Geysir & Strokkur Field Notes

Haukadalur (Geysir & Strokkur): Field Notes & Research Dossier

Series: Iceland Explained · Public Production Bible & Research Companion
Region / Municipality: South Iceland (Suðurland) · Bláskógabyggð Municipality (Biskupstungur district, Árnessýsla)
Primary Coordinates: 64.3104, -20.3024 (64°18'37"N, 20°18'09"W: Great Geysir & Strokkur Field)
Last Source Verification: November 1, 2026

1.0 Site Dossier & The Reality Check

1.1 Etymology, Names & Toponymy

Every place name across the Haukadalur geothermal basin encodes hydrodynamic violence, household tools, royal claims, or weather omens:

• Geysir (/ˈceiːsɪr/): Derived from the Old Norse and Icelandic verb geysa ("to gush, rage, or rush forth furiously"). First attested in written records in 1647 (by Bishop Brynjólfur Sveinsson after the field's 1294 seismic awakening in Oddaverjaannáll), this single vent is the nomenclatural ancestor of the English word geyser and every spouting hot spring named around the globe [S1, S2].
• Strokkur (/ˈstrɔhkʏr/): Icelandic noun for a vertical wooden butter churn: describing the rhythmic rising, falling, and plunging agitation of the water column inside its narrow sinter throat before each eruption [S1, S2].
• Haukadalur (/ˈhœiːkaˌtaːlʏr/): Compound of haukur (genitive plural hauka, "hawk" or the personal name Haukur) and dalur ("valley"). "Hawk Valley": the historic valley and medieval seat of learning (Haukadalsskóli, home to Ari Þorgilsson the Learned) at the northern margin of the southern lowlands [S1].
• Laugarfjall (/ˈlœiːɣarˌfjatn̥/): From laug (genitive laugar, "hot bathing spring") and fjall ("mountain"). The 187-meter evolved rhyolite lava dome rising immediately west of the thermal field whose silica-rich rock forms part of the geothermal system's volcanic setting [S1, S2].
• Blesi (/ˈplɛːsɪ/): From blesa ("white blaze on a horse's forehead"). "The Blazer": twin hot pools separated by a narrow natural bridge of silica sinter: a deep, crystal-clear boiling southern source basin and a cooler (~40°C), colloidal opal-blue northern runoff basin [S1, S2].
• Konungshver (/ˈkʰɔːnʏŋksˌkʰvɛːr/) & Konungasteinar: "The King's Hot Spring" and "The King's Stones": the northern cauldron and monogram-engraved boulders commemorating the state visits of Danish Kings Christian IX (1874), Frederik VIII (1907), and Christian X (1921) [S1, S2].
• Fata (/ˈfaːta/), Óþerrishola (/ˈouːˌθɛrːɪsˌhɔːla/), Litli Geysir, & Kúalaug: Supporting vents across the sinter apron: Fata ("The Bucket/Pail"), Óþerrishola ("Rainmaker Hole", which erupts when low barometric pressure precedes rain), Litli Geysir ("Little Geysir", a vigorously boiling cauldron), and Kúalaug ("Cow Pool", a muddy peripheral warm pool bordered by thin, unstable crust) [S1, S3].

