Geologists were watching this mountain very carefully because the rocks that make
up Mount St. Helens are of a composition called felsic.
And lavas with a felsic composition tend to produce very explosive eruptions.
Fast forward then, from March to May of 1980.
These earthquakes continued, the bulge kept growing.
On May 18th, at about 8:30 in the morning,
there were a couple of slightly larger earthquakes, most of them had
the previous earthquakes had been on the order of maybe a magnitude three or four.
There were a couple that were magnitude five.
Those earthquakes shook loose the rocks that made up
the bulge on the mountainside.
Those rocks began to slide downhill and that triggered a big landslide
where essentially the entire bulging side of the mountain went downhill.
In turn, the removal of all that rock
released the pressure on the liquid rock inside the volcano and
at approximately 8:32 in the morning, the eruption actually began.
It started with a blast of ash vertically upwards, and
ash reached a height of approximately 12 kilometers in the atmosphere.
At the same time, or virtually the same time,
there was another blast that came sideways out of the mountain.
And that blast knocked down forests for kilometers away from the mountain.
The ash was carried buy the winds all the way across the continent of
North America until it reached the Atlantic ocean.
By around ten in the morning on May 18th, the eruption was well underway.
Remember the picture I showed you from beforehand,
how the top of the mountain was covered with ice and snow.
Well, you combine ice and snow with hot ash and all that ice and snow melts.
Now you've got water that mixes with the volcanic ash to make mudflows.
The geological term for volcanic mud flows is lahar but
I'll just continue talking about mudflows.
Those mudflows went surging down the valley's all around the mountain and
caused severe flooding in downstream areas.
A mixture of volcanic ash and water is very thick and
it's much more powerful than just plain water.
It's able to move huge boulders as you can see in this picture.
That boulder in the middle of the picture was not there prior to the eruption and
mudflows.
Also look at the trees in background of the image and
you can see the high water mark from the mud left on the tree trunks.
So this was a really large and powerful flood down many of these rivers.
The mud flows extended for tens of kilometers from the mountain.
And, in fact, probably more property damage occurred as a result
of the mud flows than of the actual ash flow of the eruption itself.
As you work your way through this lab,
you're going to be comparing these two eruptions.
Both Vesuvius and Mount St. Helens are stratovolcanoes.
They had similar types of eruptions.
Because we have so much detail about Mount St.
Helens, we can use what we learned from that eruption in order to make
predictions about what could happen in the future around Mount Vesuvius.
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