Of all the volcanoes of the Cascades, Mount Rainier is the tallest. It towers over the surrounding mountains and dominates the horizon even in distant Seattle. But being tall in an oceanic climate can have unwanted consequences. When it rains in Seattle, here it snows, and the snow does not melt easily. Mount Rainier has grown 26 glaciers. There is an unworldly beauty hiding in those glaciers, but there is also danger. To many, the combination is irresistible.
A view of the mountain on the horizon shows how steep and rugged it is. This does not look like a Fuji or a Mount St Helens. It looks eroded, shaped like a block of granite (which it isn’t). But the summit seems out of place. The mountain suddenly comes to a halt, and has a much flatter top. In the middle of the top is another cone, but it looks too small for the size of the mountain. The highest peak is at the summit, and is called Columbia Crest (4,392 meters or 14,411 feet). On either side are two smaller peaks.
On top of the central cone are two craters, proving the volcanic nature of the mountain. In any case, if there was any doubt the sulphur smell at the summit should be conclusive. The volcanic history is the reason for the funny looking summit. Rainier used to be considerably higher, perhaps 500 to 600 meters above the current peak. That summit was lost in an ancient eruption. The flattish summit that now remains is in the floor of the ancient crater that formed at this time. Later, renewed activity build up the current, smaller summit cone.
Mount Rainier is, of course, not the original name for the mountain. In the current climate of giving a bit more recognition to those who were here before, the discussion has started whether to revert to Rainier’s original name. Perhaps one day it will again be called Tahoma.
Mount Rainier has history. It has gone through various stages of growth and decline. A volcano existed here before the current mountain grew, but it died. Rainier started to grow some half a million years ago. At times it grew rapidly, at other times its lava came out along it flanks. For a while Little Tacoma took over as the eruptive centre. 40,000 years ago the summit re-activated. Perhaps something happened in the Cascades, as this is about the same time that Mount St Helens began to grow. The rugged shape of the mountain is not only because of erosion: it is also because the lava flows of the time were stopped by the thick ice-age glaciers which covered the mountain.
There have been several eruptive episodes since the ice age. Mount Rainier is only an occasional eruptor, and is not as active as other Cascades volcanoes. The most recent confirmed eruption was 1500 years ago, a minor one which formed an ash layer near the summit. There were reports of activity in December 1894 but they were unconfirmed and contradicted and should probably be discounted. Significant eruptions have come in phases. Most recently this was in the period 2600-2200 years ago, and before that 5600-4500 years ago and 7400-6700 years ago. In between these phases, the mountain is quiet or it may have an occasional small explosion.
The magma reservoir is located between 5 and 18 km below the summit. The precise source of its magma is not well known: there is no clear conduit below these chambers. There is evidence for upwelling of deep magma some 30 km northwest, which comes from the subduction zone below. However, this is quite far from Mount Rainier and it not clear whether that magma can reach the Rainier reservoir.
Glaciers grow at altitude, above the snow line. But they don’t stay there. As the snow mass grows from the multiple snow falls, the snow turns first to firn under the weight, and then to ice. Now it begins to be slide down the slopes at glacial speed. The slide brings the ice to lower heights where temperatures are above freezing. Here, below the snow line, melting begins. But a bit of melt does not stop the ice. It continuous to slide down, until finally the melting rate equals the rate at which the glacier flows down. Here is where the glacier ends, far below the level of the everlasting snow.
The top level is called the accumulation zone: this is where the glacier grows. The lower levels, where it loses mass, is called the ablation zone. The line that separates the two is the equilibrium line. This line of separation goes up and down with the temperature – during our global warming drastically so. If ever it reaches the summit of the mountain, the glacier will disappear, as happened in Hekla.
That will not happen at Rainier where the summit is far above the line of equilibrium. If the climate gets wetter, more snow accumulates and the glacier can reach further down into the ablation zone – the glacier can grow even when the temperature increases as long as the warmth brings more snow. And snow is not in short supply here: the South Tahoma glacier, for instance, collects 16 meters of snow during an average year, and 25 meters is not uncommon.
