Imagine being an extraterrestrial geologist in geostationary orbit above the Pacific Northwest in the 1970s. You're the first explorers to reach Earth (underpants-thieving aliens aside), and you haven't got a lot of data on this little blue marble. But your own planet has plate tectonics, so you're familiar with the landforms caused by the process.
You have a look through your sensors, and see a conga line of volcanoes weaving up the continent.
Now, your image wouldn't be labeled, of course. But you can see a trench offshore, and a zig-zaggy ridge, so you can sketch in some very small oceanic plates busily subducting beneath the continent. The line of coastal mountains and that volcanic arc, less than a hundred miles from the sea, would have given the game away even if you didn't have super-cool sensors that can see beneath the sea. You know there's an active subduction zone here.
You run further scans. Some of the volcanoes in that arc are fairly young, and have been active in the recent past. You do a quick north-to-south sweep, and notice Mt. Baker still steaming away. Glacier Peak has a 300 year-old coating of ash. Mount Rainier is riddled with hydrothermal anomalies, cooking itself from the inside: it's so rotten you can see evidence of mudflows barely over a decade old. Mt. Adams doesn't seem to have erupted for the past few thousand years, but hydrogen sulfide fumaroles puff away atop it. And, a short jog to the south, Mt. Hood has also got active fumaroles. Any of these volcanoes could erupt at any time. The cities you see, located in the shadows of these restless mountains, had better watch out.
But it's one volcano in particular that's caught your eye (or eyestalk): just to the west of Mt. Adams, the most symmetrical of the lot. It's not the tallest, but it's the youngest. The others are hundreds of thousands, all the way up to a million, years old. Mount St. Helens is a barely-adolescent 40,000. And it's been extremely active.
You zoom in for a closer look. You see evidence of lahars, pyroclastic flows, ash falls, and lava flows. And you notice a dedicated geologist, scrambling over the outcrops. You don a disguise and beam down to chat him up. His name is Dwight "Rocky" Crandell, and he's been doing an exhaustive study on Mount St. Helens's eruptive history. It's almost complete. And he's concerned: this young volcano has been quite the firecracker. It's likely to erupt before the end of this century, he tells you. Being close to population centers, it could cause quite a mess. So it's important to understand its past behavior in order to assess the risks it poses.
You sit with him for hours, and learn more than you could have possibly hoped. By the end of your discussion, you know this volcano's history intimately. And you're intrigued. You can't help but to be. Dr. Crandell has an infectious passion for geology, and Mount St. Helens is just the kind of volcano that repays that interest in spades.
You learn there are two parts to this seemingly single mountain. Dr. Crandell divides it into Old Mount St. Helens, which was built mostly of volcanic domes and short, thick lava flows between 40,000 and 2,500 years ago, and the modern cone that was active as little as a century and a quarter before. He and other geologists, working diligently for many years, have divided her eruptive history into a number of stages and periods. When he's finished describing them, you have one good English word to describe her: explosive.
You give a précis of what you've learned to your team of intrepid explorers when you return to your ship.
First, you tell them, your suspicions of a subduction zone have been confirmed. Earth scientists only recently discovered plate tectonics, but they've done an excellent job figuring it out. Just off the coast near Mount St. Helens, a small plate they've named the Juan de Fuca is subducting beneath the North American continent. And this has given birth to a very lively chain of volcanoes called the Cascades.
Mount St. Helens was born during the Ape Canyon Eruptive Stage, beginning before 40,000 years ago, and going on until about thirty-six thousand. It was an explosive birth: Dr. Crandell found plenty of pyroclastic flow deposits and a lahar. Ash clouds left blankets of tephra so thick that even tens of thousands of years later, the layers can be recognized - they're called tephra set C. Both lithic (pulverized stone) and pumiceous (made of pumice) ash blasted out of the baby volcano. One or more lava domes and flows emerged.
After a noisy introduction to the world, Mount St. Helens fell silent. Then the Cougar Eruptive Stage began about 20,000 years ago, and lasted for around two thousand years. During this episode, she produced plenty of dacite eruptions. Pumiceous pyroclastic flows, the eruption of tephra sets K and M, lahars, and debris avalanches kept things hot. She filled the Lewis River Valley over a hundred meters (over 328 feet) deep - the deposits might have piled up to 115 meters (337 feet). A high-silica andesite lava flow joined the excitement. By the time she was done, she'd grown to 1,825 meters (5987.5 feet). Not bad for a youngster!
