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Basalt erosion at Silver Falls State Park

A cache by shadow101 Send Message to Owner Message this owner
Hidden : 11/26/2007
1.5 out of 5
1 out of 5

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Geocache Description:

This easy earthcache illustrates how an eroding basalt formation (lava flow) has created one of the world's great collections of waterfalls.

Between six and 17 million years ago, during the late Miocene era, what is now the Columbia River basin was covered in a series of catastrophic lava flows. The supervolcanos that caused these events were mostly in Idaho (more than 250 miles away), with one notable exception: the increasingly active supercaldera beneath what is now Yellowstone National Park. The eruptions occurred with an average of one flow every 35,000 years with an output of lava per flow of over 100 cubic miles. Some eruptions were known to have spread more than 500 cubic miles of basaltic lava in a single flow. The total volume of the spoon-shaped mass of basalt is more than 42,000 cubic miles, with the deposits reaching more than three miles deep in the area around Yakima, WA.

While there is some disagreement of how exactly the lava in the flood basalts moved, a popular theory is that as the lava moved, the surface skin may have hardened only to break due to the internal pressure of the still molten lava. This resulted in the advancing flow looking like a wall of broken rock and glass more than 100 feet tall. This scary vision gets even worse when you understand that the flows advanced at up to 30 mph. Inside the flow the lava was fluid, but at the base, top and snout it would be covered with jagged pieces of broken, cooled basalt. Lava that spreads in this manner is known as A'a. A'a translates roughly from Hawai'ian as meaning "stony with rough lava". As can be expected, A'a is common in the Hawai'ian Islands.

As the lava would spread further from its source, it would become thinner and thinner, and would eventually cease to flow, dammed by its own cooled crust. Over time, the remainder of the lava would cool, and life would return. Layers of dead plant and animal matter would accumulate atop the flow, and the ground would settle. Then there would be another eruption, and the cycle would repeat. Eventually these large eruptions ended, and the conventional pattern of wind, rain and sun would return. This group of basalt flows is known as the Columbia River Group.

Silver Falls State Park is near the southern edge of the Columbia River Group, and the flows are correspondingly thinner. In addition, this area has seen subsequent flows from nearby Mt. Jefferson, a dormant volcano and Oregon's second highest peak. This mixture of thin basalt mixed with various types of sedimentary rock have combined for a collection of spectacular waterfalls in a small area.

This Earthcache is in an Oregon state park, so there will be a fee for entry (it was $3 when I came). And because the area is both fragile and dangerous, please refrain from leaving the trails. Permission for this Earthcache was granted by the Silver Falls State Park because of Earthcache's Leave-No-Trace principles, and the fact there is no placement of a physical cache container.

The coordinates take you to South Falls, the largest and most famous of the park's 10 waterfalls. All of the information necessary for this Earthcache can be obtained by an easy and mostly flat hike of less than 1/4 mile. However, I do recommend that you take the trail down under and behind South Falls, to get the full effect of the processes occurring here. Caution: This trail is hypothetically wheelchair and stroller accessible, but may pose problems with steeper grades and wet surfaces.

To get credit for this Earthcache, email the following info to me: 1) How many layers of basalt and sandstone do you count between the top and bottom of the waterfall, 2) Given that the falls has a height of 177 feet, make an estimate of how thick the top layer of basalt is, 3) do you predict the waterfall increasing or decreasing in height in the future, and 4) Give a reason why for #3. Answers to the above provided in a log will be deleted.

Also, since this area is so absurdly scenic, you must either post a picture with both your party and the falls in your log, or write a *detailed* entry of your trip, describing the area and your impressions of the scenery. Think travelogue here. And enjoy the park. There are nine other spectacular waterfalls here, along with several miles of fantastic hiking.

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