1.2 Key Waypoints & Spatial Layout

Waypoint / Sub-Location	Coordinates (Decimal)	Elevation / Scale	Functional Role in the Haukadalur System
Strokkur (“The Churn”)	64.3103, -20.3007	~110 m a.s.l. · ~2.2 m conduit opening within a ~12 m pool · 15–30 m eruption height	Primary active geyser 50 m south of Geysir; erupts every 4–10 minutes via a bubble reservoir inferred at ~24 m depth west of the conduit [S1, S2].
The Great Geysir	64.3108, -20.2999	~112 m a.s.l. · ~18 m sinter bowl · ~20 m vertical shaft	Nomenclatural archetype of all geysers; formed ~1150–1294 AD; largely dormant between major South Iceland earthquakes (active 2000–2016) [S1, S2].
Blesi Twin Pools (“The Spectacles”)	64.3114, -20.3006	~115 m a.s.l. · Twin basins above Great Geysir	Paired hydrothermal pools demonstrating Rayleigh scattering: one clear ~90°C feed vent and one 40°C colloidal silica-blue basin [S1, S2].
Konungshver & Konungasteinar	64.3122, -20.3015	~118 m a.s.l. · Upper NW thermal terrace	Cauldron formed after the 1896 earthquake and royal monogram stones carved for Danish kings (1874, 1907, 1921) [S1, S2].
Laugarfjall Rhyolite Dome	64.3105, -20.3085	187 m a.s.l. · Silica-rich volcanic dome (>69% SiO2)	Lithic and hydrological anchor directing groundwater into the fault zone and supplying dissolved silicic acid (H4SiO4) [S1, S2].
Litli Geysir & Mud Pot Apron	64.3095, -20.3002	~108 m a.s.l. · Lower SE entrance zone	Boiling cauldron (~90–98°C) and acidic mud vents exhibiting thermophilic bacterial mats (Mastigocladus, Cyanidiales) [S1, S3].
Haukadalskirkja & Bergþór’s Grave	64.3175, -20.2980	~125 m a.s.l. · ~800 m N of the geyser field	Historic church and graveyard beside the traditional stone-marked burial site of the friendly mountain troll Bergþór of Bláfell [S1].

1.3 The Postcard Myth vs. Ground Reality

On the Golden Circle, the Geysir Geothermal Area (protected as a 1.2 km² natural monument on June 17, 2020, containing 364 mapped thermal spots across a 3 km² high-temperature zone) is treated as a natural water-park fountain where busloads of visitors gather behind a wet rope line to film a predictable spout on their phones [S2, S4].

Geological forensics and historical archives reveal a far stranger and more volatile mechanism:

1. Why Geysers Are Rare: Heat, Water, Silica & Conduit Geometry: Most of Iceland is dark basalt, which is low in silica. A true geyser requires pressure-tight underground plumbing capable of withstanding repeated violent steam explosions without collapsing into mud. Laugarfjall's rhyolite is part of Haukadalur's volcanic setting, but rhyolite is not a universal prerequisite for geyser formation; the heat supply, water, and confining geometry of the passages are essential. Deep boiling groundwater dissolves silicic acid (H4SiO4) from volcanic rocks and precipitates silica along the conduit walls and surface aprons as Opal-A (amorphous hydrated silica forming siliceous sinter / geyserite), lining the earth's throat in mineral crust that looks like a glass varnish [S2].
2. The Dormant Eve vs. The 1963 Re-Drilled Mechanic: Visitors standing around the rope line are almost never watching Geysir itself; they are watching Strokkur ("The Churn"), 50 meters to the south [S2]. The Great Geysir runs on seismic time: awakened by the 1294 earthquakes, choked into silence by its own silica precipitate and tourist stone-throwing by 1915/1916, briefly revived by a silica rim cut in 1935, forced to perform with buckets of soap in the mid-twentieth century, and jolted back to life by the June 2000 South Iceland earthquakes before subsiding again after 2016 [S2]. Even Strokkur (born in the 1789 earthquake and silenced by the 1896 quake) owes its modern 4-to-10-minute reliability to local residents who cleared its blocked bottom conduit with a drill in 1963 [S2].

2.0 Earth & Water Architecture (Geology, Hydrodynamics & Chemistry)

2.1 Tectonic Setting & Deep-Time Formation

The Haukadalur geothermal field lies at the northern edge of the South Iceland Lowland, where northeast–southwest striking normal faults of the Western Volcanic Zone intersect the transform fault stress field of the South Iceland Seismic Zone [S2, S4].

Unlike Hengill or Krafla, there has been no surface volcanic eruption in the immediate Geysir basin for ~10,000 years. Instead, Haukadalur operates as a "fossil" plutonic heat engine: meteoric groundwater descending from the northern highlands circulates 1 to 3 kilometers deep along active fault planes, absorbing residual heat from cooling Pleistocene magmatic intrusions beneath the evolved rhyolite lava dome of Laugarfjall (187 m a.s.l.) [S1, S2].

