Lab 5 Igneous Rocks

There are lots of terms here that you should review before lab…

Look closely at figures 5.2 and 5.3 before coming to lab

-Igneous rocks form when a molten lava, or magma cools and solidifies into a body of rock.

Rock Cycle pg. 78

Two main kinds of Igneous Rocks…

When magma within the earth solidifies it forms an Intrusive.

*Internal=Intrusive*

When lava is erupted and solidifies on the surface of the earth it forms an Extrusive.

*External=Extrusive*

Look at figure 5.1 on page 93 for a diagram of specific details

Textures of Igneous Rocks…

“Texture” is a description of minerals present and their sizes, shapes and arrangements.

Intrusive textures Phaneritic = coarse grained, formed from slow cooling

Extrusive textures Aphanitic = fine grained, rapid cooling (quenched)

*Porphyritic* = melt cooled for a little while, then erupted and quenched- get large xtals in glassy groundmass (groundmass?)

See Fig 5.2 on page 94 for a good chart of more textures and minerals

Minerals in Igneous Rocks…

Quartz, Muscovite, Biotite, Olivine, K-fsp, Plag fsp, Amphibole, Pyroxene...

Mafic Minerals = Dark colored, Biotite, Olivine, Pyroxene etc

Felsic Minerals = Light colored, Qtz, Muscovite, Feldspars

Intrusive and Extrusive equivalents…fig 5.3 pg 95

Intrusive -> Extrusive

Granite -> Rhyolite

Diorite -> Andesite

Gabbro -> Basalt

Ex-

If a felsic magma is allowed to cool slowly within the earth it would have a Phaneritic texture composed mainly of interlocking, coarse grained, quartz, plag fsp, K fsp and some biotite and amphibole. Using the flow chart in fig 5.2 this would be called a Granite

Conversely, if the same felsic magma were suddenly erupted onto the surface of the earth via a volcano, it would have an Aphanitic texture that would look like really small minerals <1mm. The same minerals as above may be present, but because they are so small you can infer that the rock formed rapidly, outside the earth and would be called Rhyolite

*How to identify an igneous rock*

1. Identify the rocks texture

2. Identify the rocks “color index” ie. Amt. of Mafic vs Felsic Minerals

3. Classify the rock with flowchart on figure 5.2

~sounds simple enough~