The glacier carries more than just ice. It also brings debris and rocks down, some from rockfalls on the top of the glacier and some from the grinding of the rock surface below by the ice. The debris can form lines on the glacier. Further down where the snow melts but the rock does not, the debris forms a cover on top of the ice, and now the glacier turns grey or black. The larger rocks can be deposited at the termination point of the glacier, and form a moraine – a wall of rock. Old moraines show how far the glacier reached in the past. Inside the glacier, the finely ground rock becomes compressed, and where the glacier melts it leaves behind a glacial till, an unsorted mix of clay, rocks and even boulders.
The melting ice feeds rivers, full of fine sediment ground down by the glacier. It turns the water milky white. Where the ground flattens the sediment drops out, and forms a gravel outwash plain. During the ice age, these plains were massive. Much of the soils around the Puget Sound are glacial outwash from the extensive ice-age glaciers centred on Mount Rainier. Today, Rainier’s 26 glaciers continue to feed 5 major rivers. Where a lake forms along the rivers, even the finer sediment drops out: this is called lacustrine soil, with more silt and clay, sometimes with an impermeable clay layer – our own garden (in a different part of the world) has been dug out from such a layer: gardening on such soils can be a challenge!
Ice on fire
Adding a volcano to such a frozen region may not be a good idea. An eruption causes a sudden heat pulse which melts ice well above the equilibrium line. The water cascades down the steep upper slopes while picking up clay particles. Once these particles account for a few per cent of the flow, the flood changes to a mudflow, or lahar, which beside mud can carry rocks and boulders. They are the among the most dangerous events on volcanoes. The volume for Rainier’s lahars range from 0.1 km3 to 4 km3. The countryside around Rainier that has been reached by past mudflows include places that are now densely populated. Every river valley around Rainier has had them, and 80,000 people live within reach of its lahars.
Lahars can start in two ways. The first is as described above, where an eruption melts the ice and releases massive amounts of water. The second is through a landslide near the top of the mountain. For Mount Rainier, the first type can cause lahars in any direction, but the second is more likely on the west side of the mountain where the steep upper regions of the mountains contain a lot of clay. There is in fact a third way, where a glacier collapses and a mass of broken ice slides down the valley at speeds of 200 km/hr. This has happened elsewhere (Kazbek) but is not known from Rainier.
Lahars are regular on Mount Rainier. Although no significant event has occurred in living memory, lahars at Mount Rainier come without warning, and can happen at any time. Most lahars are small and only affect regions near the mountain. But once or twice per millennium Rainier can generate a lahar which reaches the lowland near the Puget Sound. This is the most dangerous aspect of living within sight of Mount Rainier. Fire is dangerous, and so is ice. But when ice is on fire, the combination can be lethal.
Lahars are not the only type of flood caused by ice melt. When melted ice suddenly escapes from a glacier, it forms a jokulhaup. These are frequent on the South Tahoma glacier, on the southwest side of the mountain. They tend to happen in the summer after periods of very hot weather but little rain, and they can be damaging to roads and trails inide the park.
Lahars of history
The most recent major lahar at Rainier was the so-called Electron mudflow. It happened 500 years ago and reached Sumner, 100 km downstream. At the community of Electron, the deposits are 30 meters deep. 1100 years ago a lahar reached Auburn. There is no evidence that either event was caused by an eruption. Whether the trigger was a major earthquake, a very minor eruption, or a hummingbird landing on the ice is not known – it just happened.
But in other cases there is a clear link between lahars and eruptions. All eruptive periods of Mount Rainier during the holocene have coincided with major lahars. But the link is not as straightforward as it seems. Rather than eruptions causing lahars, it seems the effect may (rather unexpectedly) have gone the other way.
These large lahars are linked to collapse events. The worst event of the holocene happened around 5600 years ago when the northeastern side of Mount Rainier collapsed. It left a crater almost 2 kilometer across, open to the northeast, and caused a lahar which is now known as the Osceola mudflow. It reached the Puget Sound near Auburn, and covered areas of Tacoma and Seattle. Sumner and Auburn are both build on top of the Osceola deposits. The process repeated itself 3000 years later, when the west side of the Osceola crater collapsed sending a lahar 30 km down the Puyallup and Nisqually rivers. This one is known as the Round Pass mudflow.
