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Specifically, the heat generated during the Earth’s formation came from the following sources: accretion energy, adiabatic compression, core formation energy and decay of short-lived radio-isotopes.
- Primordial heat
The heat generated by long-term radioactive decay): main sources are the four long-lived isotopes (large half- life), namely K40, Th232, U235 and U238 that continuously produces heat over geologic time.
- Radioactive heat
The temperature increase with depth into Earth (the non-linear temperature/depth curve)
Geothermal gradient or geotherm
Temperature gradient in the crust: ~25°C/km
Based on the geotherm curve above, it can be deduced that the mantle is considerably hotter than the crust, and the core is much hotter than the mantle.
Core-mantle boundary
3,700°C
formation came from the following sources
:accretion energy, adiabatic compression, core formation energy and decay of short-lived radio-isotopes.
Inner-core – outer-core boundary
5,000°C±500°C
Earth’s center
6,400°C±600°C
How the Earth’s internal heat is redistributed:
conduction, convection and radiation
occurs at the mantle but not between the core and mantle or even between the asthenosphere and lithosphere (except at sea-floor spreadingzones).The only heat transfer mechanism in these transition zones is through conduction.
Convection
A molten and semi-molten rock mixture found under the surface of the Earth
Magma
This mixture is usually made up of four parts:
- Melt - a hot liquid base
- Minerals crystallized by the melt
- Solid rocks incorporated into the melt from the surrounding confines
- Dissolved gases
originates in the lower part of the Earth’s crust and in the upper portion of the mantle.
Magma
All other things being equal, every mineral has a distinct melting point. In the mantle, heat is brought upward by convection. As hot rocks convect upward they transfer heat to cooler rocks lying above them, which may melt.
- Temperature
All other things being equal, the greater the pressure, the less likely materials are to melt. (This explains why the asthenosphere is limited to a shallow region of the mantle and the inner core is solid despite being hotter than the liquid outer core.) When rocks experience decompression without losing their heat, they can experience decompression melting. Consider the fate of hot rocks rising through the mantle from a hot spot.
- Pressure:
Generally, the addition of substances like water or CO2 to a mineral lowers its melting point. In this case, the shape of the melting curve for peridotite changes.
- Volatile substances
The decrease in pressure affecting a hot mantle rock at a constant temperature permits melting forming magma. This process of hot mantle rock rising to shallower depths in the Earth occurs in mantle plumes, beneath rifts and beneath mid-ocean ridges.
- Melting due to decrease in pressure (decompression melting):
- When volatiles mix with hot, dry rock, the volatile decreases the rock’s melting point and they help break the chemical bonds in the rock to allow melting.
- Melting as a result of the addition of volatiles—compounds that have low boiling points (flux melting):
A rising magma from the mantle brings heat with it and transfer heat to their surrounding rocks at shallower depths which may melt.
- Melting resulting from heat transfer from rising magma (heat transfer melting):
The rising magma in mantle convection cell brings heat to the surface, transferring heat to the overlying rocks. The transfer of heat due to convection is accompanied by a decrease in pressure or “decompression” associated with the spreading of the lithospheric plates.These two work in tandem promoting the partial melting of rocks along the spreading center.
- Mid-oceanic ridges.
Similar to mid-oceanic ridges, the transfer of heat and decompression result to magma generation. The source of heat for mantle plumes is much deeper.
- Mantle plumes (hot spots).
Oceanic crustal rocks are formed along spreading centers, typically beneath several kilometers of seawater. The presence of water during generation results to the formation of hydrous minerals. As the oceanic slab is down-thrusted along subduction zones, the change in temperature and pressure conditions brings about mineral instability (e.g. hydrous minerals) and the release of water to the surrounding hot rocks. The introduction of water effectively lowers the melting temperature of rocks and therefore causes partial melting or magma generation.
- Subduction zones.
magma is less dense than the surrounding country rock. Magma rises faster when the difference in density between the magma and the surrounding rock is greater.
- Density contrast:
magma passes through mineral grain boundaries and cracks in the surrounding rock. When enough mass and buoyancy is attained, the overlying surrounding rock is pushed aside as the magma rises. Depending on surrounding pressure and other factors, the magma can be ejected to the Earth’s surface or rise at shallower levels underneath
- At deeper levels
magma may no longer rise because its density is almost the same as that of the country rock. The magma starts to accumulate and slowly solidifies.
- At shallower levels
- : a measure of a fluid’s resistance to flow. Magmas with low viscosity flow more easily than those with high viscosity. Temperature, silica content and volatile content control the viscosity of magma. Use the table below to clarify the effects of different factors on magma viscosity.
- Viscosityb
Factor
↑ Temperature
↑ Silica Content (SiO2)
↑ Dissolved water (H2O)
Mafic magma is less viscous than silicic (felsic) magma because it is hotter and contains less silica. Also, the volatiles in magma decreases viscosity. Effect to Viscosity
↓ Viscosity
↑ Viscosity
↓ Viscosity
is the process of creating one or more secondary magmas from single parent magma.
Magmatic differentiation
– a chemical process by which the composition of a liquid, such as magma, changes due to crystallization. There are several mechanisms for crystal fractionation. One that is directly related to the Bowen’s reaction series is crystal settling.
- Crystal Fractionation
*denser minerals crystallize first and settle downwhile the lighter minerals crystallize at the latter stages. Bowen’s reaction series shows that denser minerals such as olivine and Ca-rich plagioclases form first, leaving the magma more silicic.
