Minerals in Yellowstone National Park

Yellowstone National Park is widely known for its geysers, hot springs, and dramatic volcanic landscapes, but beneath and across all of these features lies an extraordinary diversity of minerals. These minerals are the physical record of Yellowstone’s volcanic history, its active geothermal system, and the slow interaction between rock, water, heat, and life. 

Minerals in Yellowstone are not just static substances locked in stone; they are constantly forming, dissolving, and transforming as hot water circulates through the Earth’s crust and emerges at the surface. Each mineral tells a story about temperature, chemistry, pressure, and time, making Yellowstone one of the most mineralogically expressive landscapes on the planet.

Quick Reference: Minerals in Yellowstone National Park

Mineral / Material	Composition	Main Locations	How It Forms	Visual Appearance
Silica (Siliceous sinter)	Silicon dioxide	Upper Geyser Basin, around geysers and hot springs	Hot silica-rich water cools and deposits minerals	White to light gray, chalky or porcelain-like
Quartz	Crystalline silicon dioxide	Volcanic rocks throughout the park	Slow cooling of magma or crystallization from fluids	Hard, glassy crystals within rock
Obsidian	Volcanic glass rich in silica	Obsidian Cliff, lava flows	Rapid cooling of silica-rich lava	Shiny black, glassy
Feldspar	Aluminum silicates	Rhyolite lava flows and volcanic rocks	Crystallization from magma	Light-colored, dull crystals
Calcite	Calcium carbonate	Mammoth Hot Springs	CO₂ release causes mineral precipitation	Bright white crystalline deposits
Sulfur	Elemental sulfur	Norris Geyser Basin, fumaroles	Oxidation of hydrogen sulfide gas	Bright yellow crusts
Iron oxides	Iron compounds	Hot spring runoff channels, soils	Oxidation of iron-rich fluids	Red, orange, brown staining
Clay minerals	Altered volcanic rock	Acidic geothermal basins	Chemical breakdown by hot acidic water	White, gray, powdery soils
Gypsum	Calcium sulfate	Evaporating hot springs	Evaporation of sulfate-rich water	White or translucent crusts
Pyrite	Iron sulfide	Subsurface hydrothermal zones	Low-oxygen mineral formation	Metallic gold-colored crystals

Silica is the most influential mineral in Yellowstone and is responsible for many of the park’s most iconic features. It originates from the volcanic rocks beneath the park, particularly rhyolite, which is rich in silicon dioxide. As groundwater heats up deep underground, it dissolves silica and carries it upward to the surface. When this hot water cools rapidly in geysers and hot springs, silica precipitates out and forms white to light gray deposits known as siliceous sinter. These deposits build up around geysers such as Old Faithful and preserve delicate textures like ripples and microbial impressions. Over time, silica creates hard, porcelain-like surfaces that record the history of water flow and geothermal activity.

Quartz is one of the most common crystalline forms of silica found in Yellowstone, though it is usually hidden within volcanic rocks rather than visible as large crystals. Quartz forms as molten rock cools slowly underground or as silica-rich fluids crystallize within fractures. In Yellowstone, quartz contributes to the hardness and durability of rhyolite lava flows and volcanic domes. Although it may not be as visually striking as other minerals at the surface, quartz is fundamental to the park’s geology and plays a key role in controlling how rocks fracture and erode over time.

Obsidian, while technically a volcanic glass rather than a true mineral, is one of Yellowstone’s most famous mineral-like materials. It forms when silica-rich lava cools so rapidly that crystals do not have time to develop. The result is a smooth, glassy substance that fractures into razor-sharp edges. Obsidian Cliff, located near Mammoth Hot Springs, is one of the largest and most significant obsidian sources in North America. For thousands of years, Indigenous peoples quarried Yellowstone obsidian and traded it across vast distances. Its presence highlights the explosive volcanic past of the park and the rapid cooling of lava flows.

Feldspar is another major mineral group found throughout Yellowstone’s volcanic rocks. These minerals form as magma cools and crystallizes, creating the framework of rhyolite and other igneous rocks. Feldspar minerals contribute to the light color of Yellowstone’s lava flows and influence how rocks weather over time. As feldspar breaks down through chemical weathering, it helps produce clay minerals that shape soils and affect plant growth. Although feldspar rarely draws attention from visitors, it is one of the most abundant and important minerals in the park.

Calcite plays a dramatic and highly visible role in Yellowstone, especially at Mammoth Hot Springs. Calcite is composed of calcium carbonate and forms when hot, calcium-rich water emerges at the surface and releases carbon dioxide into the air. This process causes calcite to precipitate rapidly, building the bright white travertine terraces that resemble frozen waterfalls. Calcite deposits can grow quickly, sometimes changing the landscape in a matter of weeks. Their brilliant whiteness reflects sunlight and gives Mammoth Hot Springs its striking appearance, while also recording subtle changes in water chemistry and flow.

Sulfur is one of the most easily recognized minerals in Yellowstone due to its bright yellow color and strong odor. It forms in areas where volcanic gases rich in hydrogen sulfide rise to the surface and react with oxygen. Sulfur commonly accumulates around fumaroles, mud pots, and acidic hot springs, particularly in places like Norris Geyser Basin. Beyond its visual impact, sulfur is a key energy source for certain thermophilic microorganisms, linking mineral chemistry directly to biological life. Its presence also signals areas of intense geothermal activity beneath the surface.

Iron oxides are responsible for many of the red, orange, and brown colors seen in Yellowstone’s soils, rocks, and hot spring runoff channels. These minerals form when iron-bearing rocks and fluids interact with oxygen, producing compounds such as hematite and goethite. Iron oxides often mix with microbial mats, enhancing color contrasts and creating layered patterns along flowing water. In places like Chocolate Pots, iron-rich waters interact with bacteria to create striking reddish-brown sediments, illustrating the close relationship between minerals and living organisms.

Clay minerals are widespread in Yellowstone, especially in areas affected by acidic geothermal activity. When volcanic rocks are exposed to hot, acidic water and gases, their original minerals break down and reorganize into fine-grained clays. These clay-rich areas often appear white, gray, or pale yellow and can be soft and unstable underfoot. Clay formation alters soil chemistry and limits plant growth, contributing to the barren appearance of some geothermal basins. These minerals are also important indicators of subsurface chemical conditions and geothermal intensity.

Gypsum is another mineral occasionally found in Yellowstone’s geothermal areas, forming when sulfate-rich waters evaporate. Gypsum crystals can appear as white or translucent crusts near hot springs and seepage zones. Although less common than silica or calcite, gypsum reflects the complex chemistry of geothermal fluids and the diverse pathways through which minerals precipitate. Its presence often indicates interactions between sulfur compounds and calcium-rich waters.

Pyrite, sometimes called fool’s gold, occurs in small amounts within Yellowstone’s volcanic and hydrothermal systems. It forms under conditions where iron and sulfur combine in low-oxygen environments. While pyrite is not widespread at the surface, its presence underground influences water chemistry as it weathers and releases sulfuric acid. This process contributes to the acidity of certain hot springs and plays a role in shaping mineral assemblages throughout the park.