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Bridge of the Gods

 

 

The Bridge of the Gods is a natural bridge that was created by the Bonneville slide. The Bonneville slide is a major landslide that dammed the Columbia River near present-day Cascade Locks, Oregon in the Pacific Northwest of the United States. The event is remembered in local legends of the Native Americans as the Bridge of the Gods.

The Bonneville landslide sent a large amount of debris south from Table Mountain and Greenleaf Peak, covering more than 5.5 square miles (14 km2). The debris fell into the Columbia Gorge close to modern-day Cascade Locks, Oregon, blocking the Columbia River with a natural dam approximately 200 feet (61 m) high and 3.5 miles (5.6 km) long. The impounded river formed a lake and drowned a forest of trees for about 35 miles (56 km). The Columbia River eventually broke through the dam and washed away most of the debris, forming the Cascade Rapids. Geologists have determined that debris from several distinct landslides in the same area overlap, forming what is called the Cascades landslide complex. The Bonneville landslide was the most recent, and perhaps the largest landslide of the complex.

The Cascadia fault is a subduction zone that stretches from northern Vancouver Island to northern California. It is a long fault that separates the Juan de Fuca and North America plates. The geological record of the Pacific Northwest reveals that “great earthquakes” occur in the Cascadia subduction zone about every 500 years on average, often accompanied by tsunamis. There is evidence of at least 13 events at intervals from about 300 to 900 years with an average of 590 years. The Cascadia fault is thought to be the cause of the massive Cascades landslide complex.

On January 26, 1700, a massive magnitude 8.7 to 9.2 megathrust earthquake occurred in the Cascadia subduction zone. Evidence supporting the earthquake has been gathered in the 2005 book The Orphan Tsunami of 1700, by geologist Brian Atwater. Atwater has spent much of his career studying the likelihood of large earthquakes and tsunamis in the Pacific Northwest region of North America. The earthquake produced a tsunami so large that contemporary reports in Japan noted it, allowing Atwater to assign a precise date and approximate magnitude to the earthquake.

After studying the coastline across the Pacific Northwest, Atwater found evidence that an enormous tsunami devastated the area around the year 1700. The earthquakes path and size are confirmed by evidence of a dramatic drop in the elevation of the northwest coastal land, recorded by buried marsh and forest soils that underlie tidal sediment. Atwater’s team found a layer of tsunami sand on the subsided landscape. The most important clue linking the tsunami in Japan and the earthquake in the Pacific Northwest comes from studies of tree rings which show that red cedar trees killed by the lowering of coastal forests into the tidal zone have outermost growth rings that formed in 1699, the last growing season before the tsunami.

Recent findings conclude that the Cascadia subduction zone is more complex and volatile than previously believed. Geologists predict a 37 percent chance of a M8.2+ event in the next 50 years, and a 10 to 15 percent chance that the entire Cascadia subduction will rupture with a M9+ event within the same time frame. Geologists have also determined that the Pacific Northwest is not prepared for such a colossal quake. The tsunami produced by such an event could reach heights of 80 to 100 feet (24 to 30 m).

The date of the Bonneville landslide is an unresolved issue among people studying it. Some researchers promote a date around 1450, while others favor a date around 1700, which would connect the landslide to the 1700 Cascadia earthquake. Native American legends from the Klickitat tribe describe an earthquake that shook so violently that a huge bridge fell into the river, creating the Cascades Rapids of the Columbia River Gorge. The legends date to the early 18th century.

 

(source: http://listverse.com/2011/08/21/top-10-recent-geological-discoveries-and-hypothesis/)

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