Aim: To investigate the properties by which igneous rocks can be identified.
PROCEDURE
1. Obtain the igneous samples from your instructor
2. Arrange your samples in the order demonstrated by your instructor
3. Complete the report sheet using your samples, the Scheme of Igneous Rock Identification in
the Appendix, and the reference material provided by your instructor
4. Observe the large igneous rock samples provided.
QUESTIONS AND ANSWERS
Sampl
Color
How
e No.
formed
1
Light
Intrusive
2
Light
Intrusive
3
Light
Intrusive
4
Medium
Intrusive
5
Dark
Intrusive
6
Light
Extrusive
7
Dark
Extrusive
8
Light
Extrusive
9
Light
Intrusive
10
Medium
Extrusive
11
Dark
Extrusive
12
Dark
Extrusive
13
Dark
Extrusive
14
Light
Extrusive
15
Light
Extrusive
Minerals present
Composition
Rock Name
Potassium Feldspar, Quartz,
Plagioclase feldspar, Biotite,
Amphibole
Potassium Feldspar, Quartz,
Plagioclase feldspar, Biotite,
Amphibole
Potassium Feldspar, Quartz,
Plagioclase feldspar, Biotite,
Amphibole
Quartz, Plagioclase Feldspar,
Biotie, Amphibole, Pyroxene
Plagioclase Feldspar, Biotite,
Pyroxene, Olivine, Amphibole
Potassium Feldspar, Quartz,
Plagioclase feldspar, Biotite,
Amphibole
Plagioclase Feldspar, Biotite,
Pyroxene, Olivine, Amphibole
Potassium Feldspar, Quartz,
Plagioclase feldspar, Biotite,
Amphibole
Potassium Feldspar, Quartz,
Plagioclase feldspar, Biotite,
Amphibole
Quartz, Plagioclase Feldspar,
Biotie, Amphibole, Pyroxene
Plagioclase Feldspar, Biotite,
Pyroxene, Olivine, Amphibole
Plagioclase Feldspar, Biotite,
Pyroxene, Olivine, Amphibole
Plagioclase Feldspar, Biotite,
Pyroxene, Olivine, Amphibole
Potassium Feldspar, Quartz,
Plagioclase feldspar, Biotite,
Amphibole
Potassium Feldspar, Quartz,
Plagioclase feldspar, Biotite,
Amphibole
Felsic
Biotite Graphite
Felsic
Granite
Felsic
Syenite
Medium
Diorite
Mafic
Gabbro
Felsic
Anorthosite
Mafic
Hornblende
Basalt Porphyry
Tracyte
Porphyry
Felsic
Felsic
Felsite
Medium
Andesite
Mafic
Basalt
Mafic
Obsidian
Mafic
Pitchstone
Felsic
Pumice
Felsic
Scoria
1. How is the size of the mineral grain affected by rate at which molten rock material cools?
It is affected for the faster the materials cool then bigger the grains
2. How can you determine if an igneous rock has had an intrusive of extrusive origin?
Underground rock and are formed from the cooling of magma is intrusive and
rocks on the surface formed from the cooling of lava is extrusive
3. In general, how does the density of a light colored igneous rock differ from that of a dark colored
igneous rock?
Light colored igneous rocks are lesser in density than dark colored igneous
rocks.
4. What is the main difference between lava and magma?
Lava is molten rock that reaches the earth’s surface and magma is molten rock
under the earth’s surface
5. Describe a porphyritic texture
Large crystals are surrounded by a fine-grained mass of rock.
6.
Describe a porphyritic texture.
Porphyritic texture is an igneous rock texture characterized by two distinctively different
crystal sizes. Porphyritic rocks are composed of at least two minerals having a conspicuous (large)
difference in grain size. The larger grains are termed phenocrysts and the finer grains either matrix
or groundmass. Porphyritic rocks are thought to have undergone two stages of cooling; one at
depth where the larger phenocrysts formed and a second at or near the surface where the matrix
grains crystallized.