Frequently Asked Questions

What type of volcano is Mount Hood?

Mount Hood is a stratovolcano, also known as a composite volcano. This type of volcano, is typically associated with more viscous (less runny) lavas like andesite and dacite. Stratovolcanoes have relatively steep sides and are made of different layers (strata) of lava, ash, tephra, pumice as well as rocks and debris from landslides and collapses.


When were the most recent eruptions of Mount Hood?

The most recent set of eruptions of Mount Hood occurred in the 1780’s (best estimate is around 1782 from tree ring evidence, although the eruption could have continued for about a decade). This phase is known as the Old Maid eruptive phase. Lava was extruded at Crater Rock to form a lava dome. This periodically collapsed to produce incandescent pyroclastic flows that moved down the White River, Sandy and Zig Zag drainages. These eruptions also initiated lahars (mud flows) that travelled as far as the Deschutes and Columbia Rivers.  Lewis and Clark observed some of the after effects of this eruption, noting the presence of a large sediment delta where the Sandy River meets the Columbia (they named the river the “Quicksand River” – which gives you and idea of what it was like).

Prior to this a large eruptive sequence occurred about 1500 AD – the Timberline eruption. This was associated with a lava dome at Crater Rock, and also with a very large collapse of part of the summit, which debris flows and lahars, and also formed the the relatively smooth “Timberline surface” that Timberline Lodge now sits on.

Deposits made by pyroclastic flows White River valley from the Old Maid eruptions of Mount Hood. At just over two centuries in age these are the youngest volcanic rocks in Oregon.

When will Mount Hood erupt next?

We cannot predict exactly when Mount Hood will erupt again, although the fact that it has erupted within the last few centuries means that it is possible it will erupt again in our lifetimes. The USGS puts the probability of eruptions in the next 30-50 years between 1 in 15 to  in 30.


What would a future eruption look like?

If we assume that the next eruptions will be similar to those that have occurred in the recent geological past then we can get some idea of what future activity will look like. In this case the eruptions will probably involve formation of lava domes or lava flows, together with ash, pyroclastic flows and other tephra produced by collapse events. Eruptions are also likely to trigger debris flows and lahars in nearby rivers. See the USGS Mount Hood pages for more information.

The USGS hazard map for Mount Hood area, showing regions near the volcano that are subject to  proximal hazards such as pyroclastic flows, as well as those areas subject to distal hazards such as lahars. Times along rivers are the estimated travel times for lahars.



Why is Mount Hood where it is?

Mount Hood, and the rest of the Cascade volcanoes overly the Cascadia subduction zone – a region where two of the Earth’s tectonic plates are colliding. The plate to the west – the Juan de Fuca plate of oceanic crust – is being pushed under the North American continental plate in a process known as subduction. As it descends the Juan de Fuca plate heats up and water and other fluids are driven off, causing the overlying parts of the Earth’s mantle to melt. Some magmas produced by melting eventually migrate to the Earth’s surface, forming volcanoes.