Igneous Rocks

Igneous rocks are made up of intergrown mineral crystals formed by the cooling and solidification of magma or lava. Igneous rocks are classified on the basis of their mineral composition (felsic, intermediate, mafic, or ultramafic) and texture (aphanitic, phaneritic, porphyritic, glassy, vesicular, or pyroclastic). The composition gives information about the type of magma from which it formed. Generally, more felsic minerals are lighter in color, including pink, while more mafic minerals are darker in color, including green. The texture provides insight into the cooling history of the magma. Coarse-grained rocks are ones that individual minerals are visible to the naked eye, cooling very slowly beneath the Earth’s surface. Fine-grained rocks contain minerals too small to be seen with the naked eye, cooling at the4 Earth’s surface very quickly. Use the information from the igneous rock classification chart below to identify the ten unknown igneous rock names (granite, basalt, gabbro, diorite, rhyolite, tuff, scoria, andesite porphyry, peridotite, and obsidian). Use sections 5.1, 5.2, and 5.3 in the textbook for assistance.


Sample NumberTexture (Aphanitic, Phaneritic, Porphyritic, Glassy, Vesicular or Pyroclastic)Mineral Composition (Felsic, Intermediate, Mafic or Ultramafic)Cooling History

(Fast, Slow, Two-Stage)




For each sample, answer the following questions by filling in the chart above.

What is the texture?

What is mineral composition?

What is the name of the rock?

What is the cooling history of this mineral?

After completing the igneous identification rock chart above, you will then take a 20-question multiple-choice quiz on the assignment in Moodle. Be sure to complete the chart before taking the quiz.

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