The Pacific Northwest NAGT meeting was supposed to have 4 days of field trips, and one day (Saturday) of meetings. This years meeting only had about a 1/2 day of meetings so Saturday afternoon we headed out to Diamond Craters,, BLM designated "Outstanding Natural Area". Diamond craters began erupting about 20 ka and according to the the Global Volcanism Program website was last active about 7 ka. But according to our guide, there may be indications of eruptive activity in archeological sites that are less than 500 years old.
Unfortunately time was limited so we were only able to make three stops in the area, but each stop represented a different kind of volcanism so it was a great overview of the eruptive history: The first stop was a pair of Maars at the western end of the road. The first was smaller and dry, but the second (Malheur Maar) was deep with an active mosquito breeding farm at the bottom.
Maars/craters are generally caused by the ejection of hot steam and other volcanic gasses. The pressure of these gasses blow a hole in the surface that becomes the crater, but the eruption containes little to know magmatic material. So at our second stop we saw an eruptive area that showed indications of fountaines of eruptive matieral. Red Crater was a lapilli crater built up to about a hundred feet on the eastern side. The rim was comprised of lapilli sized tephra that had fused while still molten. Scattered around the site were a number of "Breadcrust Bombs". Volcanic Bombs that were beginning to cool and harden on the outside while still molten and outgassing on the inside. The release of the internal gasses cause the bombs to continue to expand, stretching the outercrust creating cracks like a crust of baked bread. This bomb was found about 50 feet from the crater rim.
Our last stop was Lava Pit Craters. Actually three craters formed by drained Lava lakes. Eminating from each lake are a sereies of tubes and caves of highly basaltic lava.
One of the pits has a large cave/tube with lavacicles hanging down and a hornito on top.
Unfortunately because our time was limited, we did not make the hike into the central vent area with more than 30 vents and some "dribblet spires" where smalll amounts of molten lava is forced to the surface with just enough energy to run down the sides as it cools. Sort of like the magma version of a drip sand castle. Oh well, it gives me a reason to make the drive back.