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Chapter 6: Igneous Rocks
Up from the Inferno: Magma and Igneous Rocks
6.1 Introduction
Melt = hot molten rock.
⛰️ Igneous rock = rock that forms when hot molten rock (magma or lava) cools and freezes solid.
Igneous comes from the the Latin words ignis = fire.
6.2 Why Do Melts Form?
Melt that is underground is magma, whereas melt above ground is called lava.
Volcanic words:
Volcano = 1. A vent from which melt from inside the Earth spews out onto the planet’s surface; 2. A
mountain formed by the accumulation of extrusive volcanic rock.
Lava fountain = where lava forcefully rises meters to hundreds of meters into the air.
Lava lake = where lava pools over the vent.
Lava flow = sheets or mounds of lava that flow onto the ground surface or seafloor in molten form
and then solidify.
Pyroclastic debris = fragmented material that sprayed out of a volcano and landed on the ground or
seafloor in solid form.
We can distinguish two categories of igneous rocks:
1. Extrusive igneous rock = rock that forms by the freezing of lava above ground, after it flows or
explodes out (extrudes) onto the surface and comes into contact with the atmosphere or ocean.
2. Intrusive igneous rock = rock that forms from magma that has pushed its way (intruded) into pre-existing rock (wall rock) and
solidified underground.
Why Is It Hot inside the Earth?
The Earth's internal heat, crucial for igneous processes, originates from the Earth’s early days:
Collisions and merging of planetesimals transformed kinetic energy into heat.
Gravity-induced compression.
Chapter 6: Igneous Rocks 1
, The sinking of molten iron to form the core caused friction.
A Mars-sized object collided after Earth's differentiation. This heat made the early Earth partially molten, potentially creating a surface
lava.
The Earth has cooled since then, but the presence of radioactive elements have slowed the cooling a lot. This is because the decay of a
single radioactive atom produces a tiny amount of heat.
Geotherm = the change in temperature with depth in the Earth.
Typically, rock remains solid in the Earth due to the high pressures. There are exemptions:
1. Melting due to a decrease in pressure:
Decompression melting = the kind of melting that occurs when hot mantle rock rises to
shallower depths in the Earth so that pressure decreases while temperature remains unchanged.
It occurs at mantle plumes, beneath rifts and beneath mid-ocean ridges.
2. Melting as a result of the addition of volatiles:
Volatiles = substances that evaporate easily and can exist as gasses on Earth.
Flux melting = the transformation of hot solid to hot liquid that occurs when a volatile material
injects into a solid. It occurs above subduction zones.
3. Melting as a result of heat transfer:
Heat-transfer melting = melting that results from the transfer of heat from a hotter magma to a cooler rock. It mainly occurs along
convergent boundaries.
6.3 What’s in Molten Rock?
Geologists describe the chemical composition of molten rock in terms of the proportions of oxides (= a molecule consisting of an element
bonded to oxygen, eg. SiO₂, Al₂O₃ or FeO).
Dry melt = molten rock that contains no volatiles.
Wet melt = molten rock that does contain volatiles.
The four major compositional types of
Silica
molten rock depend on the proportion of
Felsic melts 67% - 76%
silica relative to the combination of
magnesium oxide and iron oxide. Intermediate melts 53% - 66%
Mafic melts 46% - 52%
Ultramafic melts 38% - 45%
In ‘mafic’ the ‘ma’ stand for magnesium
and the ‘fic’ comes from the Latin word for
Classes of igneous rock, distinguished by composition
iron.
(= SiO2 / (FeO + MgO)
Why do melts of so many compositions form in the Earth?
Source-rock composition: melts form from different source-rocks, so they have different compositions.
Partial melting = the melting in a rock of the minerals with the lowest melting temperatures, while other minerals remain solid.
→ Magma’s formed by partial melting are more felsic than the source rock.
Assimilation = the process of magma contamination in which blocks of wall rock fall into a magma chamber and dissolve.
Magma mixing: Different magma’s can dissolve in each other and produce a new magma.
6.4 Movement and Solidification of Molten Rock
Why Does Magma Rise? What Controls the Speed at Which Molten Rock Flows?
1. Magma is less dense than the surrounding rock and buoyant Viscosity = the resistance of material to flow
forces will drive it upward. 1. Temperature: a hotter melt is less viscous.
2. The weight of rock produces pressure at depth which 2. Volatile content: a more volatile melt is less viscous.
squeezes the magma upwards.
3. Silica content: a more mafic melt is less viscous.
Freezing or solidifying of melt usually takes place because a melt cools below its freezing temperature, this may happen when magma
rises toward the Earth’s surface. The time it takes for magma to cool depends on how fast it can transfer heat.
