Chapter 6 Magma and Igneous Rocks
6.2 Why do melts form?
We refer to melt that’s underground as magma and to melt that has
emerged1 at the Earth’s surface as lava. We distinguish three types of
products of eruptions: lava fountain – lava rises meters into the air,
lava lake – lava pools over the vent2 and lava flow- lava flows down
a slope.
Pyroclastic debris is the material and dust explosively send into the
sky and landed on the surface in solid state. Lava
We distinguish two categories of igneous rocks; extrusive igneous and intrusive igneouslake
rock. Rock
that forms by the freezing of lava above ground is called extrusive igneous rock. Intrusive igneous
rock is magma which intrudes into pre-existing rock and solidified in the underground.
Why is it hot inside the Earth?
According to the nebula theory, our planet formed from the collision and merging of planetesimals.
Every time a collision occurred, the kinetic energy transformed into heat energy. As the Earth grew,
gravity pulled matter inward until eventually the weight of overlying material squeezed the matter
inside tightly together. Such compression made the Earth’s inside even hotter. The iron inside started
to melt and the dense iron sank to the core, the friction produced even more heat. Later, a Mars-sized
object collided with the Earth. This collision generated vast amounts of heat. The Earth became a
ocean of lava, but it radiated heat into space and therefore has slowly cooled. Igneous rock solidified
and formed a crust. (so it’s the first existing rock). The Earth probably would be too cold right now if
radioactive elements would not exist. The decay of radioactive elements slowed the cooling of the
planet.
Causes of melt
1. Decompression melt
A geotherm (temperature as a function of depth) shows that temperature always increases with depth
but not always with the same rate. Pressure squeezes atoms together so they can’t easily break free
from solid crystals. So, pressure prevents melting. A decrease in pressure can permit melting.
Left from the solidus the rock is solid. Right from the solidus it starts to melt and right from the
liquidus the rock is entirely molten.
1 Emerge= oprijzen
2 Vent= opening
, Decompression melting occurs in places where the pressure decreases, so
rock rises but the temperature stays the same! It occurs
in places whrer hot mantle rock rises to shallower
depths in the Earth.
2. Flux melting
Melting due to addition of volatiles is
sometimes called flux melting. It happens at
subduction zones. The volatiles like water, they
help break chemical bonds. Volatiles decreases
a rock’s melting temperature.
3. Heat-transfer melting
It occurs when rising magma brings heat up with it
and melts overlying or surrounding rock. The
cooler magma is felsic and the hotter magma is mafic
6.3 What is molten rock made of?
An oxide is a molecule consisting of an element bonded to oxygen. Geologists distinguish between dry
melts – which contains no volatiles- and wet melts – which do contain volatiles. These volatiles come
out of the Earth at volcanoes in the form of gas.
The major chemical types of molten rock
Melts differ from one another in proportions of chemicals that they contain. Four major types of
molten rock are distinguished, depending on the proportion of silica relative to the combination of
magnesium oxide and iron oxides.
- Felsic melts high proportion of silica relative to magnesium and iron oxide
- Intermediate melts composition lies between that of mafic and felsic melts
- Mafic melts low proportion of silica relative to magnesium and iron oxide
- Ultramafic melts very low proportion of silica relative to magnesium and iron oxide
Why does the composition differ?
- Source- rock composition
- Partial melting
The temperature at sites of magma production simply never gets high enough to melt the
entire source rock, and the magma tends to migrate away from the site of melting before all of
the original rock has had a chance to melt
These magma’s are more felsic than the source rock because felsic has a low melt temperature
so it melts first.
6.2 Why do melts form?
We refer to melt that’s underground as magma and to melt that has
emerged1 at the Earth’s surface as lava. We distinguish three types of
products of eruptions: lava fountain – lava rises meters into the air,
lava lake – lava pools over the vent2 and lava flow- lava flows down
a slope.
Pyroclastic debris is the material and dust explosively send into the
sky and landed on the surface in solid state. Lava
We distinguish two categories of igneous rocks; extrusive igneous and intrusive igneouslake
rock. Rock
that forms by the freezing of lava above ground is called extrusive igneous rock. Intrusive igneous
rock is magma which intrudes into pre-existing rock and solidified in the underground.
Why is it hot inside the Earth?
According to the nebula theory, our planet formed from the collision and merging of planetesimals.
Every time a collision occurred, the kinetic energy transformed into heat energy. As the Earth grew,
gravity pulled matter inward until eventually the weight of overlying material squeezed the matter
inside tightly together. Such compression made the Earth’s inside even hotter. The iron inside started
to melt and the dense iron sank to the core, the friction produced even more heat. Later, a Mars-sized
object collided with the Earth. This collision generated vast amounts of heat. The Earth became a
ocean of lava, but it radiated heat into space and therefore has slowly cooled. Igneous rock solidified
and formed a crust. (so it’s the first existing rock). The Earth probably would be too cold right now if
radioactive elements would not exist. The decay of radioactive elements slowed the cooling of the
planet.
Causes of melt
1. Decompression melt
A geotherm (temperature as a function of depth) shows that temperature always increases with depth
but not always with the same rate. Pressure squeezes atoms together so they can’t easily break free
from solid crystals. So, pressure prevents melting. A decrease in pressure can permit melting.
Left from the solidus the rock is solid. Right from the solidus it starts to melt and right from the
liquidus the rock is entirely molten.
1 Emerge= oprijzen
2 Vent= opening
, Decompression melting occurs in places where the pressure decreases, so
rock rises but the temperature stays the same! It occurs
in places whrer hot mantle rock rises to shallower
depths in the Earth.
2. Flux melting
Melting due to addition of volatiles is
sometimes called flux melting. It happens at
subduction zones. The volatiles like water, they
help break chemical bonds. Volatiles decreases
a rock’s melting temperature.
3. Heat-transfer melting
It occurs when rising magma brings heat up with it
and melts overlying or surrounding rock. The
cooler magma is felsic and the hotter magma is mafic
6.3 What is molten rock made of?
An oxide is a molecule consisting of an element bonded to oxygen. Geologists distinguish between dry
melts – which contains no volatiles- and wet melts – which do contain volatiles. These volatiles come
out of the Earth at volcanoes in the form of gas.
The major chemical types of molten rock
Melts differ from one another in proportions of chemicals that they contain. Four major types of
molten rock are distinguished, depending on the proportion of silica relative to the combination of
magnesium oxide and iron oxides.
- Felsic melts high proportion of silica relative to magnesium and iron oxide
- Intermediate melts composition lies between that of mafic and felsic melts
- Mafic melts low proportion of silica relative to magnesium and iron oxide
- Ultramafic melts very low proportion of silica relative to magnesium and iron oxide
Why does the composition differ?
- Source- rock composition
- Partial melting
The temperature at sites of magma production simply never gets high enough to melt the
entire source rock, and the magma tends to migrate away from the site of melting before all of
the original rock has had a chance to melt
These magma’s are more felsic than the source rock because felsic has a low melt temperature
so it melts first.
