Silex Spring

Silex Spring is one of the most captivating and colorful hot springs in Yellowstone National Park’s Lower Geyser Basin. Its brilliant blue waters, surrounded by vivid orange and yellow microbial mats, make it a striking highlight along the Fountain Paint Pot Nature Trail. 

The name “Silex” comes from the Latin word for silica, a key mineral present in abundance around this thermal feature. The hot waters circulating deep beneath Yellowstone’s surface dissolve silica from the underlying volcanic rocks, and as the water cools near the surface, the silica is redeposited as sinter. These sinter deposits form delicate white terraces and crusts that border the pool and its runoff channels, giving Silex Spring both its name and its unique, otherworldly appearance.

Silex Spring

Quick Reference: Silex Spring

Attribute	Details
Name	Silex Spring
Location	Lower Geyser Basin, Yellowstone National Park, Wyoming, USA
Feature Type	Hot Spring
Geothermal Activity	Constantly active, non-eruptive thermal pool
Last Known Eruption	2006
Water Temperature	Average 174.7°F (79.3°C)
pH Level	Approximately 8.4 (alkaline)
Conductivity	Around 2000 µS/cm
Depth	Estimated 10–15 feet (3–4.5 meters)
Mineral Composition	Rich in silica (SiO₂) which forms sinter terraces
Color Origin	Deep blue center due to clear, hot water; orange and green runoff from thermophilic bacteria
Formation Process	Hot water dissolves silica from volcanic rocks, which precipitates as sinter upon cooling
Thermal Microbiology	Thermophiles in runoff channels create bright microbial mats
Accessibility	Easily accessible via the Fountain Paint Pot Trail boardwalk
Best Viewing Time	Morning or late afternoon for best light on water colors and steam
Notable Features	Vivid blue pool, striking runoff channels, silica sinter terraces
Fun Fact	“Silex” is Latin for silica — the mineral responsible for its brilliant terraces and smooth white crusts
Safety Note	Stay on designated boardwalks; water is near boiling and surfaces are fragile

What makes Silex Spring particularly captivating is its vivid color palette. The pool itself glows with a deep cerulean hue, while the runoff channels shimmer with streaks of orange, yellow, and green. These colors come from thermophilic bacteria and algae that thrive in the hot, mineral-rich water flowing away from the spring. The contrast between the clear blue pool and the multicolored channels creates a stunning natural display that captures the essence of Yellowstone’s geothermal beauty.

Silex Spring is constantly active but not eruptive, meaning it bubbles and steams continuously without shooting water into the air like a geyser. It last erupted in 2006, a rare occurrence for a feature of this type. Today, it remains a stable, boiling pool with surface temperatures averaging around 174.7°F (79.3°C). The spring’s pH level of 8.4 indicates slightly alkaline water, and its conductivity of about 2000 µS/cm reveals a high concentration of dissolved minerals. The heat and clarity of the water prevent most bacterial growth inside the main pool, leaving it crystal clear and mirror-like on calm days.

The boardwalk trail that passes Silex Spring offers visitors a close yet safe view of this geothermal marvel. From the edge, you can trace the flow of mineral-rich water as it carves smooth terraces and deposits thin crusts of silica. Along these runoff paths, thermophiles—heat-loving microorganisms—form intricate mats that sparkle with color when illuminated by the sun. It’s a living laboratory of geology and biology working hand in hand, transforming raw volcanic energy into a stunning natural mosaic.

Visiting Silex Spring feels like stepping into the heart of Yellowstone’s thermal world. The combination of vivid colors, geological artistry, and quiet steam rising from the water’s surface creates a mesmerizing and almost meditative scene. It serves as a perfect example of how geothermal processes shape Yellowstone’s landscape while supporting an ecosystem unlike any other on Earth.