If you take a walk sometime above Timberline Lodge, up towards the Palmer snowfield, and look around you are likely to come across some of these strange fractured rocks shown in the above photo. These are some of my favourite geological features at Mount Hood, and they tell an important part of the story of the Timberline eruption – the second most recent Mount Hood eruption, which occurred about 1500 AD.

The Timberline eruption started with the formation of a lava dome at Mount Hood’s summit – near the present location of Crater Rock. However the really notable thing about that eruption is that at some point a very big collapse event also occurred – basically the southern third or so of the summit crater fell off and down the mountain. This event completely changed the southern side of volcano by producing the “smooth” surface (a surface of minimum slope) that Timberline lodge sits on today.

Note that these sort of sector collapse type events are not that rare at big volcanoes (witness Mount St Helens in 1980, and a really big one happened at Mount Shasta about 300,000 years ago or so). Sometimes collapse events occur without an eruption – as the rocks of volcanoes are weakened by alteration and can’t hold the mass of edifice up. This sort of thing has happened repeatedly at Mount Rainier over the last 10,00 years, for example.

However at Mount Hood we know that the Timberline collapse DID occur during an eruption – and one important evidence are the blocks with the odd wiggly fractures in them. The fractures in these rocks, known as polygonal fractures, are the type of fracture that occurs when a rock is really hot and then cools quickly. The fractures also form after the rock has come to rest in its current position – because the rock is so broken up it would fall completely apart if it moved any more after cooling. The way that the polygonally jointed blocks form is to be part of the hot lava dome at the summit that periodically collapses down the mountain. The superheated blocks tumble down the mountain and once they come to rest they cool quickly and develop the polygonal fractures. Some of these are really quite large – Timberline Lodge would have been an interesting place to be 1500 years ago.