BNSF 7403 West (an ES44DC) Hauls Autoracks Through Crozier Canyon Beneath the Prominent Lava Cap on the South Rim
The lava cap consists of an upper layer of rhyolite and a lower layer of basalt. Rhyolite is an igneous rock with a high silica content, usually formed in continental volcanic eruptions when granitic magma reaches the surface. Basalt is a dark, fine-grained igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as a lava flow and underlies more of earth's surface than any other rock.
The lava cap above Crozier Canyon was likely poured out in fairly recent geological times -- the late Cenozoic; i.e., within the past 15 millions years. The presence of Rhyolite on top of Basalt indicates at least two separate volcanic events. Basalt is created from non-explosive eruptions flowing like hot tar across the landscape. For example, during the past two millions years, lava with that consistency flowed into the western portion of the Grand Canyon near Vulcan's Throne at least a dozen times, temporarily damming the Colorado River. Thus, the initial lava flows across what is now Crozier Canyon created large pools of hot, sticky material that cooled into basalt.
Probably a million or two years later, a second series of eruptions occurred. These were explosive, much like the eruption in 1980 at Mount St. Helens in Washington. The lava accompanying these eruptions was rich in silica and therefore less viscous (less fluid) that the basaltic flows. Most of this lava solidified as Rhyolite. Moreover, because they did not flow easily, if at all, the Rhyolite deposits lie today where they fell years ago, unlike the basalt layers, which could have flowed hundreds of yards or even a mile or more.
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