Posted on May 30, 2021
This is an update of my post published on May 30, 2010:
A volcanic peak in Northern California, Lassen Peak, formed about 27,000 years ago, but for thousands of years it just sat there looking pretty as Maidu and other American Indian groups hunted and gathered in its shadow, and as European Americans such as Peter Lassen and William Nobles led groups of settlers past as they traveled to the Sacramento Valley.
Some people visited the area and climbed the peak, but few people lived near there. (As it turns out, that was probably a lucky thing!)
On this day in 1914, a steam explosion shook the mountain. This announced to the world that Lassen was active again after all those thousands of years of being “dormant” (which basically means “sleeping”).
Humans could say, “Thanks for the warning!” Actually, the mountain gave a series of warnings—more than 180 steam explosions occurred over the course of a year. Finally, after all those warnings, in May of 1915 larger eruptions shook the area, with hot, glowing blocks of lava spewing out of the mountain, an avalanche of snow and volcanic rock roaring down one flank, and mudflows pouring down another. The “Great Explosion” of 1915 shot pumice and other rocks high into the air, and a cloud of ash and gas rose more than 30,000 feet up. People who lived more than 200 miles away could see the eruption!
And nobody died!
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| This huge rock flew through the air, landed with a mighty thunk, and stayed hot for months! It's still called "Hot Rock"! |
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| One active area near Lassen Peak is called Bumpass Hell |
What causes volcanoes?
Volcanoes are openings in the Earth's crust that allows molten rock, ash, and gases to escape from underneath.
There are two main reasons for volcanoes: the actions of tectonic plates, and “hot spots” caused by mantle plumes.
The rocky crust of the Earth is not one whole sphere of rock, but is instead eight giant plates and many smaller plates of rock.
These plates move about on the hotter upper mantle, sometimes spreading apart and letting molten rock come up through the resulting crack, sometimes running into each other and pushing up mountains, and sometimes with one plate pushing down underneath another. The areas where plates pull apart or run into each other are often edged with volcanoes and feature earthquakes.
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| The diagram above shows how Lassen Peak and other Cascade volcanoes were formed, as the Pacific Plate ran into and then pushed underneath the continental plate. The diagram below shows how the Hawaiian Islands were formed in the middle of the Pacific Ocean. |
Hot spots are quite different. They are probably caused by mantle plumes, areas in which hotter columns of molten rock rise through the mantle and cause volcanic eruptions in the crust. They can be far from the edge of the tectonic plates, but as a plate slowly glides over a hot spot, a chain of volcanic islands can form. The Hawaiian Islands are a famous example of a chain of “hot spot” volcanic islands.
Mount Lassen is an example of a tectonic-plate volcano, and it other nearby volcanic mountains make up the Cascade Range. Mount St. Helens in Washington, which exploded in a deadly and economically destructive eruption in 1980; Crater Lake in Oregon, which is the remnant of Mount Mazama; and Mount Shasta in Northern California are other volcanoes in the Cascades.
Explore Volcanoes (Virtually Speaking!)
“How Volcanoes Work” is an interesting website with lots of diagrams.
Kids' National Geographic has a page about volcanoes.
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There are several different kinds of volcanoes and volcanic eruptions. |
There are volcanoes elsewhere in the universe, other than Earth! The largest volcano that we know of is on the planet Mars: Olympic Mons, which is about the size of the state of Arizona.
Here is a website that explains why volcanoes on Mars tend to be bigger than any we have on Earth.
And here are some new hi-res photos of the Martian volcanoes and other Martian features.
Also on this date:
