Natural Geysers & Hot Springs: Earth's Eruptive Wonders & Healing Waters

Natural Geysers and Hot Springs: Earth’s Dynamic Eruptions and Healing Waters

Introduction: Earth's Hydrothermal Heartbeat

Imagine standing amidst a landscape punctuated by plumes of steam rising into the crisp air, the ground painted in vibrant hues of orange, yellow, and green by microscopic life thriving in scalding water. Suddenly, with a subterranean rumble, a column of boiling water and steam erupts skyward, a powerful testament to the immense heat simmering beneath our feet. This is the world of geysers and hot springs – dramatic and beautiful manifestations of Earth's internal heat engine interacting with its hydrological cycle.

These hydrothermal features are more than just geological curiosities. They are unique ecosystems, windows into subsurface processes, historical centers of healing and spirituality, crucial resources for geothermal energy, and sensitive indicators of environmental change. From the predictable eruptions of Old Faithful in Yellowstone to the mineral-rich soaking pools of Iceland and Japan, geysers and hot springs capture our imagination and offer profound insights into the dynamic nature of our planet.

This post delves into the physical geography of natural geysers and hot springs. We will explore their fundamental definitions, unravel the intricate geological processes driving their formation, examine their diverse types and global distribution patterns, understand their surprising ecological significance, discuss their long-standing relationship with humanity, and address the threats they face in an increasingly human-dominated world. Prepare to journey deep into Earth's plumbing system, where heat, water, and rock conspire to create some of the planet's most spectacular and fascinating phenomena.

Section 1: Defining the Phenomena - What Are Geysers and Hot Springs?

While often found in close proximity and sharing a common origin in geothermal heat, geysers and hot springs are distinct hydrogeological features.

Hot Spring: A hot spring is fundamentally a spring characterized by the emergence of geothermally heated groundwater from the Earth's crust. The standard definition classifies a spring as "hot" if its water temperature is significantly above the mean annual air temperature of the region. More technically, in the United States, the term often refers to springs with water temperatures above human body temperature (approx. 37°C or 98°F), while other definitions use thresholds like 21.1°C (70°F) or simply "above ambient ground temperature." The key aspect is that the water flows out relatively passively, without the violent, intermittent ejections typical of geysers. Water may pool, flow away gently, or bubble slightly due to escaping gases, but it doesn't explosively erupt.
Geyser: A geyser (from the Icelandic word "geysa," meaning "to gush") is a specific type of hot spring characterized by intermittent, turbulent, and often violent discharges of water and steam. It is essentially a hot spring with a plumbing system that includes constrictions, allowing pressure to build up to the point of explosive eruption. The defining feature is this cyclical eruption behaviour – periods of relative quiet or gentle steaming followed by powerful ejections.
Related Features: Geothermal areas often host other related features:
- Fumaroles: Vents emitting steam and volcanic gases (like sulfur dioxide, carbon dioxide) without significant water discharge. They represent pathways where groundwater boils completely into steam before reaching the surface.
- Mudpots: Acidic hot springs with limited water supply. The acid breaks down surrounding rock into clay and mud, which bubbles and spatters due to rising gases and steam. The acidity often comes from microorganisms metabolizing sulfur compounds.
- Travertine Terraces: Formed by hot springs carrying large amounts of dissolved calcium carbonate. As the water cools and degasses at the surface, the calcium carbonate precipitates, forming cascading terraces (e.g., Mammoth Hot Springs, Yellowstone; Pamukkale, Turkey).
- Sinter Deposits (Geyserite): Formed by hot springs and geysers rich in dissolved silica (silicon dioxide). As the hot water cools rapidly upon reaching the surface, especially during the splashing of eruptions, silica precipitates to form hard, whitish-grey deposits called geyserite or siliceous sinter. These deposits often line and help shape geyser conduits and cones.

Understanding these distinctions is crucial for appreciating the specific conditions required for each feature to form.

Section 2: The Geological Engine - How Do They Form?

