I have found that most people don't really understand just how Mount St. Helens erupted, so let me explain, using the below video and some screen captures. Before you watch the clip though, I want to explain some things about magma. First, it's hot, maybe in the range of 800 Celsius (in this particular case), ~1350 F. Second, it has a tremendous amount of material dissolved in it that would, under normal surface conditions, prefer to be gasses: carbon dioxide, sulfur dioxide, hydrogen sulfide, and others, but most importantly water. These so-called volatile components make up a small percentage by mass of the solution, but (again, under normal surface conditions) would "prefer" to make up a massive portion of the volume.
You can think of these gases as just like the CO2 in soda pop, except the soda pop is molten rock at 800 C. You know what happens when you shake a bottle of soda, then open it quickly? Even though in terms of total mass, the CO2 is only a small component of the contents of the bottle, it quickly shows that under non-pressurized conditions, it's a very large component in terms of volume. Just to be clear, we ARE talking about gasses dissolved in the magma, not little blobs of hot water under pressure, or little bubbles of gas that expand, but again like soda, gasses actually dissolved in a liquid. A very hot liquid. And under magnitudes greater pressure than those present in bottled soda. It's important to understand that the gas is actually dissolved in the molten rock.
Now watch the video:
At 8:32, 30 years ago today, a magnitude 5+ earthquake shook the mountain. A diagram I did for an earlier post would be helpful here:That red blob in the second section would have been much larger by May 18th, and the north side bulge much more steepened... the mountain had effectively passed the point where the bulge was stable. So it fell off. In the following sequence, the times are those displayed on the video; I don't know how well those times correlate with the real timing of events.t=0 sec; earthquake hits, north side starts to slide slowly; note clouds of dust over the summit.t=2 sec; Slide accelerates. Note that in the first frame, Goat Rocks, the lumpy protrusion in the middle of the mountainside we're facing, is pretty much centered over a hill in the mid-ground. In the second frame, its left edge is now over that hilltop.
t=3 sec; the entire north side of the mountain is sliding now, and tremendous amounts of dust are rising near the summit, and above and to the left of Goat Rocks.
t=5 sec; relieved of support as the first block slides lower, a second block starts to slide from the peak. This one is rooted deeper in the mountain, and the surface of failure intersects the (until now) hidden and confined magma chamber (which, as you may remember, is very hot and under enormous pressure)t=8 sec; the first (right, lower) block continues to collapse, but is slowing and spreading out laterally. The second (left, upper) block is just getting started.t=10 sec; as the second block continues to collapse, it exposes a portion of the magma chamber, and a blast is channeled from the summit.
Brief mathematical interlude... if we assume (fairly conservatively; the number was probably higher) a 2.5% volatile percentage by weight, and a density of 2500 kg/cubic meter, we come up with a mass of volatiles of 62.5 kg/m^3. If we further assume this is all water (which it mostly was), that's the equivalent of 62.5 liters, or .0625 m^3. General rule of thumb- I'm not going to run through the gas laws- a volume of water will expand 1000 fold converting to steam. Magma that until an instant before had been under tremendous confining pressure has now been exposed to normal atmospheric conditions. There's nothing keeping those gasses dissolved in magma- now lava- anymore. They want out. And to a quick and dirty first approximation, each cubic meter of lava has 62.5 cubic meters of gas that want to escape. As I said at the outset, the mass percentage of the volatile component is small, but the amount of volume it will assume at one atmosphere is enormous compared to the volume it occupies in dissolved form.t=12 sec; as the first block slides ever lower against the second, the magma chamber is exposed in a second area. Notice this new exposure is facing sub-horizontally. While the summit exposure directs a blast at the open sky, this second exposure is directed across the landscape to the north. You have probably seen the term "lateral blast" associated with this eruption. While numerous vertical blasts have been seen and recorded in human history, this was the first time people had witnessed and (fortuitously) recorded a lateral blast, though it turns out they're not as uncommon as you might think from that fact.t=16 sec; the summit blast vaults even higher, while the lateral north side blast spreads. I have never been able to tell if the white clouds on the margins are dust (as I described them in the t=3 frame) or actually nuée ardentes- glowing avalanches. (followup: I received a comment that the white areas are avalanches of snow and ice, which makes sense. They had probably been dusted and disguised by preceding ash bursts.)t=18; both blasts expand. An important feature to note is that the blasts start well after the landslides, but have higher velocities than the landslides. The consequence of this in the rock record is that close to the mountain, landslide debris was deposited first, and blast deposits on top of that. Farther from the mountain, blasted material arrived and was deposited first, while the runout and deposits from the landslide arrived later. So close in, blast deposits are on top of landslide deposits, while further out, that sequence is inverted. This was the observation that has allowed volcanologists to recognize lateral blasts at other locations, and one of the reasons Mt. St. Helens is important to the science. Despite impressions, this was not a terribly large eruption. But it was very well documented and observed. Access was (and remains) relatively easy compared to other volcanoes. As a result, an enormous amount has been learned from this eruption and subsequent activity.
I won't make a habit of embedding the same video twice in the same post, but I encourage you to watch it again.
This is based on a classic set of photos taken by a hiker who happened to be in the right place at the right time, and who had a camera ready to go. It actually continues past the conclusion of the clip, as can be seen in this clip. I'll tack on one more frame from that one.I cringe a little when I see the phrase "an earthquake caused the eruption," or words to that effect. It would be accurate to say an earthquake triggered a landslide, releasing an explosive eruption. But what caused the eruption was gas dissolved in magma, and the sudden pressure release which allowed those gasses to catastrophically exsolve. I can think of mechanisms by which this event might have turned out differently... mostly involving plenty of time for the gas to escape more gently, but I think by the time the mountain reached this point, earthquake or no, an explosive eruption was the most likely outcome.
I had intended to point out some of my favorite MSH@30 pieces from geobloggers and other sources today, but the above took longer than I expected. Maybe tomorrow.
This Week's Geo-Quiz: Other Planets
6 months ago