2) on the
lateral margins of the slab pile
are postulated to account for
the concentrated scatterers. - 39How do Subduction Zone Processes and Magmatism interact?Models of subduction zone processes continue to be refined. Magmatism is
now considered to usually originate in the mantle wedge. Slab melting can
occur but is not the dominant process in average subduction zones. Where
melting originates in the wedge, the process is an example of melt generation
by addition of water, which is a combination of H2O carried down in the
subduction system and the breakdown of minerals containing (-OH). These
“hydrous:” mineral formed by seawater interaction with oceanic crust. - 40What is Pore water in subducting slabs?Pore water in subducting slabs: Water probably descends into deeper parts of
the slab via faults to cause serpentinization, the hydration of Mg-rich,
magmatic, olivine to form the oh-bearing mineral serpentine. The figure
below comes from a 2-D modelling study to explain the fact that seismic
surveys at trenches reveal that the oceanic plate could be pervasively
hydrated for several kilometres below the crust–mantle boundary. Heat-flow
surveys indicate active fluid circulation within the slab. - 41What is a mineral ?Minerals are naturally occurring, solid substances with the following
properties:
• a composition that is either fixed (e.g., elements such as
copper, Cu; Quartz, [SiO2]) or variable within a restricted
range [e.g., Olivine, (Mg,Fe)2(SiO4)];
• a crystalline arrangement of atoms in an ordered geometric
framework; and
• usually formed by inorganic processes, with a few
exceptions such as aragonite [CaCO3]in shells and apatite
[Ca5(PO4)3(OH,F,Cl)] in bones. - 42What are Mineral Characteristics ?Characteristic crystal structure
Atoms in minerals are arranged in regular, repetitive geometric patterns. This
pattern is the characteristic and unique crystal structure. - 43What are Mineral Properties ?The external form and optical properties of crystals are determined by
crystalline structures. The crystal form of some minerals clearly reflects
their atomic structure - 44What are Crystal “Habit” (form) ?Some minerals grow in distinctive shapes, which can be used to identify
them. This is the tendency of certain minerals to break in preferential directions
along brightly reflective planar surfaces. Biotite (a mica) has one whereas
halite (salt) has three. - 45What are cleavage planes?Cleavage Planes
Cleavage planes are much more
common than crystal faces so
they are more useful. The
features may be visible in “hand
specimen” or under the
microscope. - 46What is mineral hardness ?A mineral’s resistance to scratching which is determined by crystal
structure and the nature of the bonds. It is unrelated to cleavage.
We define hardness using the Moh’s scale: - 47What is mineral density?We can measure the density or specific gravity of a mineral but it is more
useful as a comparative tool in conjunction with experience
Galena (PbS) has a high density as does barite (BaSO4). - 48How does cooling rate chane Olivine?Olivine (seen in polarised light) has a
limited capacity to change its
compositions with changing magma
conditions and may react with the
magma if left at intermediate
temperatures for long periods. - 49What is Changing Magma Compositions?This pyroxene (seen in
polarised light) exhibits sector
zoning because particular
crystal faces more or less likely
to admit certain atoms onto a
growing face.
It also displays concentric
oscillatory zoning as a result of
slight changes in magma
composition as it grew. - 50How does Plagioclase change?Plagioclase or
Ca (Na) Feldspar
Forms part of the
continuous Bowen’s
Series. If a magma
changes composition
and then slowly cools
plagioclase may reequilibrate
to a uniform
composition.
Often however, it
displays concentric
and/or complex zoning.
(Polarised light) - 51What is Melt Generation Upper Mantle Mineral Reactions ?The upper mantle is
composed of several types of
ultramafic rocks called
peridotites. They are rich in
olivine but also contain
pyroxene, plagioclase ±
garnet and minor non-silicate
minerals - 52What is the most important mineral?the most important are the silicates which contain silica tetrahedra
(SiO4). Among the silicates, there is a range of ways that Si tetrahedra may be
bonded to each other - 53How does the silica tetrahedra go down bowens reation chain ?The key point for
our purposes is
that the crystal
structure becomes
more open from
single tetrahedra to
sheets and the
temperature of
formation for
igneous minerals
decreases - 54What are the two series in Bowens reaction series ?Discontinuous (Mafic minerals) and Continuous (Felsic minerals).
- 55What is Olivines capactiy to change?Olivine (seen in polarised light) has a
limited capacity to change its
compositions with changing magma
conditions and may react with the
magma if left at intermediate
temperatures for long periods. - 56Properties of Pyroxene?This pyroxene (seen in
polarised light) exhibits sector
zoning because particular
crystal faces more or less likely
to admit certain atoms onto a
growing face.
It also displays concentric
oscillatory zoning as a result of
slight changes in magma
composition as it grew. - 57How does Plageoclase form ?Plagioclase or
Ca (Na) Feldspar
Forms part of the
continuous Bowen’s
Series. If a magma
changes composition
and then slowly cools
plagioclase may reequilibrate
to a uniform
composition.
Often however, it
displays concentric
and/or complex zoning.
(Polarised light) - 58What is the upper mantle composed of ?The upper mantle is
composed of several types of
ultramafic rocks called
peridotites. They are rich in
olivine but also contain
pyroxene, plagioclase ±
garnet and minor non-silicate
minerals - 59What does Partial melting in the upper mantl consume?The least stable minerals (plagioclase, pyroxene then olivine)
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