What might it look like from Portland if Mount Hood erupts?

A recent article in the Oregonian highlighted a youtube video showing what it might look like from Portland if Mount Hood erupted. The video shows a growing ash cloud above the mountain, as viewed from the perspective of a busy Portland street.

From what we know of past activity at Mount Hood, what did the video get right? Well – for a start the ash cloud in the video comes from near the summit, where the most recent eruptions of Mount Hood have occurred – both the Old Maid and Timberline eruptions (occurring ~220 and ~1500 years ago) were at Crater Rock, on the south side of the current summit.

The ash cloud also seems to mostly show convective transport of ash – driven by hot gases that are less dense and thus buoyant. We don’t see large ash clouds driven upward by being expelled at high velocity from a narrow magma conduit – such as would occur in a  big Plinian eruption when there is an open pathway between a pressurized magma reservoir and the surface (think of a firehose). Plinian eruptions are named after Pliny the Younger who documented the eruption of Vesuvius is 79 AD that buried Pompeii. The ash clouds that such blasts produce are much larger and can reach up into the stratosphere. Take the example below from Mount Pinatubo in 1991.

Mount Pinatubo, Philippines, June 1991 (USGS photo)

For Mount Hood there is no geological evidence of this type of large eruption that has occurred in the past. Eruptions at Mount Hood are smaller and typically involve growth of lava domes or lava flows. Explosions can still occur – via interaction between magma and water, or some sudden release of pressure within the lava dome. These smaller eruptions might produce an ash cloud similar to that in the fictional video – similar to this one from Sakurajima volcano in Japan from 2014.

The lava dome can also collapse and fall down the side of the volcano – and this could also produce pyroclastic flows (literally gravity driven flows of “fire particles”). In fact one nice addition to the video would be to add a source of ash moving down the side of the mountain to represent a pyroclastic flow – like seen in this video from Batu Tara volcano (starting about 1 minute in).

One thing that is definitely wrong – the loud sound of an explosion is heard about 30 seconds after the first ash is visible. Volcanic eruptions can produced very loud sounds, but as sound travels about 330 meters per second at sea level 30 seconds is not enough for the bang to be heard in Portland, that would take several minutes ( almost 4 by my calculations).