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Ice, Lava, Geology, And Mount Rainier

What happened to the lava at Mount Rainier? Why is the story so unusual?

A close up view of the inside of Mount Rainier

Photo: Vlad Karpinsky (Flickr)

The east wall of Mount Rainier shows the ridges left by lava.

On our last program, we introduced a mysterious geological feature that can be found on the slopes of Mount Rainier, a volcano in Washington state.

Flows of hardened lava cling to the tops of ridges as they wind down the mountainside, ignoring deep valleys to either side.

Why Didn’t The lava Flow Down The Valleys, As Any Other Liquid Would?

The key to this is the epoch in which these features formed. They formed during an ice age, which means that this curious geography is the result of a tremendous, elemental battle between fire and ice–a battle which ice won.

Imagine a volcano, besieged by glaciers. The glacial ice is thickest where it’s filling deep valleys on the mountainside, and thinnest where there are ridges that almost poke their rocky spines up through the ice sheet.

Suddenly, the volcano erupts. Molten lava pours toward the ice. If the lava were hot enough, it would melt its way through the glacial ice like a hot knife through butter, then follow the path of least resistance–flowing down into the valleys.

Cold Lava

As it turned out though, the lava wasn’t hot enough. Mount Rainier’s lava was only able to melt through the thinnest parts of the glacier–those parts that were over ridges and hilltop spines.

When the glacier retreated, it left behind the geography we see today: Hardened lava flowing along hilltops, and valleys free of lava flows.

Is there a moral to this story of patient ice and fiery lava? We can’t say, but the history seems to be written clearly enough, in flowing lines down the slopes of Mount Rainier.

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