An Idaho Geothermal History Lesson

Ninety percent of Idaho’s 340 hot springs are the result of leftover energy heating water near fault lines. That heat is not the scar of an ancient meteorite impact. It comes from three overlapping geologic stories: a mantle plume known as the Yellowstone hotspot, the granite Idaho Batholith in the center of the state, and the crust-stretching faulting of the Basin and Range. Each story heats groundwater in its own part of Idaho, and together they make Idaho the most hot-spring-rich state in the nation.

The Yellowstone Hotspot

The mainstream geologic account begins about 17 million years ago near present-day southeast Oregon and northern Nevada, where a plume of hot rock rising from deep within the Earth – a mantle plume – first reached the crust. The plume’s arrival produced enormous flood-basalt eruptions, including lavas that spread as far as the Columbia Plateau. The hotspot itself has remained nearly stationary ever since, while the North American tectonic plate drifts slowly above it.
Because the plate moves and the hotspot does not, the eruptions appear to migrate. The result is a traceable track of volcanic calderas cut diagonally across the Snake River Plain of southern Idaho: each caldera marks where the plume sat at that time, and the line of them ends at the active caldera beneath Yellowstone, just east of Idaho. Idaho sits directly on this path. Other evidence the hotspot left behind includes Craters of the Moon and the basalt lava flows visible throughout southeast Idaho, most notably off Interstate 84. The crust along the track is still abnormally warm, so groundwater circulating near fault lines there is heated and rises as hot springs. As the plate continues to shift, Yellowstone will eventually cool and look much like Craters of the Moon does now.

The Idaho Batholith

Central Idaho’s hot springs owe their heat to a different source. The Idaho Batholith is one of the largest granite bodies in North America – a vast pluton of molten rock that rose into the crust roughly 100 to 75 million years ago and slowly cooled into granite beneath what are now the mountains of central Idaho.
Granite of this kind generates a modest amount of its own heat through the slow decay of radioactive elements, and a body this large also holds the Earth’s internal heat closer to the surface. Rain and snowmelt seep into fractures in the granite and circulate deeply along faults – in places a mile or more down – picking up that heat before rising back to the surface. This deep circulation along faults in and around the batholith is why so many springs appear in central Idaho, far from the Yellowstone hotspot track.

Basin and Range Faulting

In southern and southeastern Idaho, a third process takes over. The Basin and Range province is a region where the Earth’s crust is being pulled apart and stretched. That extension cracks the crust along normal faults, dropping long valleys down and lifting mountain ranges up in an alternating pattern of basins and ranges.
Those extension cracks make ideal plumbing for hot springs. Surface water descends along the faults to depths where the normal increase of temperature with depth – the geothermal gradient – heats it, in some places close to boiling. Because hot water is less dense, it rises back up the same fault planes and emerges at the surface, often in a line along the base of a range. The town of Lava Hot Springs in southeastern Idaho, where odorless water at 102-112°F flows at roughly 2.5 million gallons a day, sits in exactly this kind of faulted Basin and Range country at the edge of the Snake River Plain.

Why Idaho Has So Many

No single cause explains all of Idaho’s roughly 340 known hot springs; the state stacks all three causes at once. The Yellowstone hotspot track crosses the entire southern part of the state, leaving warm crust the length of the Snake River Plain. The Idaho Batholith heats the mountainous center. Basin and Range faulting cracks open the south and southeast, and faults connected to all three systems give surface water a deep, heated path almost everywhere. Add abundant mountain rain and snow to recharge the system, and the result is approximately 130 soakable springs – more usable natural hot springs pools than in any other state in the nation.

Why are Idaho hot springs hot?

Idaho hot springs are hot because groundwater circulates deep underground – often a mile or more – along faults before returning to the surface. At depth, the water is heated by the Earth’s internal heat, which is locally intensified by hotspot and batholith heat: the Yellowstone hotspot track across southern Idaho and the granite Idaho Batholith in the center of the state. The faults are the delivery system; the hotspot and the batholith are the furnaces.

Are Idaho hot springs safe to soak in?

Most developed hot springs in Idaho are safe to soak in: commercial and maintained public pools are kept at comfortable soaking temperatures. Natural, undeveloped sources are a different matter – water can emerge from the ground hot enough to exceed safe soaking temperatures, sometimes near boiling. A natural pool should always be tested carefully before anyone gets in, and posted closures and warnings should always be obeyed.