Description
Cinder Cone is a 700 ft (213 m) high cone of loose scoria. Scoria forms when blobs of gas-charged lava are thrown into the air during an eruption and cool in flight, falling as dark volcanic rock containing cavities created by trapped gas bubbles. The summit of Cinder Cone has a crater with a double rim (photo), probably created by fluctuating eruptive activity late in its formation. The cone also has several associated blocky lava flows, and a related, widespread ash deposit identifiable for 8 to 10 miles (13 to 16 km) from the cone. There are actually two scoria cones at Cinder Cone—the remnants of a nearly completely buried earlier cone can be seen on the larger cone's south side. Much of the earlier cone was probably destroyed by lava flows erupting from its base. Blocks of red, cemented scoria within the Painted Dunes lava flows (photo) are pieces of this earlier cone, which were carried away by the flowing lava.
During its formation, the composition of the molten rock (magma) feeding the eruption changed from basaltic andesite to andesite and then back to basaltic andesite, but with a higher titanium content. Chemically, basaltic andesites are volcanic rocks containing 53 to 57% silica (SiO2), and andesites are those containing 57 to 63% silica. Though chemically distinct, the lava flows and scorias at Cinder Cone are similar in appearance. They are dark, fine-grained rocks, containing a few visible crystals of the minerals olivine, plagioclase, and quartz.
The earlier erupted group of volcanic deposits at Cinder Cone, which are relatively titanium-poor, consist of the older scoria cone, the Old Bench flow, the two Painted Dunes flows, and the lower part of the widespread ash layer. The later erupted, relatively titanium-rich group consists of the large, younger scoria cone, the upper part of the ash layer, and the two Fantastic Lava Beds flows. Flow 2 of the Fantastic Lava Beds (see geologic map) is the flow that everyone but Diller thought had been erupted in 1851. The ash on the Old Bench and Painted Dunes lava flows is brightly oxidized because it fell on the lava flows when they were still hot. This oxidized ash belongs to the same compositional group as the Fantastic Lava Beds flows, the last flows erupted at Cinder Cone. All the volcanic materials at Cinder Cone therefore represent a single eruptive sequence that probably spanned no more than a few months.
An unusual characteristic of the Fantastic Lava Beds is the presence of anomalous quartz crystal xenocrysts (foreign bodies in igneous rock). Geologists think that they were picked up from wall rocks by the lava as it moved toward the surface. The lava and pyroclastics of Cinder Cone and the Fantastic Lava Beds are made of basaltic andesite to andesite.
Read more about this topic: Cinder Cone And The Fantastic Lava Beds
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