BH250-243c
Title
BH250-243c
Subject
Ignimbrite/Tuff
Description
Major Minerals: Glass, sanidine
BH250-243c represents the less-welded, more porous portion of the ignimbrite (BH250-243) exposed along Charlie Brown Road near Shoshone. This unit occurs toward the upper and lower parts of the Miocene Resting Spring Pass Tuff and provides an important contrast to the intensely welded vitrophyre (BH250-243b). Together, these layers offer a clear view of how conditions changed within a single pyroclastic flow deposit. This ignimbrite formed during a large explosive volcanic eruption that spread silicic ash and pumice across eastern California and Nevada, and was later tilted and exposed by Basin and Range extension.
Like the rest of the ignimbrite, BH250-243c formed from a pyroclastic density current (nuée ardente)—a fast-moving, ground-hugging cloud of hot ash, pumice, crystals, and gas. However, in this part of the deposit, temperatures were lower compared to the central zone. As a result, the material did not fully compact and weld. Instead, it remained more open and porous, preserving more of its original fragmental character. Ash, pumice, and lithic fragments are still present, but they are less fused together than in the welded interior.
This unit records the cooler margins of the ignimbrite, both above and below the hotter core. Unlike the dense, glassy vitrophyre, BH250-243c shows limited welding, meaning pumice fragments are less flattened and may retain more of their original shapes. The rock is typically lighter in color and more friable, with visible pore spaces. These features indicate that this part of the flow cooled more quickly and experienced less compaction. When viewed together with the welded zones and vitrophyre, BH250-243c helps define the vertical thermal gradient within the deposit.
Although welding is weaker, you may still observe incipient eutaxitic fabric, where some pumice fragments show slight alignment. This suggests that the material was still moving and settling during deposition, but not at temperatures high enough to fully deform and fuse the pumice into strongly developed fiamme. In some areas, subtle alignment may still reflect flow direction and underlying topography.
Compositionally, BH250-243c is also silicic, similar to rhyolite or dacite, and contains minerals such as quartz, feldspar, and biotite. Unlike the vitrophyre, however, much of the original glass has been altered or devitrified to fine-grained crystalline material. The higher porosity and lower degree of welding make this unit more susceptible to weathering and alteration.
Even though pyroclastic flows can travel great distances, the presence of a thick, vertically zoned ignimbrite sequence—including the transition from poorly welded (BH250-243c) to densely welded zones and a central vitrophyre—indicates deposition from a hot, thick flow relatively close to its source (proximal to medial). The less-welded nature of BH250-243c reflects cooling at the margins of the flow, not a distal setting.
Although the original vent or caldera is not preserved, these relationships allow us to infer that the source was likely a nearby Miocene silicic volcanic center in the southern Death Valley–eastern California region, probably within a few tens of kilometers.
Overall, BH250-243c provides an important teaching example of the cooler, less-welded parts of a pyroclastic flow deposit and helps complete the picture of how temperature, compaction, and welding vary within an ignimbrite.
BH250-243c represents the less-welded, more porous portion of the ignimbrite (BH250-243) exposed along Charlie Brown Road near Shoshone. This unit occurs toward the upper and lower parts of the Miocene Resting Spring Pass Tuff and provides an important contrast to the intensely welded vitrophyre (BH250-243b). Together, these layers offer a clear view of how conditions changed within a single pyroclastic flow deposit. This ignimbrite formed during a large explosive volcanic eruption that spread silicic ash and pumice across eastern California and Nevada, and was later tilted and exposed by Basin and Range extension.
Like the rest of the ignimbrite, BH250-243c formed from a pyroclastic density current (nuée ardente)—a fast-moving, ground-hugging cloud of hot ash, pumice, crystals, and gas. However, in this part of the deposit, temperatures were lower compared to the central zone. As a result, the material did not fully compact and weld. Instead, it remained more open and porous, preserving more of its original fragmental character. Ash, pumice, and lithic fragments are still present, but they are less fused together than in the welded interior.
This unit records the cooler margins of the ignimbrite, both above and below the hotter core. Unlike the dense, glassy vitrophyre, BH250-243c shows limited welding, meaning pumice fragments are less flattened and may retain more of their original shapes. The rock is typically lighter in color and more friable, with visible pore spaces. These features indicate that this part of the flow cooled more quickly and experienced less compaction. When viewed together with the welded zones and vitrophyre, BH250-243c helps define the vertical thermal gradient within the deposit.
Although welding is weaker, you may still observe incipient eutaxitic fabric, where some pumice fragments show slight alignment. This suggests that the material was still moving and settling during deposition, but not at temperatures high enough to fully deform and fuse the pumice into strongly developed fiamme. In some areas, subtle alignment may still reflect flow direction and underlying topography.
Compositionally, BH250-243c is also silicic, similar to rhyolite or dacite, and contains minerals such as quartz, feldspar, and biotite. Unlike the vitrophyre, however, much of the original glass has been altered or devitrified to fine-grained crystalline material. The higher porosity and lower degree of welding make this unit more susceptible to weathering and alteration.
Even though pyroclastic flows can travel great distances, the presence of a thick, vertically zoned ignimbrite sequence—including the transition from poorly welded (BH250-243c) to densely welded zones and a central vitrophyre—indicates deposition from a hot, thick flow relatively close to its source (proximal to medial). The less-welded nature of BH250-243c reflects cooling at the margins of the flow, not a distal setting.
Although the original vent or caldera is not preserved, these relationships allow us to infer that the source was likely a nearby Miocene silicic volcanic center in the southern Death Valley–eastern California region, probably within a few tens of kilometers.
Overall, BH250-243c provides an important teaching example of the cooler, less-welded parts of a pyroclastic flow deposit and helps complete the picture of how temperature, compaction, and welding vary within an ignimbrite.
Coverage
Location: Death Valley, California, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 243c. Housed at Carleton College in Minnesota.
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-243c,” BH250 Mineralogy Teaching Collection, accessed April 24, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/364.