The formation of both hot springs and geysers hinges on the confluence of three essential geological ingredients:

An Abundant Heat Source: The primary driver is geothermal heat – heat originating from within the Earth. This heat source is typically:
- Magma Chambers: In volcanically active regions, relatively shallow bodies of molten or partially molten rock (magma) provide intense, localized heat. This is common near active or recently active volcanoes, volcanic arcs associated with subduction zones, and mantle hotspots (like Yellowstone and Iceland).
- Deep Circulation & Geothermal Gradient: Even in non-volcanic areas, water can be heated if it circulates deep enough into the Earth's crust. The Earth's temperature increases with depth (the geothermal gradient, averaging about 25-30°C per kilometer). If geological structures allow water to penetrate several kilometers down and then rapidly return to the surface, it can emerge as a hot spring (e.g., Hot Springs, Arkansas).
- Radioactive Decay: The decay of radioactive elements within the crust contributes to the overall geothermal heat flow, though it's usually less significant than magmatic heat or deep circulation in creating high-temperature features like geysers.
An Ample Water Supply: The water ejected by geysers and flowing from hot springs is predominantly meteoric water – rainwater and snowmelt that seeps into the ground. This groundwater percolates downwards, sometimes over vast distances and long timescales, until it encounters the heated rock formations. The amount of available water influences the size, frequency, and longevity of the features.
A Specialized Plumbing System: This is the network of fractures, fissures, conduits, and porous rock layers that allows groundwater to descend towards the heat source, become heated, and then ascend back to the surface.
- For Hot Springs: The plumbing needs to be relatively open, allowing heated water to rise via convection without significant obstruction or pressure build-up. Fault lines often provide excellent pathways.
- For Geysers (The Critical Difference): Geyser formation requires a specific type of plumbing system, usually featuring one or more constrictions or tight spots below the surface. This system acts somewhat like a pressure cooker:
  - Heating at Depth: Water at the bottom of the conduit system is heated by the nearby heat source.
  - Pressure Effect: The weight of the overlying water column significantly increases the pressure on the water at depth. This elevated pressure raises the boiling point of water (it needs to be hotter than 100°C/212°F to boil).
  - Superheating: The water at depth can thus be heated well above its surface boiling point without actually boiling – it becomes superheated.
  - Initiation of Boiling: As heating continues, or due perhaps to a slight pressure decrease from water convection, some water eventually reaches its pressure-dependent boiling point, usually starting near the middle or upper-middle section of the water column. Bubbles of steam form.
  - Chain Reaction: These rising steam bubbles displace some of the overlying water, pushing it out of the geyser's opening. This reduces the height and weight of the water column.
  - Pressure Drop & Flash Boiling: The reduction in pressure instantly lowers the boiling point of the entire column of superheated water below. This water is now suddenly above its new, lower boiling point. It violently and rapidly flashes into steam.
  - Eruption: The rapid expansion of water flashing to steam (a volume increase of over 1600 times) forcefully ejects the remaining water and steam mixture out of the vent, causing the geyser eruption.
  - Recharge: After the eruption expels much of the water, cooler groundwater begins to seep back into the plumbing system, refilling it, and the cycle begins anew. The duration of this cycle determines the geyser's eruption frequency.
  - Role of Sinter: The deposition of silica sinter (geyserite) within the conduits can be crucial. It helps to seal the walls of the plumbing system, preventing water leakage, and can create or maintain the constrictions necessary for pressure build-up.

The delicate balance between heat supply, water recharge rate, and the precise geometry of the plumbing system dictates whether a feature becomes a passive hot spring or a dynamic geyser, and determines the specific characteristics (height, duration, frequency) of a geyser's eruption.

Section 3: Types and Variations

Geysers and hot springs exhibit considerable diversity, reflecting variations in their geological settings, water chemistry, and plumbing systems.

Hot Spring Variations:

Temperature: Ranging from lukewarm (just above ambient) to boiling.
Mineral Content: The dissolved mineral content, derived from the rocks the water passes through, varies greatly:
- Sulfur Springs: Rich in dissolved hydrogen sulfide (H₂S), giving them a characteristic "rotten egg" smell. Often acidic.
- Calcareous Springs (Travertine): Rich in calcium carbonate (CaCO₃), leading to the formation of travertine terraces as the water cools and CO₂ degasses. These are common in limestone regions.
- Siliceous Springs (Sinter): Rich in dissolved silica (SiO₂), common in volcanic areas where water interacts with rhyolitic rocks. They deposit geyserite/sinter.
- Iron Springs: Rich in dissolved iron compounds, often staining surrounding ground reddish-brown upon oxidation.
- Saline Springs: Containing high concentrations of dissolved salts (like sodium chloride).
Flow Rate: From gentle seeps to large rivers of hot water.

