The Absaroka Volcanics are the eruptive products of a large Eocene volcanic field in Montana and Wyoming. The province has ~13 major eruptive centers, each of which includes lava flows, pyroclastic deposits, and dike swarms. The volcanics on Mt. Washburn are calc-alkaline and bimodal, with basaltic andesite predominating. Dacite is more common in the lower parts of the section. True basalts and rhyolites are not found on Mt. Washburn. Of lavas studied to the date of this trip, there is no evidence of disequilibrium among the phenocryst populations. This suggests that basalt-crustal melt magma mixing was not an important process in the generation of these volcanics, or that the magma had a long enough residence time to destroy evidence of disequilibrium. Some other Absaroka volcanics do, however, have such disequilibrium phenocryst populations. Later, the Absaroka Volcanics were here covered by Pleistocene pyroclastics from the Yellowstone calderas. These pyroclastics have been eroded away from the Mt. Washburn area, thus exposing the older, underlying volcanics. The remaining Mt. Washburn volcanics are ~1300 m thick. The southeastern flank of the Mt. Washburn volcano is missing, having been truncated by one of the Yellowstone caldara ring fractures. Presumably the missing rocks are still present, somewhere deep inside the caldara.


Topographic map of the trail we took up Mt. Washburn. NOTE: Remember that this is a national park. No sample collecting without written permission from the NPS!


1. Layered pyroclastics


Outcrop of two-pyroxene pyroclastics. Lower part of the outcrop is better consolidated and less vesicular than the upper part. The contact exposed on the outcrop is visible texturally, and the change in slope on the outcrop surface was caused by faster weathering of the upper layer.


The upper layer in the outcrop above. Darker blocks are dense rock with few or no vesicles. Pyroxenes are visible as black specks. Lighter blocks are altered vesicular fragments. The matrix material is fine-grained ash, in some cases layered.


Closeup of some of the altered matrix material (light-colored, top) and a layered block made of denser, dark-gray and browner, more vesicular layers.


Dense gray rock with black pyroxene crystals and some vesicles filled with white and tan material.


2. Dike and lava flow in pyroclastics


Andesite dike crosscutting pyroclastic rocks. Columnar jointing in the dike is poorly developed, but perpendicular to the dike contacts. Dikes like this are more or less radial from the presumed volcanic center, represented by a some small tonalite bodies just to the southeast of Mt. Washburn.


Pyroclastic rocks cut by the dike in the image above.


Lava flow (or possibly a sill) in the pyroclastics shown in the two images above. The flow has sharp and rather irregular contacts top and bottom, and doubtless represents the dense rock of a flow core, with its own aa rubble above and overridden rubble below. The flow seems to pinch out just short of the dike.


3. Lava flow and ramp structure


Exceptionally colorful layer of pyroclastic debris, or possibly a mudflow deposit.


This outcrop is interpreted to be a cross section of a basaltic andesite lava. The dense rock at the bottom is the lava flow core that solidified after movement ceased. It is overlain by its rubbly aa lava top.


A view of the same lava from higher up. The red material is the rubbly aa flow top, which is partially overridden by the dense, gray core rock at the flow front, in the center-right of this image. This is known as a ramp structure. Click to see the outline of the flow core.


Close-up view of the lava flow, showing the dense flow interior rock in the lower half of the image, and the rubbly aa flow top material in the image top half.


This may be the same flow as seen in the three images above, farther up the trail. Irregular layering like this was said to be typical of "spatter-fed flows". These are flows fed in large part by air fall of molten lava spatter, as from a spatter cone.


Scenic overlook and another lava flow and ramp structure


Looking south along the extended ridge of Mt. Washburn from a scenic switchback in the trail. On the near hill some of the beds can be seen with their rather steep northward (right) dips.


View to the southeast from Mt. Washburn. The Grand Canyon of the Yellowstone River is in the middle ground. The white patches in the near ground are hydrothermally altered rock in the Washburn hydrothermal field. The Sulphur Creek tonalite stocks, thought to mark volcanic feeders for some of the local Absaroka volcanics, are near, but much older than and unrelated to this hydrothermal field.


View to the east, also showing the Washburn hydrothermal field and the Grand Canyon of the Yellowstone River.


A flow front behind the scenic overlook, which again consists of a dense lava flow core overlain by and overriding its own rubbly aa flow rubble. Click here to see the dense flow core outlined.


The rubbly material at the front to the lava flow in the image above.


5. Radially fractured block


Radially fractured lava block. This was interpreted as evidence that this pyroclastic block was hot at the time of deposition. The fractures formed during cooling of the block.


6. Fragmented lava and clastic dike


Layered, light gray, apparently water-lain volcanoclastic sediments that overlie dark-red pyroclastics. Above the water-lain sediments is an odd mix of volcanoclastic sand, in part layered, and fragmented lava. The suggestion is that this represents a lava flow that flowed into a stream flanking the volcano, causing boiling, lava fragmentation, and mixing of some of the lava and unconsolidated sediment. A clastic dike can be seen just to the right of center, apparently extending downward from the mixture above into the pyroclastics below the light-gray unit.


Closer view of the light-gray volcanoclastic deposit, composed of sand and pebbles. At the top left is the lowest part of the sand-fragmented lava mixture, and in the center and extending to the right and downward is the clastic dike. There was some controversy as to whether or not the dike extends from the top downward or the reverse. I think it extends from the top downward, because the dike material, with its ~50% sand component, looks more like the mixed material above than the pyroclastic layer below.


Closeup view of the fragmented lava surrounded by layered sandy sediment. Such thin layering and clean sandy sediment would not be expected in a mudflow.


7. Scenic overlook near the summit


Northwest (left)-dipping flows on the east side of Mt. Washburn. The pyroclastics on the top left of this image are at a crosscutting relationship to the pyroclastics and lavas in the center and right. The contact may be an erosional channel or fault. The trip leader suggested that it was a landslide scarp, later covered by eruptives.


North flank of Mt. Washburn. The deepest notch marks the path of a wide composite dike that crosscuts the pyroclastics and lavas. A few tiny bighorn sheep can be seen near the center of the image.


8. Mt. Washburn summit


Bighorn sheep near the summit of Mt. Washburn. The valley below is to the west of the mountain, and the road to the right is on the northern side of the mountain, whereas we hiked up the southwestern side.


View from the summit of Mt. Washburn toward the southeast, into the broad, flat center of the Yellowstone caldara complex. The Grand Canyon of the Yellowstone can be seen cutting across the middle ground.


View to the northwest of Mt. Washburn from the summit. The high peak in the middle horizon, is Electric Peak, another of the Absaroka volcanic centers. Electric Peak is barely within the northern boundary of Yellowstone Park.