| Contains 15 representative specimens of intrusive and extrusive
igneous rocks, each approximately 11/2" x 11/2. Rocks, which have
formed from the molten state, are called igneous rocks. Igneous
rocks that have cooled and crystallized deep beneath the surface of
the earth are called INTRUSIVE or PLUTONIC rocks. Plutonic rocks
usually display PHANERITIC or PORPHYRITIC texture. Rocks that were
formed when lava reached the surface of the earth through fissures,
vents or volcanoes are called EXTRUSIVE or VOLCANIC rocks. Volcanic
rocks will usually exhibit APHANITIC texture.
PHANERITIC TEXTURE
Mineral grains of nearly equal size and visible to the naked
eye.
1. PEGMATITE is a very coarse grained plutonic rock with a
similar composition to granite.
2. GRANITE is a crystalline rock consisting of potassium
feldspar and quartz with hornblende and mica as common accessory
minerals.
3. GRANODIORITE is a crystalline rock consisting of quartz,
plagioclase feldspar, potassic feldspar, hornblende, and biotite.
4. ANORTHOSITE is a plutonic rock consisting of 90 to 100
percent feldspars and minor amounts of quartz.
5. GABBRO is a crystalline plutonic rock consisting of
plagioclase with minor amounts of augite.
PORPHYRITIC TEXTURE
Mineral grains of two distinct sizes.
6. RHYOLITE PORPHYRY is a light colored igneous rock
containing alkali feldspar crystals in an aphanitic groundmass
of quartz and feldspar.
7. BASALT PORPHYRY is a dark colored rock containing feldspar
crystals in an aphanitic groundmass of plagioclase feldspar and
hornblende.
APHANITIC TEXTURE (Microcrystalline)
Mineral grains too small to be seen by the naked eye.
8. TRACHYTE is composed of potassium feldspar, minor
plagioclase feldspar, and accessory hornblende.
9. RHYOLITE is a light colored rock consisting of micro
crystalline potassium feldspar, quartz, and minor accessory
minerals.
10. ANDESITE is a light colored rock consisting of
plagioclase feldspar and hornblende, biotite or augite.
11. TUFF is compacted volcanic materials smaller than 4mm in
size.11. TUFF is compacted volcanic materials smaller than 4mm
in size.
APHANITIC TEXTURE (Glassy)
12. OBSIDIAN is a glassy volcanic rock formed by the rapid
cooling of rhyolitic lavas.
13. PITCHSTONE is formed by the rapid cooling of rhyolitic
laves, and has a resinous or pitchy appearance.
14. PUMICE is a light colored volcanic rock of rhyolitic
composition; the texture results from bubbles formed by escaping
gasses.
15. SCORIA is a dark colored volcanic rock of basaltic
composition with a porous texture due to escaping gasses.
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