Stratigraphic and mineralogical dating of the siliceous sinter (hverahrúður / geyserite) apron around the Great Geysir shows that the mound began accumulating around 1150 AD, with the first documented historical geyser eruptions recorded in the Oddaverjaannáll chronicle following the violent earthquakes of 1294 [S6]. Over centuries, freshly precipitated hydrated amorphous silica (Opal-A) coats pebbles, sedges, and living thermophilic bacteria, preserving microbial structures in silica, and slowly matures through diagenetic dehydration into Opal-CT (cristobalite-tridymite) and microcrystalline quartz.

2.2 Hydrodynamics, Thermal Systems & Optical Physics

In 1846, German chemist Robert Wilhelm Bunsen traveled to Haukadalur, measured eruptions of the Great Geysir reaching 45 to 54 meters, lowered thermometers into its 20-meter vertical shaft, and formulated the foundational scientific theory of geyser mechanics (decompression boiling along the hydrostatic boiling-point curve) [S2].

At Strokkur, modern seismic and camera studies resolve a multi-phase eruptive cycle. A simplified three-part account follows, inside a roughly 12-meter surface pool with a 2.2-meter central opening that narrows into an elliptical sinter conduit fed by a bubble reservoir inferred from seismic data at 23.7 ± 4.4 meters depth, 13–23 meters west of the conduit [S5]:

1. Phase 1: Refill & Hydrostatic Suppression: Immediately after an eruption empties the upper throat, cooler water (~85–90°C) from the surface basin rushes back down the conduit. The weight of this 20+ meter water column exerts hydrostatic pressure that raises the boiling point of water at the bottom of the shaft well above 100°C (to ~120–130°C), trapping superheated liquid water like a coiled spring.
2. Phase 2: Nucleation & Bubble-Collapse Pulses: As deep heat saturates the base of the column, steam bubbles nucleate and rise into cooler upper water, where they initially collapse implosively (cavitation): generating low-frequency seismic pulses. Published 10–30 Hz signals describe ground motion; they do not establish an airborne infrasonic note or a universal chest sensation [S5].
3. Phase 3: Unloading, The "Blue Bubble" & Flash-Boiling: Once a large slug of steam escapes the laterally offset bubble reservoir and pushes the water column upward, water spills over the rim, instantly reducing hydrostatic pressure at the bottom of the shaft (unloading). The superheated deep water flashes violently into steam, expanding ~1,600 times in volume. At the surface, for a fleeting moment before rupture, surface tension holds the rising water column inside a hemispherical dome, the Blue Bubble, which appears neon azure before shattering upward into a 15-to-30-meter (occasionally 40-meter) jet. The cited mechanical studies do not establish colloidal-silica scattering as the cause of the dome's colour [S5].

2.3 Geochemistry & Surface Alteration

The thermal waters of Haukadalur are predominantly alkaline-chloride to bicarbonate-sulfate fluids (pH ~8.0–9.2 in the main spouting vents) saturated with dissolved silicic acid (H4SiO4) leached from volcanic rock, accompanied by dissolved hydrogen sulfide (H2S), carbon dioxide (CO2), and sodium/potassium ions [S2, S3].

This chemistry produces two contrasting optical and physical signatures across the field:

• Blesi's Dual Optics (Clear vs. Colloidal Opal Blue): At the twin pools of Blesi, water wells up in the deep southern basin near boiling (~90°C), where silica remains completely dissolved in molecular solution, making the pool crystal-clear. As that water overflows across a thin sinter bridge into the northern basin and cools toward ~40°C, the silica polymerizes into millions of suspended sub-micron colloidal spheres (Opal-A sols) that scatter short blue wavelengths, turning the northern pool opaque milky turquoise ("spectacles with one clear lens and one frosted blue lens") [S1].
• Mineral Residue on Optics: Water spray can leave mineral deposits on lenses, eyeglasses, or windshields. Loosen dried residue before wiping to avoid dragging grit over optical coatings. Opal-A is amorphous silica, not a collection of microscopic glass crystals.

