Snake River Canyon tells a story as the volcanic cliffs would suggest. The eruptive force of volcanic activity spewing molten lava and mantle rocks from deep within the earth. The canyon you see before you as the Snake River canyon is what happenes when thousands of years of water erode the volcanic rock that lines the canyon walls. Many of the low mounds and buttes you see on the surrounding sagebrush-covered plateaus, such as Initial Point, Sinker Butte and Kuna Butte, are remnants of shield volcanoes. Shield volcanoes are the largest volcanoes on Earth that actually look like volcanoes. They are named for their large size and low profile, resembling a warrior's shield lying on the ground (the Hawaiian shield volcanoes are the most famous examples).
Volcanic Rock Types:
Rocks are not all the same. Some are heavy, some are light. Others are dark, while some can be almost pure white. Even igneous rocks that are all formed from magma in the Earth’s mantle can look very different.
Volcanic rock is an igneous rock of volcanic origin. Volcanic rocks are usually fine-grained or aphanitic to glassy in texture. They often contain clasts of other rocks and phenocrysts. Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye. They were created during fractional crystallization of magma before extrusion. Volcanic rocks are named according to their chemical composition. Basalt is a very common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has the same chemical composition as granite and basalt is compositionally equal to gabbro. Intermediate volcanic rocks include andesite, dacite, and latite. Volcanic rocks often have a vesicular texture, which is the result voids left by volatiles escaping from the molten lava. Pumice is a rock, which is an example of explosive volcanic eruption. It is so vesicular that it floats in water. Pyroclastic rocks are the product of explosive volcanism. They are usually felsic (high in silica). Examples of pyroclastic rocks are tuff and ignimbrite. Shallow intrusions, which possess structure similar to volcanic rather than plutonic rocks are also considered to be volcanic.
What are igneous rocks? Igneous rocks are formed from the solidification of molten rock material. There are two basic types: 1) intrusive igneous rocks such as diorite, gabbro, granite and pegmatite that solidify below Earth's surface; and 2) extrusive igneous rocks such as andesite, basalt, obsidian, pumice, rhyolite and scoria that solidify on or above Earth's surface.
ROCK TYPES:
Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene and biotite.
Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene.
Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes quartz
Gabbro is a coarse-grained, dark colored, intrusive igneous rock that contains feldspar, augite and sometimes olivine.
Granite is a coarse-grained, light colored, intrusive igneous rock that contains mainly quartz and feldspar minerals.
Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form.
Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals.
Scoria is a dark-colored, vesicular, extrusive igneous rock. The vesicles are a result of trapped gas within the melt at the time of solidification. It often forms as a frothy crust on the top of a lava flow or as material ejected from a volcanic vent and solidifying while airborne.
The Cache:
This cache is located on the edge of a large volcanic crater overlloking the Snake River. There is a nearby geoart that brought me to this spot, it was a breathe taking view. Looking down into the canyon you will notice large piles of rocks littering the floor. The cliffs across the way are sheer and formed of the same material. Understanding volcanic activity and knowing the the eruptie force of the explosion brings minerals from deep within the earth, and the consequent flow leaves deposites. Overlooking the river and knowing the history of this valley would shed some light onto the creations, that clash of fire and water.
To log this cache:
1. Looking at the cliffs, the scattered rocks on the floor below and even the scatterenings of rocks at your feet, you will see darker rocks, some rather large. These are volcanic rocks. Based on the descriptions provided, and possible volcanic knowledge, what kind of volcanic rocks are these?
2. At the cache sight there is a natural rock formantion that I found very intriging and beautiful. You will see this natural formation as you look down into the canyon. Describe the formation.
Please do not post you answers for this cache in your log. Email me, tell me the number in your party and who they are, one email per party should be fine. Have fun, thanks.
Sources:
"Igneous Rocks." Igneous Rocks. geology.com.
"Types of Volcanic Rock." Science Learning Hub RSS. http://sciencelearn.org.nz/
"What Are Volcanic Rocks?" What Are Volcanic Rocks? http://www.volcanodiscovery.com/
BLM is aware of this earthcache and has been notifiede about its placement. They have given permission for the placement of this and bid geocachers good journies and good times. Be careful and mind the little ones and the pets.