Scientists have documented that Yellowstone’s rare acidic Echinus geyser has resumed erupting after more than five years of inactivity.
The return of one of the park’s few acid-driven geysers highlights a fragile underground balance that allows eruptions to occur where corrosive water normally destroys them.
Echinus in Norris Geyser Basin
Within Yellowstone’s Norris Geyser Basin, Echinus is once again sending columns of hot water roughly 20 to 30 feet into the air.
Temperature spikes recorded in runoff channels led Michael Poland of the U.S. Geological Survey (USGS) to confirm the geyser’s renewed eruptions beginning on February 7, the first activity observed since 2020.
Subsequent monitoring showed the bursts repeating every two to five hours and lasting several minutes, closely resembling an earlier active phase seen in 2017.
That pattern suggests a temporary revival of the underground conditions that allow this unusual acidic geyser to erupt, a system whose chemistry now demands closer explanation.
Why Echinus geyser is acidic
Most acidic hot springs never become geysers because harsh water eats away the underground channels that let pressure build.
Echinus escaped that fate because acidic gases mix with more neutral groundwater, leaving water sour enough to sting rock less aggressively.
Inside those mineral-lined passages, trapped heat and rising steam can still build pressure before the pool suddenly unloads upward.
That unusual balance makes Echinus rare even in Yellowstone, where strange hot features are common and chemical extremes often stay quiet.
Colors on the rim
Around the pool, jagged mineral crust and spiny rocks explain why early geologists named this feature after sea urchins.
At about 66 feet across, the geyser looks rough and almost armored, not smooth like many famous Yellowstone pools.
Red, orange, and yellow staining comes from iron, aluminum, and arsenic that settled around the rim as hot water cooled.
Those colors are not decoration alone, because they record the same chemistry that helped preserve Echinus instead of dissolving it.
Very colorful and acidic Echinus Geyser in the Norris Geyser Basin in Yellowstone National Park Wyoming. Click image to enlarge.Heat without panic
Above all, the real danger here is heat, since the water can approach Yellowstone’s surface boiling point, near 200º Fahrenheit.
“That sounds a bit scary, but the acid is not concentrated,” wrote Poland. USGS scientists compared the water’s acidity to orange juice or vinegar, mild enough to be less alarming than the temperature.
Nearby Steamboat Geyser also erupted in February, yet the broader Yellowstone system stayed at background levels of activity.
A stop-start past
For much of recorded history, Echinus behaved like a reluctant performer, spending long stretches quiet before brief active runs.
Before 1948, observers saw only occasional outbursts, but regular eruptions arrived in the 1970s at 40 to 80 minute intervals.
During the 1980s and 1990s, some episodes stretched beyond 90 minutes, and the tallest columns reached roughly 75 feet.
By the early 2000s, temperatures had dropped, eruptions weakened, and the old rhythm started slipping away again.
2017 echo from Echinus geyser
Then came another comeback in October 2017, when repeated eruptions arrived every two to three hours for weeks.
From October 18 to November 10 that year, the geyser held a consistency it had not shown for years.
Afterward, activity faded to a few scattered events between 2018 and 2020, and then the basin went quiet.
That earlier burst matters now because the new pattern looks less like a random fluke and more like a rerun.
Watching the signals
Since 2010, temperature instruments in Echinus’s runoff channel have given scientists a simple way to catch hidden eruptions.
At Norris, sensors record every two minutes, and sharp jumps in outlet temperature usually mean the geyser has fired.
Because the readings are posted quickly, researchers and curious visitors can follow changes without standing beside boiling water.
That steady record turned a hard-to-predict attraction into a feature scientists could track hour by hour.
What powers geysers
Deep heat from slowly cooling magma warms groundwater under Yellowstone, and fast-rising water can flash into steam and erupt.
When that hot water loses heat more gently, it reaches the surface as a spring instead of a geyser.
Across Norris, small changes underground can reroute water, change pressure, and wake one feature while another stays quiet.
Even with new eruptions on the books, “change is constant at Norris Geyser Basin,” wrote Poland when explaining why forecasts stay fragile.
Importance of rare geyser eruptions
Echinus matters because it shows how local chemistry can decide whether underground heat becomes a spring or an erupting column.
In one place, acid can chew rock into clay, while a slightly different mix leaves the channels standing.
That fine margin helps explain why Yellowstone has many acidic features but very few acidic geysers.
Watching Echinus now gives scientists a live test of how water, gas, and rock keep remaking the basin.
Echinus has come back with the same short, forceful rhythm that marked its last serious wake-up, and that alone makes it worth watching.
No one can say how long this run will last, but Yellowstone can change fast without signaling a larger emergency.
Credit: Photo by MA Bellingham.
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