3.0 Living Ecosystem & Seasonal Biology

3.1 Botanical Succession & Micro-Habitats

• Thermophilic Microbial Mats: Along the sinter runoff aprons, pigments create orange, yellow, green, and sometimes darker growth. Temperature matters, but so do pH, light, and community composition; colour does not provide a calibrated thermometer or a species identification.
  • Hotter Sinter Surfaces: Some surfaces remain pale, while heat-tolerant microbes inhabit others. A universal 63–65°C photosynthetic ceiling is incorrect: photosynthetic communities in alkaline hot springs elsewhere can persist around 72–73°C [S7].
  • Orange/Yellow Mats: Microbial pigments can create vivid streamers in cooling runoff. Assigning a particular Haukadalur mat to Mastigocladus or Chloroflexus requires local sampling; Chloroflexus is not a cyanobacterium.
  • Green Mats: Chlorophyll-rich communities can make cooling runoff look emerald, without fixing a universal 45°C boundary or identifying Calothrix and Phormidium by colour alone.
  • Acidic Margins: Thermo-acidophilic red algae such as Cyanidiales occur in acidic geothermal habitats, but the cited inventory does not establish their identity on particular Haukadalur crusts.
• Native Geothermal Specialists vs. Invasive Campylopus introflexus: The warm, frost-free soils around Haukadalur support rare native geothermal bryophytes such as clay earth-moss (Archidium alternifolium). In recent decades, however, the invasive Southern Hemisphere heath star-moss (Campylopus introflexus) has spread into Icelandic geothermal habitats; a specific introduction to Haukadalur on tourists' boots is not established. It is an aggressive invasive moss that exploits the geothermally heated winter turf to form dense mats that crowd out native specialists.

3.2 Fauna, Aquatic Life & Migration Windows

While the boiling alkaline vents are lethal to vertebrates, the snow-free geothermal turf and warm drainage trickles attract wintering Eurasian wrens (Troglodytes troglodytes islandicus), meadow pipits (Anthus pratensis), and spring arrivals of the European golden plover (Pluvialis apricaria, lóa) and common snipe (Gallinago gallinago, hrossagaukur), which forage for invertebrates along the heated soil margins weeks before the surrounding Biskupstungur heath thaws.

4.0 Human Stratigraphy: History, Sagas, Folklore & Material Culture

4.1 Settlement, Chronicles & Political/Religious History

• 1294: Birth in the Chronicles (Oddaverjaannáll): Although Haukadalur was a major chieftaincy and ecclesiastical school in the eleventh and twelfth centuries (home to Hallr Þórarinsson and historian Ari the Learned), neither Landnámabók nor the early sagas mention a spouting geyser there. The first historical record appears in the Oddaverjaannáll for the year 1294, reporting that violent earthquakes in South Iceland caused massive hot springs to burst forth in Haukadalur while others vanished [S2].
• 1647–1896: The Global Archetype & Royal Monograms: Bishop Brynjólfur Sveinsson of Skálholt first used the proper name Geysir in writing in 1647, and after Eggert Ólafsson and Bjarni Pálsson described its thunderous eruptions in their 1750s Ferðabók, Haukadalur became the primary destination for European scientists and monarchs visiting Iceland [S2]. Following the 1896 earthquake, Geysir hurled water 70 to 80 meters into the air several times a day [S2]. Meanwhile, visiting Danish monarchs treated the valley as a royal inspection ground, carving their crowned monograms into the boulders of Konungasteinar: King Christian IX in 1874 (after whom the erratic, color-shifting cauldron of Konungshver was named), King Frederik VIII in 1907, and King Christian X in 1921 [S1, S2].
• 1935–June 17, 2020: From Private Purchase to Protected Monument: In 1894, the core vents (Geysir, Strokkur, Blesi) were sold to foreign whiskey distiller James Craig (and later owned by Einar Benediktsson and Sigurður Jónasson), before Sigurður Jónasson donated the Geysir site to the Icelandic people in 1935; following decades of legal negotiations over the surrounding land, the entire 1.2 km² field was formally proclaimed a protected Natural Monument on June 17, 2020 [S1, S4].

