Is this an intrusive or an extrusive igneous rock?
From this view the amazing structure of rocks that make up Devil's Tower doesn't really indicate whether the structure formed slowly or quickly. A close up view would show small crystals in a mafic rock, indicating a rapid cooling from a basalt lava. Cooling was slow enough that the hexagonal "posts" could form.
Igneous Rock Classification
Igneous rocks are first classified by their composition, from felsic to ultramafic. The characteristics and example minerals in each type are included in Table below.
|Felsic||Light||Low||Quartz, orthoclase feldspar|
|Intermediate||Intermediate||Intermediate||Plagioclase feldspar, biotite, amphibole|
|Ultramafic||Very dark||Very high||Olivine|
Second to composition in igneous rock classification is texture. Texture indicates how the magma that formed the rock cooled.
|Type||Amount of Silica||Extrusive||Intrusive|
Some of the rocks in Table above were pictured earlier in this concept. Look back at them and, using what you know about the size of crystals in extrusive and intrusive rocks and the composition of felsic and mafic rocks, identify the rocks in the photos in Figure below:
These are photos of A) rhyolite, B) gabbro, C) peridotite, and D) komatiite.
- Composition is the first criteria on which to classify igneous rocks, with categories from felsic to ultramafic; color is a first order indicator of composition.
- Texture is the second criteria for classifying igneous rocks because texture indicates how a rock cooled.
- Igneous rocks are categorized in pairs with the same composition but different textures: gabbro-basalt, diorite-andesite, and granite-rhyolite.
Use this resource to answer the questions that follow.
Types of Igneous Rocks
1. Describe andesite.
2. What is basalt?
3. What is diorite?
4. Describe gabbro.
5. What is granite?
6. How is obsidian formed? What is it?
7. What is pegmatite?
8. What is peridotite composed of?
9. What is pumice?
10. What is rhyolite?
11. Describe scoria. How does it differ from pumice?
12. What is tuff?
1. Describe the formation of the igneous rock pair gabbro-basalt. What makes the rocks the same and what makes them different?
2. How does the composition of a rock affect its color?
3. What are ultramafic rocks and where are they likely to be found?