Volcanic Dam - Lava Dam

Lava Dam

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 usually is associated with less explosive eruptions; more viscous lavas with lower mafic content, like dacites and rhyolites can also flow, but tend to be more closely associated with eruptions of greater explosiveness and the formation of pyroclastics.

Once initially established, continued lave 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. Other locations include near American Falls, Idaho, and numerous others. Many of these were overtopped, washed out or skirted by the outburst flood originating from ancestral Lake Bonneville.

Many other examples exist globally including, Caburgua Lake in Chile, Mývatn in Iceland and Lake Reporoa in New Zealand. Examples in western Canada and others in northwestern United States include, Lava Lake and The Barrier, which still impounds Garibaldi Lake, and Lava Butte.