The Geo-Chemistry of Igneous Rocks

The Chemistry-Geology FrontierThe Geo-Chemistry of Igneous Rocks

Edwin Law3rd December, 2013.

Intended Learning Outcome

Define ‘rock’, ‘rock cycle’, ‘igneous’, ‘magma’, ‘lava’, and the other relevant terms.

Explain how igneous rocks are formed.

Describe the classification of igneous rocks by mineral size and composition.

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!

THE ROCK!

The Searchers (Ford) Vertigo (Hitchcock)

Kagemusha (Kurosawa)

The Gift of Nature.

Rocks!!

What are the common features of these images?

There are 3 major rock types: igneous, sedimentary, and metamorphic.

The Rock Cycle

A rock is the naturally occurring combinations of one or more minerals.

They are formed through the cooling and solidification (crystallization) of magma and lava.

Igneous Rock Formations

Magma – molten rock within the Earth. A fluid mixture of liquid, solid crystals and dissolved gases.

Lava – molten rock that flows above ground.

Size of mineral crystals in the rock (Are the crystals visible?)

How are igneous rocks classified?

Basalt Granite

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.

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.

Igneous Mineral Size

Diorite Andesite

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. Volcanic rocks.

T. L. A.: What is a porphyritic rock?

They hide in disguiseAnd boast of their size

They flirt with the viceAnd that comes with a price

He stabs with the iceAnd that surrounds like lice.

E. L., 2nd December, 2013.

Porphyritic – large crystals are surrounded by regions with much smaller or even indistinguishable grains.

First, gradual underground cooling produces that grow slowly within a magma. 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.

The Porphyritic riddle - Answer

Porphyritic Granite

Igneous Rock Classification Chart

Silicon Dioxide (Silica)

Ultramafic Igneous RocksIron-, magnesium-rich.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 RocksContains 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 RocksConsist of iron and magnesium silicates and sodium and aluminium rich minerals, and a small amount of quartz. (a bit of everything)

Felsic Igneous RocksRich in potassium feldspar, aluminium-rich mica, and quartz.

Different types of igneous rocks

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 required letter.

1. Felsic-type igneous rocks contain a rich amount of potassium -------- (1st letter).2. A coarse-grained plutonic rock, starting with D. (5th letter).3. The composition type which has little silica content, but rich in iron. (4th letter).4. This coarse-grained rock has a pinky color (7th letter).

Answer:

1. Potassium Feldspar2. Diorite3. Ultramafic4. Granite

F-I-R-E

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?

Summary and Learning Outcomes

THANK YOU!

'Geology : an introduction to physical geology' by Chernicoff, Stanley

http://www.bbc.co.uk/bitesize/ks3/science/environment_earth_universe/rock_cycle/revision/10/

http://www.ucl.ac.uk/es/impact/geology/london/glossary/rocks/igneous/basalt

http://library.thinkquest.org/05aug/00461/granite.htm

http://geology.about.com/od/rocks/ig/igrockindex/rocpicdiorite.htm

http://geology.about.com/od/rocks/ig/igrockindex/rocpicandesite.htm

http://www.sciencephoto.com/media/169458/view

http://www-personal.umich.edu/~jmpares/Igneous.html

http://www.chemguide.co.uk/inorganic/group4/oxides.html

References