Download SGES 1302 Lecture14

SGES 1302
INTRODUCTION
TO EARTH SYSTEM
LECTURE 14: Rock Cycle & Magmatism
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Lecture 14: Rock Cycle & Magmatism
• Rock is defined as any solid
mass of mineral, or mineral-like
matter that occur naturally as
part of the Earth.
• Rocks can be divided into 3
broad groups:
• Igneous Rocks
• Sedimentary Rocks
• Metamorphic Rocks
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Rock cycle helps us to understand the interrelationships among different
parts of the Earth system
It help us to understand the origin of igneous, sedimentary and metamorphic
rocks
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Rock Cycle
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Magma is a molten material that forms inside the Earth.
Eventually magma cools and solidifies : crystallisation
that forms igneous rocks
Can occur at the surface (volcanism) or below the
surface (plutonism)
Igneous rocks exposed at the surface will undergo
weathering (disintegration & decomposition)
Materials formed often move downslope and transported
Most sediment ultimately deposited in the ocean, others
along floodplains, swamps, etc.
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Rock Cycle
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Sediments undergo lithification (compaction &
cementation) to form sedimentary rock
If a sedimentary rock is buried deep enough or intruded
by magma, it will be subjected to high pressure &
temperature (metamorphism).
It will turn into a metamorphic rock.
If temperature is high enough, melting will occur, creating
magma.
The magma will crystallise into igneous rocks and the
cycle continues.
Other paths of the rock cycle (see rock cycle diagram).
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Igneous Rocks: Magma
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Igneous rocks were the first rocks to form as Earth cooled from a molten
mass to the crystalline rocks of the early crust.
Igneous rocks are the rocks that form when molten material (magma)
cools and crystallizes.
Magma is often a slushy mix of molten rock, dissolved gases, and
mineral crystals.
The common elements present in magma are the same major elements
that are in Earth’s crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe),
magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na).
Magma crystallise to form mainly silicate minerals.
Of all the compounds present in magma, silica is the most abundant and
has the greatest effect on magma characteristics.
Magma is classified as basaltic (42-52% SiO2), andesitic (52-66% SiO2),
or rhyolitic (>66% SiO2), based on the amount of silica it contains.
Silica content affects melting temperature and impacts how quickly
magma flows (viscosity).
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Igneous Rocks: Magma
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The type of igneous rock that forms depends on the composition
of the magma.
Different minerals have different melting points.
For example, rocks such as basalt, which are formed of olivine,
calcium feldspar, and pyroxene, melt at higher temperatures than
rocks such as granite, which contain quartz and potassium
feldspar.
Granite has a melting point that is lower than basalt’s melting
point because granite contains more water and minerals that melt
at lower temperatures.
In general, rocks that are rich in iron and magnesium melt at
higher temperatures than rocks that contain higher levels of
silicon.
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Igneous Rocks: Magma
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Source of magma: partial melting of rocks, mainly in
the mantle, minor in lower crust
Rocks do not entirely melt at one temperature &
pressure because each mineral in the rock has a
particular melting point.
Some minerals remains solid, while the melted portion
may flow away as magma.
As each group of minerals melts, different elements
are added to the magma mixture thereby changing its
composition.
If temperatures are not high enough to melt the entire
rock, the resulting magma will have a different
composition than that of the original rock.
This is one way in which different types of igneous
rocks form.
Main factors controlling the creation of magma: heat,
pressure, water & composition of the source rock.
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Creation of Magma
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Lithosphere
Increasing Depth
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Temperatures in the Earth increases with
depth (geothermal gradient)
Average geothermal gradient is 30°C per km
(eg. At the depth of 10 km, the temperature
is ~300°C hotter than the Earth’s surface)
With increasing depth, the pressure also
increases, and the melting point of rocks
generally increases with increasing pressure.
This is because pressure squeezes atoms
and ions closer together, therefore requires
higher temperature to break their bonds.
A rock that melts at 1100°C at Earth’s
surface will melt at 1400°C at a depth of 100
km.
Temperature
Asthenosphere
Geothermal
Gradient
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Creation of Magma
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If pressure of a hot rock is removed or reduced, the rock may reach a
condition where melting occurs : decompression melting.
Decompression melting occurs at divergent plate boundaries, where
partial melting of asthenosphere occurs below a thinning lithosphere at
oceanic or continental rift zones.
Water, even a small amount lowers the melting point of rocks, especially
at high pressures.
Water enhances melting because it dissolves more easily in magmas
than in minerals.
The partial melting aided by dehydration of subducted oceanic plate is
important mechanism of magmatism at subduction zones.
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Motion of Magma
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Once partial melting produces magma, the magma rises towards the
Earth surface due to its bouyancy (less dense than the surrounding
rocks).
A magma’s ability to rise is large controlled by its fluidity (viscosity),
which is governed by its temperature and composition.
Increasing temperature decreases viscosity of any material: hot
magmas flows more easily than relatively cool magmas.
Viscosity generally increases with silica content: silica-rich magma
such as felsic magma is more viscous and tends to relatively cool (it
crystallise at lower temperature) compared to low-silica magma such
as basaltic magma.
Silica-rich magma is less likely to reach the surface compared to low
silica magma. Silica-rich magma is more likely to crystallise within
the Earth crust.
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Motion of Magma
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When magma reaches the surface, it is called lava.
Volcanic eruption can be violent or quiet.
Crystallisation of lavas forms volcanic or extrusive rocks.
Violent volcanic eruption ejects materials high into the atmosphere,
and subsequently these pyroclastic materials fall on the Earth
surface to form pyroclastic rocks.
Most magma loses its mobility before reaching the surface. It
crystallise at depth to form plutonic or intrusive rocks.
Intrusive rocks will only be exposed at the surface after being
uplifted and overlying rocks eroded away.
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Kilauea Volcano, Hawaii
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Mt Pinatubo, Philippines
Mt St Helen, USA
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