Chapter 6: Igneous Rocks 2
Chapter 6: Igneous Rocks
Up from the Inferno: Magma and Igneous Rocks
6.1 Introduction
Melt = hot molten rock.
⛰️ Igneous rock = rock that forms when hot molten rock (magma or lava) cools and freezes solid.
Igneous comes from the the Latin words ignis = fire.
6.2 Why Do Melts Form?
Melt that is underground is magma, whereas melt above ground is called lava.
Volcanic words:
Volcano = 1. A vent from which melt from inside the Earth spews out onto the planet’s surface; 2. A
mountain formed by the accumulation of extrusive volcanic rock.
Lava fountain = where lava forcefully rises meters to hundreds of meters into the air.
Lava lake = where lava pools over the vent.
Lava flow = sheets or mounds of lava that flow onto the ground surface or seafloor in molten form
and then solidify.
Pyroclastic debris = fragmented material that sprayed out of a volcano and landed on the ground or
seafloor in solid form.
We can distinguish two categories of igneous rocks:
1. Extrusive igneous rock = rock that forms by the freezing of lava above ground, after it flows or
explodes out (extrudes) onto the surface and comes into contact with the atmosphere or ocean.
2. Intrusive igneous rock = rock that forms from magma that has pushed its way (intruded) into pre-existing rock (wall rock) and
solidified underground.
Why Is It Hot inside the Earth?
The Earth's internal heat, crucial for igneous processes, originates from the Earth’s early days:
Collisions and merging of planetesimals transformed kinetic energy into heat.
Gravity-induced compression.
Chapter 6: Igneous Rocks 1
, The sinking of molten iron to form the core caused friction.
A Mars-sized object collided after Earth's differentiation. This heat made the early Earth partially molten, potentially creating a surface
lava.
The Earth has cooled since then, but the presence of radioactive elements have slowed the cooling a lot. This is because the decay of a
single radioactive atom produces a tiny amount of heat.
Geotherm = the change in temperature with depth in the Earth.
Typically, rock remains solid in the Earth due to the high pressures. There are exemptions:
1. Melting due to a decrease in pressure:
Decompression melting = the kind of melting that occurs when hot mantle rock rises to
shallower depths in the Earth so that pressure decreases while temperature remains unchanged.
It occurs at mantle plumes, beneath rifts and beneath mid-ocean ridges.
2. Melting as a result of the addition of volatiles:
Volatiles = substances that evaporate easily and can exist as gasses on Earth.
Flux melting = the transformation of hot solid to hot liquid that occurs when a volatile material
injects into a solid. It occurs above subduction zones.
3. Melting as a result of heat transfer:
Heat-transfer melting = melting that results from the transfer of heat from a hotter magma to a cooler rock. It mainly occurs along
convergent boundaries.
6.3 What’s in Molten Rock?
Geologists describe the chemical composition of molten rock in terms of the proportions of oxides (= a molecule consisting of an element
bonded to oxygen, eg. SiO₂, Al₂O₃ or FeO).
Dry melt = molten rock that contains no volatiles.
Wet melt = molten rock that does contain volatiles.
The four major compositional types of
Silica
molten rock depend on the proportion of
Felsic melts 67% - 76%
silica relative to the combination of
magnesium oxide and iron oxide. Intermediate melts 53% - 66%
Mafic melts 46% - 52%
Ultramafic melts 38% - 45%
In ‘mafic’ the ‘ma’ stand for magnesium
and the ‘fic’ comes from the Latin word for
Classes of igneous rock, distinguished by composition
iron.
(= SiO2 / (FeO + MgO)
Why do melts of so many compositions form in the Earth?
Source-rock composition: melts form from different source-rocks, so they have different compositions.
Partial melting = the melting in a rock of the minerals with the lowest melting temperatures, while other minerals remain solid.
→ Magma’s formed by partial melting are more felsic than the source rock.
Assimilation = the process of magma contamination in which blocks of wall rock fall into a magma chamber and dissolve.
Magma mixing: Different magma’s can dissolve in each other and produce a new magma.
6.4 Movement and Solidification of Molten Rock
Why Does Magma Rise? What Controls the Speed at Which Molten Rock Flows?
1. Magma is less dense than the surrounding rock and buoyant Viscosity = the resistance of material to flow
forces will drive it upward. 1. Temperature: a hotter melt is less viscous.
2. The weight of rock produces pressure at depth which 2. Volatile content: a more volatile melt is less viscous.
squeezes the magma upwards.
3. Silica content: a more mafic melt is less viscous.
Freezing or solidifying of melt usually takes place because a melt cools below its freezing temperature, this may happen when magma
rises toward the Earth’s surface. The time it takes for magma to cool depends on how fast it can transfer heat.
Chapter 6: Igneous Rocks 2