- Crystal settling
as described in Bowen’s reaction series, quartz and muscovite are basically formed under low temperature conditions, making them the first ones to melt from the parent rock once exposed in higher temperature and/or pressure. Partial melting of an ultramafic rock in the mantle produces a basaltic magma.When solid mixtures partially melt, it is the lower melting point materials that melt first.
Separation can occur in partial melts, with the high melting point materials sinking to the bottom and the liquid from the lower melting point materials flowing to the top. These two different materials, that have different chemical compositions and different physical properties, may then be further separated, e.g., by the liquid rising further through overlying materials, leaving the solid behind.
Oxygen/silicon-rich rock-forming minerals have lower melting points than iron/magnesium-rich minerals.
Each stage of partial melting produces rocks enriched in oxygen/silicon (and depleted in iron/magnesium)
- Partial Melting
this may occur when two different magma rises up, with the more buoyant mass overtakes the more slowly rising body. Convective flow then mixes the two magmas, generating a single, intermediate (between the two parent magmas) magma
- Magma mixing –
a reaction that occurs when the crust is mixed up with the rising magma. As magma rises to the surface, the surrounding rocks which it comes in contact with may get dissolved (due to the heat) and get mixed with the magma. This scenario produces change in the chemical composition of the magma unless the material being added has the same chemical composition as the magma
- Assimilation/contamination of magma by crustal rocks
As a response to heat, pressure, and chemically active fluids, minerals become unstable and change into another mineral without necessarily changing the composition. For example, coal, which is composed entirely of carbon, will turn into a diamond (also composed of carbon) when subjected to intense pressure.
METAMORPHISM
- The mineral composition of the resulting metamorphic rock is influenced by the following:
- Mineral composition of the original or parent rock
- Composition of the fluid that was present
- Amount of pressure and temperature during metamorphism
- Factors controlling the mineral assemblage of metamorphic rocks include:
- Bulk composition of the original rock
- Attained pressure during metamorphism
- Attained temperature during metamorphism
- Composition of fluid phase that was present during metamorphism (Nelson, 2011).
Certain minerals identified are good indicators of the metamorphic environment or zone of regional metamorphism in which these minerals are formed (Tarbuck and Lutgens, 2008).
index minerals
Earth Science, is an alteration of the size or shape of rocks. It is caused by stress, the scientific term for force applied to a certain area.
DEFORMATION
the relative change in shape or size of an object due to externally applied forces
STRAIN
the internal force per unit area associated with a strain
STRESS
stress denoted by squeezing or pushing an object from outside towards itself. The stress that squeezes rocks together. Compression causes rocks to fold or fracture (break).
- Compressive stress (compression)
stress denoted by pulling or stretching. Rocks are being pulled apart. Tension causes rocks to lengthen or break apart.
- Tensional stress (tension)
stress denoted by wrenching or formed by a force vector component parallel to the cross section. Forces act parallel to each other but in opposite directions. Shear stress causes two planes of materialto slide past each other.
- Shear stress
- adeeply buried rock is pushed down by the weight of all the material above it. Since the rock is trapped in a single spot, it is as if the rock is being pushed in from all sides. This pushing causes the rock to become compressed, but it cannot deform because there is no place for it to move.
- Confining Stress
irreversible, resulting in a permanent change to the shape or size of the rock that persists even when the source of stress stops.
- Ductile
temporary and reversed when the source of stress is removed.
- Elastic
also known as fracture, is irreversible and results in the breakage of rock
- Brittle
returns to its original shape once the stress that deforms it is removed
Elastic
does not return to its original shape after it is deformed
Inelastic
Inelastic material category
- Brittle 2. Ductile
to stress by breaking or fracturing
- Brittle
respond to stress by bending or deform without breaking
- Ductile
Factors that affect deformation:
- Confining stress
- Temperature
- Strain rate
- Composition
direction of line formed by the intersection of a rock surface with a horizontal plane
STRIKE
acute angle that a rock surface makes with a horizontal plane
DIP
Features of brittle deformation:
- Joints 2. Faults
a crack in a rock along which no appreciable movement has occurred.
- Joints
a plane of dislocation where rocks on one side of the fault have moved relative to the rocks on the other side.
- Faults
where the crust is being pulled apart, normal fault occurs
- Normal fault
crustalblocksmay also move sideways past each other, usually along nearly-vertical faults
- Strike-slip fault
where the crust is being compressed, reverse fault occurs
- Reverse fault
a dip slip fault in which the upper block, above the fault plane, moves up and over the lower block
- Thrust fault
involves various combinations of these basic movements
- Oblique slip fault
of ductile deformation of rocks in response to external forces
Folds
upward fold (A)
- Anticlines
downward fold (B)
- Synclines
gently dipping bends in horizontal rock layers (C)
- Monoclines
metamorphic rocks are formed when heat is the main agent of metamorphism. Generally, non-foliated rocks are composed of a mosaic of roughly equi-dimensional and equi-granular minerals.
Non-foliated
which is generally caused by a preferred orientation of sheet silicates (silica minerals with sheet-like structures), such as clay minerals, mica and chlorite.
foliation
Slate, phyllite, schist, and gneiss are foliated
Hornfels and granulite are examples of non-foliated metamorphic rocks.
is formed when the pressure applied to a rock at depth is not equal in all directions. If present during metamorphism, effects of differential stress in the rock’s texture include the following (Nelson, 2012):
Differential stress
Metamorphic Process
Minerals convert to new high temperature minerals
Fluids are released (ex: clay = mica + water)
Crystals grow larger
Rocks become weaker and easier to deform
Agents of Metamorphism
High Temperature
Minerals may recrystallize into more compact/stable forms
Platy or elongate minerals may align in a preferred direction
High Pressure
