We tend to look at the landscape around us and think it's unchanging because it hasn't changed in our lifetimes.

We tend to look at the landscape around us and think it's unchanging because it hasn't changed in our lifetimes.

That, like the story of the Tooth Fairy, isn't quite true (sorry Virginia), especially considering geologic time.

In 1980, Mount St. Helens gave us a jolting wake-up call. Volcanic earthquakes set off a huge landslide, a catastrophic debris avalanche, followed by a spectacular explosion. The St. Helens' landslide, though, was a piker compared to what occurred at Mount Shasta between 300,000 and 360,000 years ago.

Unlike at Mount St. Helens, "juvenile" volcanic ash (produced during eruption) hasn't been identified, so likely no major eruption occurred. However, researchers believe that a large earthquake or earthquakes destabilized Mount Shasta, triggering the largest landslide in the world younger than 1.8 million years old! The catastrophic debris avalanche moved 43 kilometers (about 26 miles) from the base of ancestral Mount Shasta north-northwest across part of the Shasta Valley past Yreka and into the Shasta River Gorge. Its estimated volume dwarfed Saint Helens' debris avalanche.

What does it look like? Actually, it's best seen from the air as I saw it in 1982 while flying with a student taking my volcanoes course. Looking down at the Shasta Valley, I remarked, "That looks like a bodacious landslide! Nah, too big."

There's a sad lesson about limited minds somewhere in those words.

As seen from Interstate 5, the landslide is composed of hundreds of scattered mounds, hills and ridges surrounded by flat grassy areas supporting methane-producing cows. Rock fences, most made by Chinese workers, scale some of the hills. About five miles south of Yreka, these hills pop up like chunks of fruit in melted ice cream. Those "chunks" are actually detached pieces of ancestral Mount Shasta that were carried relatively intact off the volcano.

We're not sure how the chunks managed to remain intact, but they might have trapped a cushion of air beneath them (think of an air hockey game) and moved over a nearly frictionless surface. The materials that make up the flat areas between mounds were deposited as water-saturated slurries of silt, sand, cobble and larger boulders. This layered material can be seen in I-5 road cuts just west of Weed. Lake Shastina lies within the landslide and, indeed, all of Weed was built atop it.

Could a similar volcanic landslide happen in the Bear Creek Valley? Put on your nightcap and slip contentedly into bed. The hard ridges between Mount Shasta and our valley are a formidable barrier. Likewise, the tilted volcanic ridges of the humble western Cascade volcanoes would impede any volcanic landslide from our nearest volcano, Mount McLoughlin.

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.