Geysers and hot springs are both hydrothermal features where heated groundwater rises to the Earth’s surface. The fundamental difference lies in their visible behavior: hot springs maintain a continuous, passive flow, while geysers intermittently and violently erupt water and steam. This variation is not due to a difference in their heat source but entirely to the architecture of their underground plumbing system and the resulting physics of water pressure and boiling.
Shared Foundations: The Geothermal Heat Source
Both geysers and hot springs require two primary geological components: a steady supply of groundwater and an intense source of heat. The water source is typically surface water, such as rain or snowmelt, that percolates deep into the earth through porous rock and fissures. This water is heated by a geothermal source, often a shallow body of magma or hot, recently solidified rock located a few kilometers beneath the surface in volcanically active regions.
In areas without recent volcanic activity, heat comes from the normal geothermal gradient, where crust temperatures increase with depth. If water descends far enough, it reaches rock hot enough to significantly raise its temperature before circulating back toward the surface. This heat transfer creates a subterranean convection system, driving the hot, less dense water upward.
The Hot Spring Mechanism: Open Circulation and Passive Flow
A hot spring does not erupt because of the open and unrestricted nature of its underground channels. Hot springs possess wide, relatively straight conduits and fissures that allow heated water to rise with minimal impediment. This open structure facilitates continuous convection: hot water rises, releases heat and pressure efficiently at the surface, and is replaced by cooler, denser water sinking from above.
As the heated water ascends, pressure is constantly released, preventing the water from reaching the superheated state required for an explosive event. The water emerging at the surface is generally near or slightly below the boiling point for that altitude, maintaining a stable, passive flow.
The Geyser Mechanism: The Critical Plumbing System
The defining difference for a geyser lies in its highly specific and restricted subterranean architecture. A geyser’s plumbing system is a deep, narrow, and often tortuous network of tubes, fissures, and side chambers. These constricted passageways prevent the free, continuous circulation and heat dissipation seen in a hot spring.
The narrow geometry traps the water and steam, creating bottlenecks. The conduit walls are often lined with geyserite, a form of silica deposited by the hot water, which makes the plumbing even more constricted. This restricted system forces the water at deeper levels to remain under intense pressure from the weight of the water column above it, a condition necessary for the eruption cycle.
The Physics of Eruption: Pressure and Flash Vaporization
The restricted plumbing allows water deep within the system to become superheated. The elevated hydrostatic pressure at depths can raise the boiling point significantly, allowing the water to reach high temperatures without turning into steam. This pressure-temperature balance is highly unstable, storing immense thermal energy.
The eruption cycle begins when a small disturbance, such as boiling water or a steam bubble release, reduces the hydrostatic pressure on the superheated water below. This sudden drop causes the superheated water to instantly “flash” into steam, a process known as flash vaporization. Since water expands by a factor of about 1,600 times when converting to steam, this rapid phase change creates an explosive volume expansion that violently ejects the remaining water and steam into the air.