Geyser Variations:

Geysers are primarily classified based on their eruption style, largely determined by the shape of their vent and upper plumbing:

Cone Geysers: Erupt from a nozzle-like cone made of accumulated geyserite. Their eruptions are typically steady jets of water and steam, often reaching great heights but relatively narrow. The cone restricts the opening, focusing the eruption.
- Example: Old Faithful Geyser, Yellowstone National Park, USA. Known for its relatively predictable, tall, jet-like eruptions from a distinct sinter cone.
Fountain Geysers: Erupt from a pool of water, often explosively and in a series of bursts rather than a continuous jet. The eruption splashes outward in various directions. These occur when the vent opens into a wider basin or crater at the surface.
- Example: Grand Geyser, Yellowstone National Park, USA. Erupts from a large pool in powerful, fan-like bursts, considered one of the tallest predictable geysers globally.

Other Geyser Characteristics:

Regularity: Some geysers erupt at highly predictable intervals (like Old Faithful), while others are highly irregular, with intervals ranging from minutes to decades or becoming dormant altogether. Regularity depends on the stability of the heat supply, water recharge rate, and plumbing system integrity.
Duration: Eruptions can last from a few seconds to several hours.
Height: Eruption heights vary dramatically, from less than a meter to over 100 meters (e.g., historically, Waimangu Geyser in New Zealand; currently, Steamboat Geyser in Yellowstone during major eruptions).
Dormancy: Geysers can become dormant due to earthquakes altering the plumbing, changes in water supply (natural or human-induced), or shifts in geothermal activity. They can sometimes reactivate later.

Section 4: Global Distribution - Where Do We Find Them?

Geysers are rare geological phenomena, requiring the precise combination of heat, water, and plumbing described earlier. Consequently, they are concentrated in a few specific regions of the world, almost always associated with active volcanism or mantle hotspots. Hot springs are more common but still favor areas with elevated geothermal gradients.

Major Geyser Fields:

Yellowstone National Park, USA: By far the world's largest concentration of geysers (and hydrothermal features in general), containing over half of the world's active geysers (estimated 300-500). Located over a massive continental hotspot. Home to iconic geysers like Old Faithful, Steamboat Geyser (world's tallest active), Grand Geyser, and numerous hot springs, mudpots, and fumaroles within distinct geyser basins (Upper, Midway, Lower, Norris).
Valley of Geysers (Dolina Geizerov), Kamchatka Peninsula, Russia: A remote and stunning valley containing the second-largest concentration of geysers. Part of the Pacific Ring of Fire, associated with active subduction zone volcanism. Severely impacted by a massive mudflow in 2007, but many geysers remain active or have recovered.
El Tatio, Atacama Desert, Chile: Located high in the Andes mountains, associated with subduction zone volcanism. It is the largest geyser field in the Southern Hemisphere and the third largest globally. Characterized by numerous steaming vents and relatively lower-eruption geysers, most active at dawn when cold air temperatures enhance steam visibility.
Taupō Volcanic Zone, North Island, New Zealand: An area of intense geothermal activity related to subduction. Historically hosted many powerful geysers (like Waimangu), though many were significantly impacted or destroyed by geothermal power development in the mid-20th century. Significant features remain at sites like Te Puia (Pohutu Geyser) and Orakei Korako.
Iceland: Situated astride the Mid-Atlantic Ridge (a divergent plate boundary) and over a mantle hotspot, Iceland is a hotbed of geothermal activity. While the original "Geysir" (from which the name derives) is now largely infrequent, the nearby Strokkur geyser erupts reliably every few minutes. Numerous hot springs, fumaroles, and geothermal power plants dot the landscape.

Other Notable Areas:

Minor Fields/Individual Geysers: Small fields or individual geysers exist in places like Umnak Island (Alaska, USA), parts of Indonesia, Japan, Kenya (Lake Bogoria), and potentially other remote volcanic regions.
Abundant Hot Springs: Hot springs are found more widely, including regions of tectonic extension (Basin and Range Province, USA), along major fault systems, and in areas with deep groundwater circulation, even far from active plate boundaries (e.g., Bath, England; Hot Springs, Arkansas, USA; various locations in the Alps, Himalayas, Japan, Costa Rica, Turkey).

The global distribution pattern strongly reinforces the link between these hydrothermal features and areas where Earth's internal heat can readily access near-surface groundwater systems, primarily driven by plate tectonics and mantle plumes.