There is a pattern here. For both lahars were at the start of an eruptive period. After the collapse, there were a series of eruptions over a period of several hundred years, some explosive, some with lava. After the Round Pass mudflow, the focus of the eruptions was the east summit crater. Near the end of that period there was another major lahar, called the National lahar, which came down the Nisqualla river reaching as far as 100 km. This event coincides with a layer of tephra which indicates it happened during an explosive eruption. Since that time Mount Rainier has been mostly asleep, although there was a minor explosive eruption 1500 years ago which left some ash layers high up the mountain.
This suggests that the two major collapses may have caused the eruptive period that followed. The removal of so much material opened up pathways for the magma. The reduction in weight may also have helped new magma to reach the magma chambers. Over a few hundred years the summit rebuild itself, until the conduits closed under the weight. In this model, the lahars were the cause, not the consequence, of the eruptions. It may also explain why Rainier isn’t more active. This is a marginal volcano. The magma is unable to reach the summit: the magma pressure is insufficient to overcome the weight of the mountain. Only when something destabilizes the mountain does the magma manage to move up. Once stability has been regained, the pathway shuts. This could be the reason for the funny summit: the summit cone is a bit small compared to what it should be because this is all the magma is currently capable of. If Rainier had been smaller, perhaps it would have been more active. History can be a weight.
Based on this, we can make tentative predictions. Smaller lahars can come at any time. There will be little or no warning, other than the systems that the USGS has in place to detect the mudflow coming down the rivers. If you live along one of these rivers, be familiar with the warning systems (sirens and automated messages), have your escape route ready and make sure you flee upward, away from the valley. (Be aware that the flow can come a long way up the sides of the valley, so get away from its direction of travel.) Large lahars happen much less frequently, but that doesn’t mean it couldn’t happen. Such an event may give more warning if any creep along the flanks of the mountains can be detected in time. Most spontaneous major landslides (e.g. not caused by earthquakes or eruptions) happen after a prolonged period of creep, and with INSAR (such as on the European Sentinel satellites) we should now be able to see this.
A large lahar might signal the start of a new eruptive period. It hasn’t happened for 2600 years, and over the holocene eruptive periods have happened roughly every 3000 years so we may have another one in the next several centuries. However, volcanoes are not easily predictable and for all we know Rainier may sleep in for another 3000 years. Or perhaps it will wake up just after Covid, to add a disaster to a crisis.
If Mount Rainier does wake up, it may start with a flank collapse and lahar followed by one or more eruptions. Over a period of a few hundred years there are will be several summit eruptions, both effusive and explosive, perhaps decades apart. The explosions will be limited to VEI 3, perhaps with a marginal VEI 4 (Rainier hasn’t produced a VEI-5 for almost 100,000 years; the largest explosion in the holocene had a tephra volume of 0.11 km3). The effusive eruptions will produce andesite lava. Eventually calm returns, until the next collapse, perhaps a few thousand years later.
Mount Rainier is particularly susceptible to collapse because of the steep sides and weak surface and clay at high altitude. The map shows the areas at risk. USGS is upgrading the warning systems along the rivers. Let’s hope it will never be needed.
Rainier is a dangerous mountain. But ice on fire also creates beauty. To see this, one needs to scale the mountain, past the dirty-black lower reaches of the glacier, up the steep sides, across the summit plain and up the summit cone. It is a challenging mountain climb, made more difficult by the steep and slippery glaciers. It takes several days, and requires skill, experience, a permit, and luck with the weather. At the top, exhausted and sick with altitude, you will find the crater – and its secrets.
The summit cone is 300 m high above the surroundings, and is 2 km wide. The cone itself has a flat top, with two overlapping craters, each about 300 m diameter.
The two craters are filled with ice, to such a degree that it can be hard to see them on satellite images. Radar can help and it shows the circular depression in the ice caused by one of the craters. The geothermal heat does funny things to the ice. During summer, the heat from below keeps the rim of the summit crater free of snow. The heat of the fumaroles is brought up by water circulating through the rock: this is a hydrothermal area. There are also fumaroles on the western flank of the summit cone which keep an additional area free of ice.