4.2 Micro-History, Architecture & Domestic Survival

• 1915–1981: Silica Choking, The 1935 Ditch, and the Soap Era: By 1915–1916, the Great Geysir had largely stopped erupting as silica deposition narrowed its internal plumbing and cooling water in its wide 18-meter surface basin suppressed deep boiling (exacerbated by tourists throwing turf and stones down the shaft to provoke it) [S2]. In 1935, engineers cut a drainage notch through Geysir's silica rim to lower the surface water level and reduce hydrostatic pressure, successfully reviving eruptions for several years [S2]. When it grew sluggish again, authorities and guides resorted to soaping: dumping tens of kilograms of soft soap into the 20-meter shaft on special occasions or for visiting dignitaries [S2]. By lowering surface tension and altering bubble coalescence, soap triggered artificial flash-boils as described in the local historical account, which dates the end of soaping to 1992. Hrafn Gunnlaugsson's 1981 film intervention involved reopening and enlarging the drainage furrow, rather than a documented soap-induced eruption [S1].
• June 2000: The Seismic Comeback & Reported Heights: When the twin Mw 6.5 South Iceland earthquakes struck in June 2000, the seismic shock cleared Geysir's subterranean fractures; the Great Geysir roared back to life, with heights reported as high as 122 meters before gradually settling back into dormancy (with its last minor natural eruption recorded in 2016) [S2, S4].
• 1963: Unblocking Strokkur: Born during the 1789 earthquake and blocked by the 1896 earthquake, Strokkur sat quiet for decades until 1963, when the Geysir Committee and local residents drilled a borehole to 39.4 meters to reactivate the geyser [S5]: restoring the 4-to-10-minute cycle that carries the entire tourist workload today [S2].

4.3 Folklore, Ghosts & Hazard Encoding

• Bergþór of Bláfell: The Troll Beside the Churchyard: The defining folk legend of Haukadalur centers on Bergþór, a benign, prophetic giant/troll (bergrisi) who lived in a high mountain cave on Mount Bláfell to the north with his wife Hrefna. Unlike the man-eating troll Jóra of Hengill, Bergþór lived in peace with Christians and loved the sound of the church bells at Haukadalskirkja. Growing old, Bergþór asked the farmer of Haukadalur to give him a resting place where he could hear the bells ring and the river flow, promising that his giant iron-shod walking staff (broddstafur) would appear outside the farmhouse door as the sign of his death. One morning the staff stood by the door; the farmer hauled Bergþór's massive body down from the mountains in a coffin and buried him north of the churchyard at Haukadalskirkja, where a large stone by the churchyard still marks the troll's grave today [S1].

4.4 Cultural & Pop-Culture Footprint (Verified vs. Myth)

• João Salaviza's Short Film Strokkur (2011) & Hrafn Gunnlaugsson (1981): Portuguese director João Salaviza filmed his experimental acoustic/visual short Strokkur on site in 2011, exploring the geyser's sound and imagery, three decades after Hrafn Gunnlaugsson reopened Geysir's drainage furrow for filming in 1981 [S1].
• The "Game of Thrones Filmed at Geysir" Claim: Unverified in the supplied bibliography. Known Icelandic locations include Hengill/Nesjavellir and Grjótagjá, but their use does not by itself prove that no filming occurred at Haukadalur.

5.0 Scout & Local Insider Readout (Light, Sound, Seasons & Vicinity)

5.1 Tactile & Acoustic Profile

• The Three-Part Acoustic Signature: Standing upwind of Strokkur, the observer registers three distinct mechanical beats: (1) the Boiling Prelude, a low, rhythmic thump from the buried water-and-steam system (the 10–30 Hz range reported by researchers concerns seismic signals, not a measured chest-felt airborne tone); (2) the Surface Rupture, a sharp, concussive thump as the neon-blue hemisphere shatters; and (3) the Steam Exhalation: a tearing, broadband white-noise roar followed by the heavy, wet slap of cooling water sheets striking the sinter apron.
• Olfactory Persistence: The sulfurous odour from the boiling shafts can linger on hair and wool: clinging to hair and wool sweaters hours after leaving the valley.

