Two fundamental types of volcanism were involved in the formation of the Snake River Plain, as the wave of volcanism swept across southern Idaho. The first was the eruption of rhyolite from magmas with the same composition as granite, being rich in silica and alkali elements. These magmas mainly were materials melted out of the Earth's crust, so its no wonder they have a granitic composition. The other type of volcanism was the eruption of basalt. These magmas came from deeper in the Earth, from the region geologists call the mantle, and were hotter than the rhyolite magmas. Basalt is much richer in iron and magnesium and poorer in silica than rhyolite.
Rhyolite, extrusive igneous rock that is the volcanic equivalent of granite. Most rhyolites are porphyritic, indicating that crystallization began prior to extrusion. Crystallization may sometimes have begun while the magma was deeply buried; in such cases, the rock may consist principally of well-developed, large, single crystals (phenocrysts) at the time of extrusion. The amount of microcrystalline matrix (groundmass) in the final product may then be so small as to escape detection except under the microscope; such rocks (nevadites) are easily mistaken for granite in hand specimens. In most rhyolites, however, the period of such crystallization is relatively short, and the rock consists largely of a microcrystalline or partly glassy matrix containing few phenocrysts. The matrix is sometimes micropegmatitic or granophyric. The glassy rhyolites include obsidian and pumice.
Basalt, extrusive igneous (volcanic) rock that is low in silica content, dark in colour, and comparatively rich in iron and magnesium. Some basalts are quite glassy (tachylytes), and many are very fine-grained and compact. It is more usual, however, for them to exhibit porphyritic structure, with larger crystals (phenocrysts) in a finely crystalline matrix (groundmass). Basaltic lavas are frequently spongy or pumiceous; the steam cavities become filled with secondary minerals such as calcite, chlorite, and zeolites. Often it exhibits a vesicular quality - having small depressions in the rock.
The banks of the Snake River consist of high quantities of both of these lava rocks. But here, you will find larger quanties of one. Based upon the above descriptions and your own indpendent research, please answer the following questions about these lava relics:
1. Are the rocks primarily one color or a mixture of colors? What color(s) are most predominate?
2. Are the rocks jagged or smooth? If jagged, describe what caused the jagged edges. If smooth, describe wht caused the rock to be smooth.
3. Study the surface of the rocks. Describe for any porphylitic qualities that you see - including the color of any crystals.
4. Does the rock show any vesicular qualities? If so, please describe these for me - are there many, or a few? Large or small? Uniform in size?
5. Based upon your observations, are you looking at Rhyolite or Basalt? Explain your answer.