Section 5: Ecological Significance - Oases of Life in Extreme Environments

At first glance, the scalding, often mineral-laden waters of geysers and hot springs might seem inhospitable to life. However, these environments host unique and surprisingly diverse ecosystems dominated by extremophiles – organisms adapted to thrive in extreme conditions.

Thermophiles: The most prominent inhabitants are thermophilic (heat-loving) bacteria and archaea. These microorganisms can survive and flourish at temperatures that would instantly kill most other life forms, some thriving even above the boiling point of water under pressure.
Microbial Mats: These thermophiles often form thick, colorful communities called microbial mats. The vibrant colors frequently seen in hot spring runoff channels (yellows, oranges, reds, greens, browns) are typically due to the pigments (like carotenoids and chlorophyll variants) within different species of cyanobacteria and other microbes, arranged along temperature gradients. Each color band often represents a different microbial community adapted to a specific temperature range.
Chemosynthesis: In environments rich in chemical compounds like hydrogen sulfide or methane (common in geothermal systems), some microbes derive energy through chemosynthesis (using chemical reactions) rather than photosynthesis (using sunlight). These form the base of unique food webs.
Specialized Eukaryotes: While less common at the highest temperatures, some specialized eukaryotic organisms can live in the cooler margins of hot springs, including certain types of algae, fungi, protozoa, nematodes, and even insects (like the Ephydrid flies whose larvae graze on microbial mats) and crustaceans. In some outflow channels where water has cooled sufficiently, certain fish species may be found.
Astrobiological Relevance: Studying extremophiles in geothermal environments on Earth provides analogues for potential life in similar extreme environments hypothesized to exist elsewhere in the solar system, such as on Jupiter's moon Europa or Saturn's moon Enceladus, which are thought to harbor subsurface liquid water potentially warmed by tidal forces or geothermal activity.

These geothermal ecosystems are not just scientifically fascinating; they play roles in nutrient cycling within their immediate vicinity and represent unique reservoirs of biodiversity adapted to Earth's extremes.

Section 6: Human Interactions and Significance

Humans have interacted with geysers and hot springs for millennia, drawn to their warmth, perceived healing properties, and awe-inspiring power.

Cultural and Spiritual Significance: Many cultures have revered hot springs as sacred sites or portals to the underworld. They have been centers for spiritual rituals, cleansing ceremonies, and communal gathering places (e.g., Māori traditions in New Zealand, Japanese onsen culture, Roman baths).
Therapeutic Uses (Balneotherapy): The practice of bathing in mineral-rich hot springs for health benefits (balneotherapy) is ancient and continues today. Different springs are claimed to alleviate various ailments, attributed to the heat (relaxing muscles, improving circulation) and dissolved minerals (though scientific evidence for many specific mineral benefits varies).
Tourism and Recreation: Geyser fields and accessible hot springs are major tourist destinations worldwide, attracting millions of visitors annually. This brings economic benefits but also poses management challenges. Iconic sites like Yellowstone, Iceland's Blue Lagoon, and Japanese onsens are prime examples.
Geothermal Energy: Hot springs and deeper geothermal reservoirs are tapped as a source of renewable energy. Geothermal power plants use the steam or hot water to drive turbines and generate electricity. Direct uses include heating buildings (district heating, common in Iceland), warming greenhouses, and industrial applications. However, energy extraction must be managed carefully to avoid depleting the resource or damaging surface features.
Scientific Research: These features are invaluable natural laboratories for:
- Geology and Geophysics: Studying subsurface heat flow, hydrogeology, volcanology, and earthquake precursors (changes in eruption patterns can sometimes precede seismic activity).
- Microbiology and Astrobiology: Discovering and studying extremophiles, understanding the limits of life, and searching for enzymes with industrial applications (e.g., Taq polymerase, used in PCR, was originally isolated from Thermus aquaticus found in Yellowstone hot springs).
- Geochemistry: Analyzing water and gas chemistry to understand water-rock interactions and mineral transport.

Section 7: Threats and Conservation

Despite their robust geological origins, geysers and hot springs are remarkably fragile systems, highly sensitive to environmental changes, both natural and human-induced.

