Belknap and Little Belknap Crater

For a Google Earth .kmz file to explore Belknap and Little Belknap Crater, scroll to the very bottom of the page.

One of the most interesting drives over the Cascades is through McKenzie Pass on Oregon Route 242. Most people don’t necessarily head that way to get to Sisters or Bend because the road closes early in the winter, it’s extremely narrow, and it is very slow going in terms of traffic. However, once you get to the crest you are in for a pretty amazing sight. The top of McKenzie Pass is an area extremely dense with volcanic vents and thick lava flows covering the ground surface (Figure 1). These particular lava flows are very young in age and, as a result, there is very little vegetation on the surface. These particular basalt flows are actually situated on top of the Sand Mountain volcanic field, which is a volcanic field that is composed of 22 cinder cones and 41 separate vents (Orr and Orr, 1999). Even though there are so many vents in the Sand Mountain volcanic field the Belknap lava flows are much more substantial than the Sand Mountain volcanics. The Belknap lava flows began flowing about 3,000 years ago from Belknap Crater as well as the other surrounding vents (Taylor, 1987). Belknap is a shield volcano, meaning it is broadly shaped, with shallow flanks and is formed mostly from basalt (Figure 2). Belknap is much smaller than other well known shields, like the Newberry Volcano outside of Bend, Oregon. Its main shield is 5mi in diameter approximately 35 square miles in size, 1700ft at its thickest, and 1.3mi^3 in volume (Taylor, 1987). These flows happened many times and covered an area of over 40 square miles (Orr and Orr, 1999). There are many vents that cover the surfaces of this shield but the most recent eruptions that occurred from the area were erupted from Belknap Crater, Little Belknap Crater and South Belknap Cone.

The first phase of volcanism came from Belknap Crater and started about 3,000 years ago. This phase consisted of mainly tephra that spread both to the northeast and the southeast as well as creating a basalt flow that travelled east over a 10 mi (6 km) area (http://volcanoes.usgs.gov/volcanoes/belknap/). The second phase of volcanism occurred around 2900 year ago and created the smaller shield known as Little Belknap Cater (http://volcanoes.usgs.gov/volcanoes/belknap/, Figure 3). The third phase of volcanism occurred 1500 years ago from two different cones and erupted the majority of the basaltic andesite flows present. The two cones involved in these eruptions were the central cone, Belknap Crater, and a cone about 1 mile to the south, creatively called South Belknap Crater (http://volcanoes.usgs.gov/volcanoes/belknap/). The final flows came from the base of Belknap Crater and flowed 15 miles west to the McKenzie River (http://volcanoes.usgs.gov/volcanoes/belknap/).

McKenzie Pass is a great place to see many different basalt flow features in a very small area. There are essentially two types of basaltic lava flows, a’a and pahoehoe, and their basic look and related features are due to their temperature. The temperature of a lava flow will many times determine the lava flows viscosity. Viscosity is defined as a fluid’s resistance to flow, meaning the more viscous something is, the less likely it is to easily flow. A good way to think about viscosity and temperature is with honey; heat up honey and it flows very easily and put that honey in the fridge and it barely flows. Pahoehoe is hotter and therefore flows more easily and smoothly. As pahoehoe flows the surface of the flow cools to a solid and rolls up on itself creating a ropy texture. A’a is cooler in temperature and therefore does not flow as easily as pahoehoe. A’a is the type of basalt flow that is found on McKenzie Pass. It’s thicker and doesn’t really flow like a liquid but more like a chunky solid. A’a flows more like a tank wheel, with the top of the flow falling over the front. Its also more jagged and crumbly and is usually fairly thick. As you go over McKenzie Pass the highway follows the edges the flows allowing you to see the thickness of the flow front.

These a’a flows of McKenzie Pass also show many different types of flow features. The flows are so young in age that their surfaces are very well preserved. One of the main features of lava flows you can see immediately are to the west of the observatory are these two islands of land surrounded by channels of lava flows. The name for an island surrounded by lava is a kipuka (Figure 4). After walking along the kipukas towards Belknap Crater you come across several lava channels through which lava once flowed. Along the edges of these channels lava has spilled over the sides creating lava levees. There are also several pressure ridges to be seen in the lava flows. Pressure ridges almost look like the the bulge of a muffin as it bakes in the oven. The lava domes upwards and many times cracks a bit on the surface.

Pressure ridges are thought to be formed due to the upward pressure of liquid lava or perhaps trapped gases that are expanding upwards below the solidified lava surface (Cas and Wright, 1987, Figure 5). Another noticeable feature found while walking in the lava flows of McKenzie Pass are a multitude lava tubes in different sizes and states of preservation. Lava tubes form when lava flows solidify on the top, bottom, and sides insulating the inner, still liquid lava. This insulation prevents the flow from cooling allowing lava to travel further. These tubes are well preserved in some places but many of the lava tubes have collapsed leaving skylights and allowing hikers to see into old channels of the lava flows (Figure 6).