The heat has melted an extensive system of ice caves along the bottom of the craters. At the deepest points of both the west and the east crater lies a small crater lake, below 30 meters of ice making them invisible to all but the most intrepid ice explorer. The west crater lake was identified in the 1970’s but it is now reported to have disappeared. Only the east lake remains. In spite of the inaccessibility and invisibility, it has been given a name: Lake Adelie. This is reportedly because the blue water reminded the discoverer of the blue eyes of the Adelie penguin. However, this runs into a fact check as Adelie penguins have dark brown irises. The only penguin with a blueish eye colour is the Little (or Fairy) Penguin, so perhaps this should have been called Fairy Lake. The lake measures some 30 by 10 meters.
Exploring the ice caves is dangerous. The 3 kilometres of caves are melted by the fumaroles, and the fumarole gasses collect in the caves. That includes CO2, which is heavier than normal air and sinks to the bottom. At the deepest points of the ice caves are deadly pockets of CO2: an invisible and silent killer. And if you escape this, the stifling smell of sulphur is never far away.
But they are also beautiful. The light that comes through the ice is blue. The roofs of the ice caves are mottled, shaped by the melt. The combination seems like being inside a piece of modern art. A dangerous one.
There is beauty on the surface too. On the slopes of the craters are dense packs of white pinnacles, up to a meter tall and made of snow. It creates an unreal landscape, seemingly alive and growing. The pinnacles are called penitentes. Mount Rainier is not the only place where they are found. Penitentes form on high altitude glaciers, in very dry air and especially in the tropics. In the Andes they can grow to 4 meters height. Typically, they have the shape of bowl-shaped depressions with edges that rise up as spires. Often they occur in lines. The name comes from the appearance of a crowd or line of kneeling people.
How can ordinary snow make such an extraordinary landscape? In the cold, dry air snow doesn’t melt – it sublimates under the rays of the sun. The snow changes directly to vapour without forming melt water, and the vapour is blown away. Once a part begins to sublimate, a small depression forms. The sides of the depression reflect the light down, where it becomes trapped. This makes the sublimation go faster there, and the depression grows more pronounced. It happens all over the snow pack. Soon the depressions become gaps and the original pack forms a forest of remaining spikes. It is an unusual type of erosion by the sun. If you want to speed it up, spread some soot over the snow. The reflected illumination can be very effective: one measurement found that while the temperature at top of a penitente was -5C, in the hollow it was +10C.
The underworld above
And this brings us from Mount Rainier to a place that is far more inhospitable and difficult to reach. Just like Rainier gets its energy from the subterranean magma chambers, so the penitentes also have their link to an underworld. Pluto, to be precise.
We have all seen the images that New Horizons send back from Pluto, now 5 years ago. There was the heart-shaped nitrogen glacier, the icebergs, the snow drifts, the ice volcano. Among the variety, one type of surface was not understood at the time. It was called the ‘bladed terrain’ and was found in a band along the (for want of a better word) ‘tropics’, at higher altitude plateaus which lie 3 km or more above (for want of a better word) ‘sea level’. The plateaus show dense fields of parallel sets of steep ridges with sharp crests. The blades are spaced 4 to 5 km crest-to-crest, and the crests reach up to 500 meters above the base.
The explanation for these blades became clear when people realized that they looked like extreme penitentes. There are times that Pluto has a ‘summer’ (again for want of a better word) with relatively high air pressure. Pluto’s orbit varies on a time scale of millions of years, and this ‘summer’ can happen during part of that cycle. At this time, the polar caps evaporate, the atmosphere thickens and methane snow falls at altitude around the equator. The snow turns to ice. Later the climate worsens and the atmosphere collapses in a Pluto-wide freeze-out. In the dry climate the tropical methane snow begins to sublimate, and the process described above kicks in. The result is the largest penitentes known in the solar system, formed over millions of years.
An interesting complication is that Pluto has both methane and nitrogen snow, but models show that only methane can form penitentes. Indeed, the bladed terrain is only seen in methane areas. The current growth rates are expected to be around 1 cm per year, but would have been much slower initially. The formation may have taken some tens of millions of years. Some things are worth waiting for.
Living near Mount Rainier means living with dangerous beauty. One day, the mountain will show its temper and lahars will suddenly come down the rivers. Climbing the mountain shows another world, equally dangerous but also with hidden wonders. And in the end, the summit’s ice on fire is a pointer to a world much further away. What a mountain.
Albert, July 2020