She slept for around 5,000 years. The Swift Creek Eruptive Stage, beginning around 13,000 years ago, produced more explosive eruptions. Tephra sets S and J blasted out of the mountain during this time. Lithic and pumiceous pyroclastic flows roared down her slopes and into her valleys. Lahars buried streams in mud and rock. Dacite domes pushed their slow, thick way to the surface. By the end, about 10,000 years ago, fans of rocky debris flanked those domes.
Five and a half thousand years of quiet followed. Then she began her Spirit Lake Eruptive Stage, which is still ongoing. Deposits are abundant enough to begin dividing her antics into periods. And during this time, things changed.
The Spirit Lake Eruptive Stage began with the Smith Creek Eruptive Period, which followed the familiar pattern: energetic eruptions spreading tephra all over the place (tephra set Y in this case). Pyroclastic flows. Lahars. Domes. The only thing really different about this time was the amount of pumiceous tephra she erupted: only the Ape Canyon Eruptive Stage could match it.
Mount St. Helens napped for a brief three hundred years, then woke up 3,600 years ago for her Pine Creek Eruptive Period. This explosive episode produced several pyroclastic flows, tephra set P, and several domes. Dr. Crandell found a remnant of a dome of that age at 2,200 meters (7,218 feet), so she'd attained quite a respectable height. She was still basically a mound of domes and piles of debris. Folks back then might not have been calling her symmetrical.
She took another three hundred year nap before the Castle Creek Eruptive Period, which began 2,200 years ago and marked a change in her style. She mixed it up, starting with dacite eruptions (her old favorite), followed by some andesitic activity that produced ash clouds and a lava flow. After that andesitic interval, she returned to her tried-and-truce dacite style, producing her patented air-fall tephra, pyroclastic flows, and likely a dome. She switched back to andesite for a bit, then really mixed it up with some respectable basalt flows. By the end, she had produced tephra set B, and boasted some very nice lava tubes within her basalts. She'd become a true stratovolcano, attained nearly her full height, and begun to assume her classic symmetrical shape.
Silence fell for about six hundred years. Then, around 850 AD, she began her Sugar Bowl Eruptive Period and emitted her first lateral blast while building Sugar Bowl Dome. Dacite blocks up to 30 centimeters (nearly 12 inches) in diameter were hurled as far as 4.5 kilometers (2.8 miles). Pyroclastic flows and lahars roared down her flanks. She deposited tephra set D, along with a warning: sometimes, she didn't blow straight up. This brief episode ended within fifty years, but she didn't stay quiet for long.
In 1480, the Kalama Eruptive Period began with explosive eruptions. In 100 years, she deposited tephra sets W and X, built domes, and released andesitic lava flows that all combined to build her to her pre-1980 shape. All she needed was a few finishing touches.
She put those on during the Goat Rocks Eruptive Period, which lasted from 1800 to 1857. Explosive eruptions of pumice, an andesite lava flow, and the emergence of Goat Rocks Dome, accompanied by hot avalanches and lahars, completed her picture-perfect shape. She also gave a traveling artist something to paint home about.
And that was it, as far as hard-working geologists could determine: a history as complete as erosion and repeated explosive activity would allow. Before you left him, Dr. Crandell had impressed upon you the certainty that Mount St. Helens would erupt again. And you knew from 40,000 years of her habits that she liked going off with a bang. So, while you continued your surveys of Earth, you periodically returned to her, just to check. Other volcanoes erupted around the world, but Mount St. Helens slept on. You began to believe she might not erupt before your survey mission was done.
And then, in mid-March, earthquake counts began to rise. At first, it didn't seem like much, just a spike in the background tectonic activity one expects in a subduction zone studded with restless volcanoes. But then came March 20th. At 3:47 pm Pacific Standard Time, the earth rumbled and shook with the force of a magnitude 4.1 earthquake. It was just a smidge to the northwest of the summit. It was very, very shallow. And it wasn't like anything seismologists (or we imaginary extraterrestrials) had ever seen round her before.
Geologists and seismologists looked at that earthquake trace, and knew: this could be it.
Crandell, Dwight R. (1987): Deposits of Pre-1980 Pyroclastic Flows and Lahars from Mount St. Helens Volcano, Washington. USGS Professional Paper 1444.