Geothermal Energy Development: Large-scale extraction of hot water or steam for energy can lower the water table, reduce reservoir pressure, and decrease heat flow, potentially causing geysers to cease erupting and hot springs to diminish or dry up (as seen historically in parts of New Zealand and Nevada, USA). Sustainable management practices are crucial.
Groundwater Extraction: Pumping groundwater for agriculture, industry, or municipal supplies in areas hydrologically connected to geothermal systems can similarly reduce the water available to feed hot springs and geysers.
Tourism Impacts: Unregulated tourism can lead to:
- Vandalism and Trash: Throwing objects into geysers or pools can damage delicate plumbing systems and alter eruption behaviour.
- Pollution: Soaps, lotions, litter, and wastewater can contaminate sensitive waters and harm microbial ecosystems.
- Trail Erosion and Compaction: Foot traffic off designated paths can damage fragile sinter formations and surrounding vegetation.
- Infrastructure Development: Building roads, hotels, and facilities too close can disrupt surface and subsurface water flow.
Climate Change: Changes in precipitation patterns (affecting groundwater recharge) and rising air temperatures (subtly altering surface conditions) could potentially impact the long-term behaviour of these water-dependent features.
Earthquakes: While a natural process, major earthquakes can instantly and drastically alter the underground plumbing, potentially destroying existing features, creating new ones, or changing eruption patterns unpredictably.
Land Use Changes: Urban development, mining, or agriculture near geothermal areas can alter surface drainage and groundwater flow patterns.

Conservation efforts typically involve establishing protected areas (like National Parks), regulating geothermal development and groundwater extraction, managing tourism sustainably (boardwalks, education, limiting access), conducting ongoing scientific monitoring, and raising public awareness about the fragility and value of these unique features.

Conclusion: Dynamic Wonders Worth Protecting

Natural geysers and hot springs are captivating expressions of Earth's restless interior energy meeting its surface water systems. They are born from a precise interplay of heat, water, and geology, resulting in phenomena ranging from tranquil, mineral-rich pools to Earth's most powerful natural fountains. Their distribution highlights the planet's most geothermally active zones, intimately linked to plate tectonics and volcanism. Beyond their geological spectacle, they host unique life forms adapted to extreme heat, hold deep cultural significance, offer recreational and therapeutic benefits, provide a vital source of renewable energy, and serve as invaluable sites for scientific discovery.

Yet, their existence hangs in a delicate balance. The very conditions that create them also make them vulnerable to changes in water supply, heat flow, and the integrity of their subterranean conduits. As we continue to utilize geothermal resources and as human activity increasingly impacts hydrological systems, the need for careful stewardship and robust conservation strategies becomes ever more critical. These dynamic eruptions and healing waters are not just geological wonders; they are irreplaceable components of our planet's natural heritage, demanding our respect and protection for generations to come.

Interactive Q&A / Practice Exercises

Test your understanding of geysers and hot springs with these questions!

Part 1: Multiple-Choice Questions (MCQs)

Which of the following is the essential characteristic that distinguishes a geyser from a regular hot spring? a) The water temperature is above 100°C (212°F). b) The water contains high levels of dissolved silica (sinter). c) The water discharge is intermittent and eruptive. d) It is primarily found near active volcanoes.
What are the three fundamental ingredients required for geyser formation? a) Magma, deep ocean water, and limestone rock. b) Heat source, abundant water, and a constricted plumbing system. c) Volcanic gases, acidic water, and porous sandstone. d) Tectonic pressure, glacial meltwater, and a wide-open vent.
Cone geysers (like Old Faithful) typically have which eruption characteristic? a) Eruptions from a large pool, splashing outwards. b) Continuous, gentle flow of hot water. c) Short, explosive bursts of steam only. d) A focused, jet-like eruption from a sinter mound.
Which type of microorganism is most characteristic of the high-temperature environments of hot springs and geysers? a) Psychrophiles (cold-loving) b) Halophiles (salt-loving) c) Thermophiles (heat-loving) d) Acidophiles (acid-loving)

Part 2: Scenario-Based Questions

Scenario: Imagine a region with a known shallow magma chamber providing significant heat. Groundwater is plentiful due to high rainfall percolating through fractured volcanic rock. However, seismic monitoring shows the fractures are generally wide and interconnected, without significant bottlenecks below the surface. What type of hydrothermal feature(s) would you most expect to dominate this landscape, and why?
Scenario: A previously stable geyser field experiences a major earthquake (magnitude 7.5) along a nearby fault line. In the weeks following the earthquake, observers note that several famous geysers have stopped erupting entirely, while a few hot springs have become noticeably hotter and more vigorous. Explain the likely physical geography reasons behind these changes.

