A volcanic dam is a type of natural dam produced directly or indirectly by volcanism, which holds or temporarily restricts the flow of surface water in existing streams, like a man-made dam. There are two main types of volcanic dams, those created by the flow of molten lava, and those created by the primary or secondary deposition of pyroclastic material and debris.
Volcanic dams generally occur world-wide, in association with former and active volcanic provinces, and are known to have existed in the geologic record, in historic times and occur in the present day. Their removal or failure is similarly recorded. The longevity, and extent varies widely, having periods ranging from a few days, weeks or years to several hundred thousand years or more, and dimensions ranging from a few meters to hundreds, to several thousand.
Lava dams are formed by lava flowing or spilling into a river valley in sufficient quantity and height to temporarily overcome the explosive nature (steam) of its contact with water, and the erosive force of flowing water to remove it. The latter depends on the quantity of water flow and stream gradient. The lava may flow during numerous successive or repetitive eruptions and may emanate from single or numerous vents or fissures. Lava of this nature, like basalt, is usually associated with less explosive eruptions; more viscous lavas with lower mafic content, like dacites andrhyolites, can also flow, but tend to be more closely associated with eruptions of greater explosiveness and the formation of pyroclastics.
Once initially established, continued lava flow creates a steeper upstream face as it battles the rising water, but with most lava flowing unimpeded downstream covering the now-dried river bed and its alluvial sediments, sometimes for miles. Thus emplaced, the shape of a lava dam resembles an elongated blob, wedged in the valley bottom. At the same time, the water continues to flow, the lake continues to rise and accumulate sediment, which previously had migrated unimpeded downstream. Sediment filling, over-topping, downward erosion, waterfalls and under-cutting inevitably follow, unless an alternative outlet is established, for water and sediment elsewhere in the drainage.
Large examples of lava dams from the geologic record include those repeatedly developed from the western side of the Grand Canyon, with the largest remnant now termed Prospect Dam, and in several locations within the Snake River drainage. The former 'Lake Idaho', which existed for more than 6.5 million years, filled the western portion of the behind such a structure and created the western section of the Snake River Plain, and accumulated 4000 ft of lake sediments.
In the recent geologic past, volcanic activity dramatically impacted the Grand Canyon. In the western Grand Canyon hundreds of volcanic eruptions occurred over the past two million years. At least a dozen times, lava cascaded down the walls of the Inner Gorge, forming massive lava dams that blocked the flow of the Colorado River. Three of these lava dams were over 1,000 feet high, forming lakes similar to reservoirs such as Lake Powell or Lake Mead. Some of the lakes were over 100 miles long and filled the lower portion of the Grand Canyon for many years before finally over-topping the dam and eroding much of it away.
How To Log this Cache:
Please message the answers to the following questions to the Cache Owner. This can be done from the cache page. You may log the find once you have sent the answers. If I have any questions regarding your answers I will contact you. If you do not email me the answers your log may be deleted without warning.
1) There were many lava dams in the Grand Canyon over the last few millenia. From what mountain range did the volcanic activity originate?
2) Given what you've learned about Lava Dams, are the ones that formed here caused by lava flow or by the secondary deposit of pyrochlastic debris? Why do you suspect this is true?
3) At what height may the highest of the lakes formed at?
4) According to scientific estimates how long would the largest of the lava-blocked lakes taken to fill? How long would it have taken to clog with mud?
5) At the current location how wide would you estimate a lava Damn would have to be to form a lake? A million years ago would this have been a smaller width than it is today? Why or why not?
6) (Optional) Post a photo of yourself at GZ. Please don't allow the answers to your questions to be seen!
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First To Find: this2shalpas
This Earthcache was placed with permission from the Grand Canyon National Park's Permitting Office. A copy of the permit is available upon request to the CO.