To get to the earthcache, take state route 124 to the turnoff for Ice Harbor Dam at Monument Road. Follow the road to the dam and when the paved road bends right to the dam, proceed straight toward the river on the gravel road past the warehouse to a barrier of yellow painted pipe. Turn left (west) and follow the gravel road along the yellow pipe downriver to the parking area (coordinates provided). Please do not block the gate. From there you will take a short 0.3 mi hike downriver on the gravel road to the earthcache. Stay on the trails and do not climb the rocks. The road to the parking area, the hike and the earthcache location are all on Corps of Engineers public land.
At the earthcache coordinates, you will be looking at the core of a small volcano that erupted about 8.5 million years ago during the Miocene Epoch near the end of the volcanic eruptions that formed the Columbia River Basalt Group. The basalt that erupted from the volcano belongs to the Ice Harbor Member of the Saddle Mountains Basalt Formation. The volcano measured about 650 feet wide and 150 feet high and was one of many volcanoes that fed the nearby lava flows.
The volcano is composed of tephra and lava. Tephra is defined as volcanic material that is ejected into the air before falling back to the earth. Tephra is made up of different sized particles ranging from very fine volcanic ash particles, to larger pea to walnut-sized pieces called lapilli, to pieces larger than about 2.5 inches in diameter that are called volcanic bombs. Lava, by definition, flows from a volcano instead of being ejected into the air.
In the volcanic core that you are looking at, the first layer of the tephra is the tan-colored layer at the base and is made up of mostly volcanic ash particles. This layer also has several larger volcanic bombs visible in this exposure. The volcanic bombs that were ejected by the volcano included pieces of basalt from older buried lava flows that were ripped loose and brought to the surface by the eruption.
On top of the tan layer is a second layer of tephra composed of dark basalt called spatter. These are large pieces of lava that were thrown a short way into the air during the eruption, fell to the ground while still liquid, and cooled near the volcanic vent. The blobs of lava have a viscosity somewhat like taffy, so they splash down and over the sides of the developing mound around the volcano to form a steep-sided cone called a spatter cone. Because the lava is not fully solid when it lands, the individual deposits are very irregular in shape and weld together as they cool.
Overlying the spatter basalt layer is basalt that flowed from the volcano. Rather than being ejected into the air like the spatter basalt, this basalt flowed from the vent and would be called lava instead of tephra.
To get credit for this earthcache:
Send me an e-mail with the answers to the following questions. (NOTE: Do not post the answers in your log or it will be deleted.)
- What is the approximate maximum thickness of the tephra (tan layer)?
- What is the approximate maximum thickness of the darker basalt overlying the tan tephra layer?
- Approach the base of the tephra layer and at about eye level you will see a volcanic bomb in the tephra (local graffiti artists have conveniently marked it with blue paint). What are the width and length of this volcanic bomb? Describe the appearance of the vocanic bomb.
- Closely examine the tan tephra. Describe the texture and appearance (size of particles, shape of particles, solid or crumbly, etc.)
Pictures are always welcome, but please do not post any that could provide information for the answers.
For a two-fer, be sure visit our Volcano Cache Twofer (GC5J16R) cache placed near the parking coordinates.
References: Reidel, S. P., 2004, The Geologic Development of the Pasco Basin, South-Central Washington, Northwest Geological Society Field Trip Guide, October 16-17, 2004.
Congratulations to kiekefretter for the FTF! 