Download Igneous Processes

GL1,
I Matter, Key idea 2 (b)
II Energy, Key Idea 1 (a), Key Idea 2 (a)
Igneous Processes
The Earth’s heat
Volcanoes show us that the Earth is hot inside. Anything to do with heat in the Earth is
called geothermal, for example in some places, people get their energy from hot water
from underground – this is called geothermal energy.
We can estimate the temperature inside the Earth and from these data you can draw a
graph called the geotherm.
Exercise 1
Use the data in the table and the graph paper below to plot the geotherm.
Depth (km)
0
50
100
200
250
300
0
Temperature (C)
0
900
1250
1400
1460
1490
500
1000
1500
2000
0
50
100
150
200
250
300
Use the graph to estimate the temperature at a depth of:
30 km …………….
180 km ……………
Calculate the geothermal gradient for the first 100 km. Show your working.
Answer …………….C/km
AS Geology, Module GL1, Igneous
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The centre of the Earth is very hot indeed. The main source of the Earth’s heat is
radioactive decay. Some elements have unstable atoms, when they break down (decay)
they change into different atoms and give out energy.
Like any hot object, the Earth loses its heat and slowly cools down. The outer part of the
Earth is rigid (it doesn’t flow) so it cools by conduction. Deeper layers are ductile (they
can flow) so they lose heat by convection.
As you go down into the Earth, the temperature that rocks melt changes. We can show
this on the geotherm graph using a melting line. If the geotherm crosses the melting line,
then the rocks melt at that depth and form magma (liquid rock)
0
500
1000
1500
2000
0
50
100
150
200
250
300
350
400
Exercise 2
Sketch the melting line on the graph above (see page 34 Geoscience). Estimate the depth
at which magma is formed.
Magma
Magma is the name given to liquid rock under the surface. In reality it is a complex
mixture of substances, mostly silicate minerals plus water and gases. The amount of
silica is a very important factor in controlling the behaviour of the magma. Magma rich
in silica (>66%) is called acidic, medium amounts of silica (54 – 66%) is intermediate,
low proportion of silica (<54%) is called basic.
The amount of silica controls the viscosity of the magma or lava. The higher the
proportion of silica, the more viscous (sticky) the magma is.
The magma will tend to rise through the crust towards the surface. If it breaks out onto
the surface, the gases are lost into the atmosphere and the liquid rocks is now called lava.
Volcanism
Lava can erupt at the surface through cracks called fissures or from a single hole called a
vent. In the case of a central vent, the lava will build up a cone shaped mountain as it
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cools – this is a volcano. The size and shape of the volcano is controlled by the type of
eruptions and the products of the eruption. Some volcanoes erupt basic lava which has
lower viscosity, the gases are able to escape easily and the eruptions tend to be gentle
(effusive). The volcano produced in this way is broad and flat, this is called a shield
volcano.
Exercise 3
Sketch a typical shield volcano and give an example.
Volcanoes formed from more viscous (usually intermediate) lava tend to be taller and
steeper. These are called composite cone volcanoes (stratovolcanoes). The eruptions are
much more violent.
Exercise 4
Sketch a composite cone volcano and give an example. (e.g. page 65 in Geoscience)
.
The lava cooling from a volcano, loses its heat very quickly. The result is a solid rock
made of randomly arranged, small, interlocking crystals. This is a description of the
texture of the rock. In geology we use the word “fine” for small crystals, these are
crystals less than 1 mm across.
The main fine grained rocks are:
Acidic
Intermediate
Basic
Rhyolite
Andesite
Basalt
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If lava cools very quickly there is not enough time for the crystals to form. Instead
volcanic glass is created, this is called obsidian.
Lava can be erupted under water – there are many volcanoes at the bottom of the ocean,
following the ocean ridges. When the lava comes into contact with the water it cools
almost instantly – there is no time for crystals to form, the rock is glassy. The shape of
this lava is a “blob” called a pillow. The pressure of the lava inside causes the pillow to
break and another pillow forms.
Exercise 5
Sketch the formation of pillow lava.(e.g. McLeish)
Intrusions
Sometimes magma never reaches the surface, so a volcano does not form. Instead, the
magma cools within the crust to form an igneous intrusion.
Small scale intrusions quite near to the surface are called hypabyssal. There are two main
types. A sill is a tabular (like a table top – flat sides) intrusion which follows the beds of
rock around it (we say it is concordant with bedding). A dyke is a tabular igneous
intrusion which cuts across the beds (we say it is discordant with bedding).
Exercise 5
Use Geoscience page 66 to sketch a sill and a dyke.
The rocks in a sill or dyke have medium sized crystals because the magma cools more
slowly than lava at the surface.
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Medium grained rocks:
Acidic
Intermediate
Basic
(Don’t need to know)
(Don’t need to know)
Dolerite
Sills and lava flows may both have columnar jointing. This is a type of structure caused
by tension in the crystallising magma. The columns are perpendicular to the base and
have a polygonal shape.
Exercise 6
Sketch the appearance of columnar jointing.
Deep, large scale intrusions are called plutonic intrusions. The main example is a
batholith. Batholiths are made of coarse grained rocks, formed by the slow cooling and
crystallisation of the magma.
Coarse grained rocks:
Acidic
Intermediate
Basic
Granite
(Don’t need to know)
Gabbro
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Exercise 7
Using a textbook, complete the table below to describe the appearance of the rocks you
need to know. Try to say the main minerals present in each.
Acidic
Granite
Intermediate
X
Coarse
Size =
Medium
Size =
X
X
Fine
Size =
Rhyolite
Andesite
Glassy
No crystals
O B
S I D
Basic
Gabbro
Dolerite
Basalt
I
A N
Describing igneous rock textures
When you describe the texture of an igneous rock, you need to describe the following
features:
Grain size - Coarse Medium Fine Glassy
Grain shape - Euhedral = well formed crystals
Subhedral = moderately well formed crystals
Anhedral = poorly formed crystals
If the rock has two distinct grain sizes, it is described as porphyritic. Large, well formed
crystals are called phenocrysts. Smaller crystals surrounding the phenocrysts are called
the groundmass. Porphyritic texture is evidence for two stages of cooling.
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Exercise 8
Using diagrams, explain how two stages of cooling can be used to explain the formation
of porphyritic texture.
Gas escaping from lava may leave bubbles in the solidifying lava – these are called
vesicles. A rock with vesicles is called vesicular.
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