Part 3: Diagram-Based Exercise

(Imagine a simple cross-section diagram of a geyser system is shown here. It would depict:

A surface vent/opening (perhaps with a small cone or pool).
An underground conduit (tube) leading downwards.
Constrictions/narrow points within the conduit.
Porous rock/fractures allowing groundwater inflow.
A heat source (labeled 'Magma' or 'Hot Rocks') at the bottom.)

Task: Label the following key components on the diagram: a) Heat Source b) Groundwater Inflow Path c) Underground Conduit d) Constriction(s) e) Surface Vent / Eruption Point

Explain the role of the 'Constriction(s)' (d) in the geyser's eruption cycle.

Answer Explanations

Part 1: MCQs - Explanations

Correct Answer: (c) The water discharge is intermittent and eruptive.
- Explanation: While high temperatures (a), silica content (b), and volcanic association (d) are common in geyser areas, they don't define a geyser. Many hot springs share these features. The defining characteristic is the cyclical, eruptive discharge caused by the specific plumbing and pressure dynamics.
Correct Answer: (b) Heat source, abundant water, and a constricted plumbing system.
- Explanation: These are the three universally recognized requirements. Heat provides the energy, water is the medium, and the specific constricted plumbing allows the pressure build-up and flash boiling necessary for eruption. The other options include incorrect elements or miss essential ones.
Correct Answer: (d) A focused, jet-like eruption from a sinter mound.
- Explanation: Cone geysers build up mounds of sinter (geyserite) around their vents. This structure acts like a nozzle, focusing the erupting water and steam into a relatively narrow, often tall, jet. Fountain geysers (a) erupt from pools.
Correct Answer: (c) Thermophiles (heat-loving).
- Explanation: Geysers and hot springs are defined by their high temperatures. Life forms adapted to these conditions are called thermophiles. While other extremophiles might be present depending on specific water chemistry (like acidophiles in mudpots or halophiles if the water is saline), thermophiles are the most characteristic group for these high-temperature environments.

Part 2: Scenario-Based Questions - Explanations

Expected Features & Reasoning: In this scenario, you would primarily expect abundant hot springs and perhaps fumaroles, but few, if any, geysers.
- Reasoning: The region has the necessary heat source and water supply. However, the lack of significant constrictions in the wide, interconnected fracture network means there is no mechanism for pressure to build sufficiently for superheating and explosive flash boiling. Heated water would simply rise via convection through the open pathways and emerge relatively passively as hot springs. If the water is hot enough to boil before reaching the surface due to the intense heat, fumaroles (steam vents) might also form. Geyser formation requires the specialized constricted plumbing, which is absent here.
Earthquake Impact Explanation: The earthquake likely caused significant changes to the subsurface plumbing system.
- Geysers Stopping: The ground shaking could have collapsed or blocked the delicate constrictions within the geysers' conduits, or it might have opened new, wider pathways for water to escape. This would disrupt the pressure build-up cycle necessary for eruptions, causing the geysers to revert to passive hot springs or become dormant.
- Hot Springs Becoming Hotter/More Vigorous: The earthquake could have opened new fractures or widened existing ones, creating more direct pathways from the deep heat source to the surface for some hot springs. This improved connection allows hotter water to ascend more rapidly and in greater volume, increasing the temperature and flow rate observed at the surface. Earthquakes fundamentally rearrange the 'pipes' of the geothermal system.

Part 3: Diagram-Based Exercise - Explanation

(Labeling would depend on the specific diagram, but placement should be logical based on the descriptions.)
Role of Constriction(s) (d): The constriction(s) are crucial for geyser eruption because they impede the free convective flow of water within the conduit. This allows the water below the constriction to be heated significantly by the heat source while being subjected to high pressure from the overlying water column. This pressure raises the boiling point, allowing the water to become superheated (hotter than 100°C but still liquid). When boiling eventually starts (often slightly above the constriction), rising steam bubbles lift the water above, reducing pressure. The constriction slows down the immediate release of this pressure. When the pressure drops sufficiently, the superheated water below the constriction instantly flashes into a large volume of steam, violently expelling the water above it in an eruption. Without the constriction, pressure wouldn't build sufficiently, and the system would likely behave as a continuously flowing or pulsing hot spring.