BH250-95
Title
Subject
Description
Rhyolite (Welded Ignimbrite)
Location: St. Francois Mountains, Ozark Plateau, Missouri, USA
Major Minerals: Quartz, sanidine, orthoclase, perthitic feldspar, biotite
Minor Minerals: Devitrified volcanic glass, Fe-Ti oxides
BH250-95 is one of the most pedagogically valuable volcanic rocks in the collection because it initially deceives students into thinking they are looking at an intrusive felsic rock. In hand sample, the rock appears massive, relatively dense, and contains visible feldspar and quartz phenocrysts that can resemble fine-grained granite or granophyric intrusive rocks. However, thin section analysis reveals that this specimen is actually a rhyolitic welded ignimbrite—formed during explosive volcanic eruptions associated with Mesoproterozoic silicic magmatism in the St. Francois Mountains of southeastern Missouri, one of the most important Proterozoic volcanic provinces in North America. This region is visited regularly during departmental field trips, making BH250-95 an especially meaningful teaching specimen because students can connect laboratory observations directly to field relationships.
The St. Francois Mountains preserve volcanic rocks that erupted approximately 1.45–1.48 billion years ago, during a major period of intracontinental felsic magmatism that affected the Midcontinent region. These eruptions produced extensive rhyolitic lava flows, ash-flow tuffs, caldera deposits, and welded ignimbrites. BH250-95 likely formed during one of these explosive eruptions when silica-rich magma became highly enriched in dissolved volatiles. As pressure decreased during eruption, gases rapidly expanded, fragmenting the magma into volcanic ash, pumice, and crystal fragments that were expelled as pyroclastic flows. These extremely hot mixtures of gas and volcanic debris moved rapidly across the landscape and eventually settled as thick deposits of ash-flow tuff. Because the deposits remained at high temperatures after deposition, volcanic glass shards and pumice fragments became compacted and welded together, producing ignimbrite.
Geochemically, BH250-95 confirms its strongly felsic composition. The rock contains 76.96 wt.% SiO₂, placing it firmly within the rhyolite field. It also contains elevated alkali contents (Na₂O = 4.15%, K₂O = 3.61%), consistent with alkali feldspar-rich rhyolitic magmas. Very low concentrations of MgO (0.20%) and CaO (0.26%) indicate strong magmatic differentiation, while trace elements such as elevated zirconium (298 ppm Zr) and rare earth elements reflect extensive fractional crystallization prior to eruption. These chemical characteristics demonstrate that BH250-95 represents an evolved magma that underwent significant differentiation before erupting explosively.
Under the microscope, the rock reveals its true volcanic origin. The thin section displays a porphyritic texture, with large phenocrysts of quartz and feldspar suspended in an extremely fine-grained groundmass. Quartz phenocrysts are commonly rounded, embayed, or partially resorbed, indicating disequilibrium between early-formed crystals and evolving melt compositions. Several quartz grains display overgrowth textures that suggest later recrystallization. Feldspar phenocrysts include sanidine, orthoclase, and perthitic feldspar, many of which preserve twinning and exsolution textures. Biotite occurs in minor amounts but remains easily identifiable.
The most important feature of this rock is its groundmass. What was originally volcanic glass has now undergone extensive devitrification, producing a fine-grained mosaic of quartz and feldspar. In several photomicrographs, the groundmass appears cryptocrystalline and crowded with tiny recrystallized grains. This is one of the clearest examples in the collection for teaching students how volcanic glass evolves through time. Instead of preserving pristine glass shards, the rock records post-eruptive recrystallization that obscures some original volcanic textures while still preserving enough evidence to reconstruct its volcanic history.
Some feldspar phenocrysts also display alteration and replacement textures, while opaque minerals likely represent iron-titanium oxides crystallized during late-stage cooling. These secondary changes provide an opportunity to discuss both primary volcanic processes and later diagenetic or low-grade metamorphic overprinting.
BH250-95 is particularly valuable because it teaches students an important lesson in igneous petrology: hand sample appearance alone can be misleading. Many students initially classify this specimen as intrusive because of its coarse phenocrysts and massive appearance. Only through petrographic analysis do they recognize its volcanic origin. This makes it an outstanding specimen for introductory students learning the distinction between extrusive and intrusive rocks, while advanced students can use it to discuss pyroclastic processes, devitrification, volcanic stratigraphy, and geochemical evolution of silicic magmas.
The rare earth element (REE) pattern provides additional evidence that BH250-95 represents a highly evolved felsic magma. When normalized to chondritic values, the REE plot shows strong enrichment in light rare earth elements (LREEs) such as La, Ce, Nd, and Sm relative to heavy rare earth elements (HREEs), a pattern commonly observed in highly differentiated continental rhyolitic magmas. The most striking feature of the diagram is the pronounced negative europium (Eu) anomaly, where Eu concentrations drop sharply relative to neighboring Sm and Gd. This strong Eu depletion is significant because europium can substitute for calcium in plagioclase feldspar during crystallization. The anomaly therefore indicates that substantial plagioclase fractionation occurred before eruption, removing Eu from the melt and leaving the residual magma increasingly silica-rich and chemically evolved. This interpretation aligns well with the abundant feldspar phenocrysts observed petrographically and supports the conclusion that BH250-95 formed from a highly fractionated rhyolitic magma prior to explosive eruption. For upper-level petrology students, this REE pattern provides an excellent opportunity to discuss incompatible element behavior, feldspar fractionation, and the geochemical evolution of silicic magmatic systems.