The Chemistry-Geology Frontier
The Geo-Chemistry of Igneous Rocks
3rd December, 2013.
Intended Learning Outcome
Define ‘rock’, ‘rock cycle’, ‘igneous’, ‘magma’, ‘lava’, and the other
Explain how igneous rocks are formed.
Describe the classification of igneous rocks by mineral size and
Describe the properties of different types of igneous rocks:
ultramafic, mafic, intermediate and felsic.
Understand the importanance of rocks in Nature.
This is NOT the intended learning outcome!
What are the common features of these images?
The Searchers (Ford)
The Gift of Nature.
The Rock Cycle
A rock is the naturally occurring combinations of one or more minerals.
There are 3 major rock types: igneous, sedimentary, and metamorphic.
Igneous Rock Formations
They are formed through the cooling and solidification (crystallization) of
magma and lava.
Magma – molten rock within the Earth. A fluid mixture of liquid, solid
crystals and dissolved gases.
Lava – molten rock that flows above ground.
How are igneous rocks classified?
Size of mineral crystals in the rock (Are the crystals visible?)
Composition (What is it made of?)
A rock’s texture refers to the size and shape of its minerals. It is
controlled by the rate which magma or lava cool.
A slower rate of crystallization will give us a purer, larger crystal.
Igneous Mineral Size
Coarse-grained rock – rock crystals are large enough to be seen visibly.
Slow cooling typically occurs when magmas intrude pre-existing solid
rock underground; so rocks with coarse-grained textures are called
intrusive rocks. a.k.a. Plutonic rocks.
Fine-grained rock – rock crystals are so small that they are not visible.
These igneous rocks solidify so quickly that their crystals have little time
to grow. They are called extrusive rocks because they generally form
from lava that has flowed out, or extruded, onto the Earth’s surface. a.k.a.
T. L. A.: What is a porphyritic rock?
They hide in disguise
And boast of their size
They flirt with the vice
And that comes with a price
He stabs with the ice
And that surrounds like lice.
E. L., 2nd December, 2013.
The Porphyritic riddle - Answer
Porphyritic – large crystals are surrounded by regions with much smaller
or even indistinguishable grains.
First, gradual underground cooling produces that grow slowly within a
Then the mixture of remaining liquid magma and the early-formed
crystals rises nearer to the surface , when it encounters cooler rocks or
erupts into the cooler environment at the surface.
The remaining liquid magma cools rapidly to produce the enveloping
body of smaller grains.
Igneous Rock Classification Chart
Silicon Dioxide (Silica)
Different types of igneous rocks
Ultramafic Igneous Rocks
Contain little silica and no free quartz.
Ultramafic rocks crystallize slowly deep in the Earth’s interior,
developing their typically coarse-grained structure.
Mafic Igneous Rocks
Contains calcium feldspar.
e.g. Basalt is the most abundant rock on the Earth’s crust, it is the
dominant rock of the oceanic plates.
Intermediate Igneous Rocks
Consist of iron and magnesium silicates and sodium and aluminium rich
minerals, and a small amount of quartz. (a bit of everything)
Felsic Igneous Rocks
Rich in potassium feldspar, aluminium-rich mica, and quartz.
Assessment Task – Crack the Code!!
Marvin has hijacked a space-travelling
shuttle. In order to save the passengers, you
need to break a 4-lettered code by answering
the following questions, and picking up the
Felsic-type igneous rocks contain a rich amount of potassium -------- (1st letter).
A coarse-grained plutonic rock, starting with D. (5th letter).
The composition type which has little silica content, but rich in iron. (4th letter).
This coarse-grained rock has a pinky color (7th letter).
Summary and Learning Outcomes
Understand the meaning of ‘rock’ and ‘rock cycle’.
Understand that igneous rocks are formed by cooling and crystallization
of magma and lava.
Appreciate that we can classify igneous rocks by mineral size and
composition through a systematic manner.
Compare and contrast ultramafic, mafic, intermediate, and felsic igneous
rocks with regards to their silica content and metal element content.
The Rock: Who wants to wrestle with me?
'Geology : an introduction to physical geology' by Chernicoff, Stanley