[a]Clear to light-colored silicon dioxide.
[]Light-colored aluminum silicate minerals.
[c]Dark-colored iron and magnesium silicate minerals.
[d]Very dark colored iron oxide mineral.
The magmas which formed the Absaroka volcanoes erupted mainly through large central vents ([fig. 16]). Most of the eruptions were fairly quiet, with the molten rock welling up to the surface and cascading down the sides of the volcanoes chiefly as viscous lava flows and breccias. Rain, seeping into these porous rocks, caused huge landslides of mud and broken rock to stream down the mountainsides. Hence, many of the rocks seen today are volcanic breccias—jumbled but crudely layered deposits of large and small angular blocks embedded in a sandy matrix, much like man-made concrete except that the rock fragments are considerably coarser ([fig. 17]). Viewed from a distance, however, most of the breccia deposits have a distinct layered appearance ([fig. 18]). The predominant extrusive igneous rock in the Absaroka volcanic sequence is andesite, but basalt also occurs in places ([fig. 15]).
ABSAROKA VOLCANOES and their rocks. Lava (mostly andesite) poured from central vents and formed volcanoes, some steep sided and others broad and relatively flat. As the lava spilled out, much of it quickly solidified, broke up into large angular blocks (breccia), and then either tumbled down the slopes of the volcanoes as individual boulders or slid down in mudflows and landslides. Some of the material was also explosively blown out as rock bombs, cinders, and ash. The more fluid lava (mostly basalt), on the other hand, flowed quietly down the volcanic slopes and onto the surrounding lowlands. The rocks near the volcanic centers therefore include thick crudely layered coarse breccias, thin fine ash and dust falls, and thin to thick lava flows. The volcanoes were repeatedly attacked by erosion, and the eroded material was redeposited by streams and mudflows in widespread layers of volcanic conglomerate and sandstone across the flat-floored valleys and plains between the volcanoes. Forests, which grew luxuriantly in these lowland areas, were repeatedly buried by volcanic eruptions and are now preserved (see inset) as the fossil forests of Yellowstone. (Based on information supplied by H. W. Smedes and H. J. Prostka.) (Fig. 16)
Chiefly lava flows of shield volcano
Chiefly volcanic sandstone and conglomerate of lowland areas
Chiefly volcanic breccias and thin lava flows of cone-type volcano
At times the Absaroka volcanic eruptions were violently explosive, showering the countryside with rock bombs, cinders, and ash. The finer debris that reached the lower slopes of the volcanoes was reworked and carried by streams into the intervening valleys, where it was deposited as sand and gravel ([fig. 16]). Eventually the entire Yellowstone region was choked with volcanic debris, the material from one volcano mixing with that from neighboring volcanoes. Even the mountain masses uplifted during the preceding Laramide orogeny were covered by the vast accumulation ([fig. 18]).
