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How Rocks Form - Igneous Rocks


    1.    Differentiate between the three types of rocks.

    2.    Compare and contrast the processes in the rock cycle.

    3.    Distinguish between intrusive and extrusive igneous rocks and how they form.

    4.    Contrast the types of plutons that form as the result of igneous activity.

Key Terms:

    rock        igneous        magma        sedimentary        metamorphic        sediment        rock cycle        felsic        mafic        pluton        batholith

Notes: (6-1 &6-2)

What is a rock?  A rock is a mixture of minerals, mineraloids, glass and organic matter.  They can be found on the crust and upper mantle (lithosphere).  All rocks can be classified in the following three ways.

Sedimentary - formed by the compaction and cementing of sediments (anything lying on the ground)

Igneous - formed by the cooling and heating of magma

Metamorphic - formed with heat and pressure 

The Rock Cycle

The rock cycle is a constant cycle of breaking down and reformation.  

Magma from within the mantle is the source of all rocks and is what makes up igneous rocks.  

The reformation is the result of processes such as 

        Weathering, erosion, melting - create sediment

        Compaction, cementation, cooling - sedimentary rock formation

Sedimentary rocks get buried and heated under pressure - metamorphic rock

Plate movements push the rocks toward the asthenosphere where the melt into magma and reform as igneous rocks








(imnh.isu.edu/digitalatlas/ geo/basics/imgs/rc1.jpg)


Igneous Rocks

Igneous rocks are formed from the cooling of magma (lava).  They are classified by their mineral content and texture.  

Magma is the molten rock material beneath the ground.  It can be divided into two distinct types. 

Felsic - thick, slow moving, high in silica

Mafic - thin, hot, easy flowing

o     Temp – 1400oC

o     60-200km below the Earth’s surface

o     Formed under great pressure

o     Is less dense than surrounding rock

o     When it reaches the surface it becomes lava

o     Types: Basaltic, Andesitic, & Granitic

Intrusive (underground) rocks

Form below the Earth - texture depends on the area of cooling

Found when the Earth’s surface has been eroded or been pushed upward

Have large mineral grains

Types: Gabbro, Diorite, & Granite


Form above ground

Fine grain texture

Not arranged into crystal patterns

Many have air pockets

Types: Basalt, Scoria, Andesite, Rhyolite, Pumice, & Obsidian


o     Gabbro - Basaltic

High Density  

Formed from mafic lava

Rich in Iron & Magnesium

From HI - aa & pahoehoe

o     Granitic & Andesitic - formed from felsic magma

Lower density

high in silicates - quartz & feldspar

Thick lava is under great pressure  

Formed as an intrusive rock

found in mountainous areas near the coast







Obsidian is considered part of the granitic family but lacks the crystal formation


Pumice - hardened in the presence of steam

Diorite (Andesite)- Combination of both the granite and gabbro families

    Click to view: Andesite lava rock.




Igneous structures:

A pluton is a label that can be given to any intrusive igneous structure.  Plutons generally only become visible after a period of uplift or erosion.  Here are some common pluton structures.
Dike - vertical igneous intrusions into formed rock

(www.landsat4u.com/Merchant/ Contact2.jpg)

Sill - horizontal igneous intrusions into formed rock

Laccoliths - thick magma hardens into a dome under rock before reaching the surface.  They are usually very large and are exposed after long periods of erosion.

volcano.und.nodak.edu/.../north_america/ bear_butte.html

Volcanic necks - Magma that has hardened in the neck of the volcano can be exposed after many years of erosion.

(geoweb.tamu.edu/faculty/herbert/ bigbend/ig/b19b.jpg)

Batholith - The largest of the intrusive structures.  Formed from the solidification of magma chambers.

(seis.natsci.csulb.edu/.../ Intrusive_soCal_batholith.jpeg)

Caldera - the eroded base of a volcano

(www.educeth.ch/stromboli/glossary/ icons/caldera.jpg)


Last modified: February 05, 2003