Physical Geology 101
9. Igneous Rocks II
(p. 104-114; 116-119; 149-153)
Textures of Igneous Rocks
The rate of cooling and crystal size determines the texture of an igneous rock.
What is the definition of rock texture? _______________________________________
______________________________________________________________________
We define two types of igneous rocks on the basis of grain size:
1. ____________________: rapid cooling at the Earth's surface  small mineral
grains. Forms from ______________.
Also called: __________________
2. ____________________: slow cooling inside the Earth  large mineral grains
Forms from ______________.
Also called: __________________
Types of Rock Textures
Magma that rises to the Earth's surface erupts as lava and immediately starts to cool due to contact with
either the atmosphere or sea water.
What are the two ways lava erupts onto the Earth's surface?

_______________________________ (volcanic mountains)

_______________________________ (big cracks in the crust)
Bulbous pods of chilled lava are formed whenever lava erupts onto the ocean bottom or in a lake. What
do we call this type of lava? ______________________
What is the general name given to the volcanic glass that forms due to very rapid cooling of lava?
_______________. What sort of texture does it have? _____________
When small crystals develop from cooling of lava, the texture is called:
______________________ (also called ___________________)
When big crystals develop from cooling of magma, the texture is called:
______________________ (also called ___________________)
Crystals start to grow in magma before it erupts, so volcanic rocks often contain these larger-sized
crystals, called __________________, that then become embedded in a matrix of much smaller grained
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Physical Geology 101
crystals (called ___________________) that grew after the lava erupted. This type of texture is called a
__________________ texture.
What are gas bubbles in lava called when they are frozen in place? ________________
What texture is produced in the rock? ______________________
e.g. pumice
 What is unusual about pumice? __________________________________
Volcanic rocks in which pyroclastic material fragments got welded together have a ________________
texture, also called a ________________ texture (e.g., ________).
If the mineral crystals are very big (>1 inch to several feet across), we say the rock has a
_________________ texture. These rocks are usually the last portion of the magma body to crystallize
due to the presence of silica-rich fluids that allow large crystals of quartz, feldspar, and muscovite to grow
(i.e., not due to slow cooling).
Mineral Assemblages
Most igneous rocks are composed of one of more of the following six minerals or mineral groups:
___________________
___________________
___________________
___________________
___________________
___________________
The types and amounts of minerals in an igneous rock define its mineral assemblage.
The growth of minerals is determined by the original chemistry of the magma and the temperature of
the magma at each point during its cooling history.
What are the four different magma types based on differences in silica content?
_______________, _______________, _______________, _______________
Temperature an important control on what type of rock forms because different minerals crystallize at
different temperatures as a melt cools and crystallizes. Because minerals have explicit chemical formulae,
we know the temperatures that each one forms at. As a result, we know there is a predictable sequence
of mineral growth from magma or lava.
This sequence of mineral growth from molten magma or lava is called: ___________________________
This series of mineral development has two branches:
continuous
and
discontinuous
Crystallization occurs at the same time along both branches.
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Physical Geology 101
What is the order of mineral crystallization along the discontinuous branch?
1. ______________________
2. ______________________
3. ______________________
4. ______________________
The magma chemistry changes as each mineral crystallizes out and removes "ingredients" from the
remaining magma. The ingredients left behind in the magma form ever different minerals further down
Bowen's Reaction Series.
What crystallizes out of magma along the continuous branch? ____________________
What are the last three minerals to crystallize out of magma or lava?
_____________________
_____________________
_____________________
Do we form the entire mineral assemblage on Bowen’s Reaction Series (all 9 minerals) every time we
cool a magma? (in other words, do all igneous rocks contains all 9 minerals?)
YES or NO
Characterizing Igneous Rocks
Now we know the following controls on the formation of igneous rocks:


the mineral assemblage is determined by the chemistry of the original melt
the texture is determined by how fast the melt cools
This implies that two igneous rocks with different textures could actually have the same mineral
assemblage. All that would differ is the size of the mineral grains in the rock. Nonetheless, these rocks
would have different names.
Volcanic Rocks
There are three main types of volcanic rocks:

_____________: it is a mafic rock, so it is dark colored - green or black.

_____________: forms from intermediate lava. Often contains light and dark minerals together.

_____________: forms from felsic lava. Contains mostly quartz and K-feldspar. If the rock contains
Na-plagioclase instead of K-feldspar, it is called ____________
Plutonic Rocks
The three equivalents of the volcanic rocks in the plutonic category are:
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Physical Geology 101

_____________: like basalt, but with big mineral grains. If there is a lot of olivine present instead of
plagioclase, the rock is called _________________.

_____________: like andesite, but with big mineral grains.

_____________: like rhyolite, but with big quartz and K-feldspar crystals. If it contains Naplagioclase, it is called ______________________.
Plutonism
All bodies of intrusive igneous rocks are referred to as _____________. They form when magma
migrates upwards through the crust and eventually starts to solidify. The rock around the intrusive rock is
referred to as ___________________.
Examples: batholiths, dikes, sills, laccoliths, volcanic pipes and necks.
Batholiths
The largest kind of pluton is a batholith. They are irregular shaped bodies of igneous rock that cut
across the layers of the country rock around it. They generally do not extend deeper than about 20 miles.
Examples: (1) _______________________________________________
(2) _______________________________________________
Dikes and Sills
When magma moves up towards the Earth's surface, it may do so by pushing its way through fractures in
the rock. If the magma then cools and crystallizes inside the fracture, it forms a dike or a sill.
The feature that forms when magma cuts across rock layers is a _______________.
The feature that forms when magma intrudes between rock layers is a ____________.
Example of local basalts that erupted from dikes as fissure eruptions:
__________________________________________________
Underneath volcanoes, dikes and sills form a network of conduits for magma to move up towards the
volcano and sometimes allow lava to erupt out along the flanks of a volcano instead of from inside its
crater.
Laccoliths
Sometimes, very thick sills push up the overlying rock layers, forming laccoliths.
What is the general shape of a laccolith? _______________________________
Volcanic Pipes and Necks
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Physical Geology 101
Magma moves up into a volcano from a magma chamber through a tube-like conduit called a volcanic
pipe. The magma may eventually crystallize inside the pipe. If it is later exposed by erosion, it forms a
volcanic neck (e.g., Ship Rock, NM).
FINAL QUESTION:
What famous example of a volcanic neck was featured in the movie Close Encounters of the Third Kind?
__________________________________________
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