5.2 Seasonal Morph & Optical Windows

• The 02:00 AM July "Midnight Glitch": Between mid-May and late July, when coach crowds vanish after 21:00, lingering northern twilight glimmers across the wet white Opal-A sinter aprons and backlights the neon-blue dome of Strokkur in an empty valley, offering unobstructed 1/2000s shutter captures of the Blue Bubble before rupture.
• Midwinter Sub-Zero Steam Pillars (December–February): In -10°C winter darkness, escaping steam freezes into ice needles on eyelashes and coats the downwind sinter walkways in glaze ice (svell), while the Blue Bubble is hidden inside a dense white condensation column until the concussive thump shakes the ground.

5.3 The Vicinity Scan (Connected Nearby Sites)

1. Gullfoss Waterfall (EP_11, 9 km E on Route 35): The 32-meter double cascade on the glacial river Hvítá at the eastern terminus of the 1907 King's Road.
2. Haukadalskirkja & Haukadalur Forestry (800 m N): The historic timber church, the grave of the troll Bergþór, and one of South Iceland's largest wooded trail networks.
3. Brúarfoss Waterfall (EP_09, 14 km SW on Route 37): The electric-blue spring-fed slot waterfall on the river Brúará.

6.0 Field Realities, Hazards & Verified Bibliography

6.1 Physical Hazards & The Stupid Tax

• Downwind Boiling-Column Strikes: The low rope barrier around Strokkur marks the sinter rim, not the fallout zone in a shifting wind. When a sudden crosswind catches a 25-meter eruption column, near-boiling alkaline water (80–95°C) sticky with dissolved silica is driven horizontally across the downwind path: causing first- and second-degree scalds to visitors standing with the wind at their face. Always check the steam drift and stand strictly upwind.
• Thin Crust Near Boiling Vents: Fragile ground beside hot springs can conceal scalding water or mud; stay on marked paths and respect barriers. Kúalaug ("Cow Pool") is a separate warm pool near Haukadalskirkja and the forest, not a boiling mud vent beside Strokkur [S1].

6.2 Practical Field & Equipment Notes

• Camera Lens Survival (Never Dry-Wipe Silica): Equip lenses with a sacrificial UV/clear filter and carry a small bottle of fresh water. If downwind spray from Strokkur mists your lens, rinse with fresh water before wiping; rubbing dried deposits with a cloth can drag grit across optical coatings. Use clean water or a suitable optical cleaner on the glass surface, following the equipment manufacturer's instructions. To freeze the split-second Blue Bubble hemisphere, shoot in burst mode at 1/1000s to 1/3200s (f/8).
• Footwear: Wear high-traction boots and carry steel microspikes (broddar) from October through April, when condensing steam coats the boardwalks and sinter paths in black ice.

6.3 Open Research Notes & Source Discrepancies

• Maximum Eruption Height of Great Geysir (70–80 m in 1896 vs. 122 m in 2000 vs. 170 m in 1845): Historical accounts from 1845 reported an exceptional eruption reaching ~170 meters, whereas Robert Bunsen's instrumental measurements in 1846 recorded 45–54 meters, post-1896 earthquake observations recorded 70–80 meters, and popular accounts report up to 122 meters after the June 2000 earthquakes. These historical height claims should be attributed; the present bibliography does not establish comparable measurement methods for all of them.

6.4 Verified Sources & Archives

• [S1] Guide to Iceland (Regína Hrönn Ragnarsdóttir): The Extraordinary Geysir Geothermal Area: Strokkur and All the Other Hot Pots in South Iceland · Verified passages: Spatial layout of Geysir, Strokkur, Blesi, Konungshver, Konungasteinar (1874, 1907, 1921 royal visits), Fata, Litli Geysir, Laugarfjall, and the surrounding features. For Kúalaug and the church, see the author's separate Haukadalsskógur account: https://guidetoiceland.is/connect-with-locals/regina/the-extraordinary-geothermal-area-of-geysir-strokkur-with-a-dip-in-kualaug-hot-pool . For Bergþór, see Jón Árnason, Bergþór í Bláfelli (1862), including burial north of the churchyard: https://is.wikisource.org/wiki/Íslenzkar_þjóðsögur_og_æfintýri/Goðfræðisögur/Bergþór_í_Bláfelli .
• [S2] Golden Circle Day Tours: Geysir Geothermal Area, Iceland: visitor overview. Detailed history should be distinguished from this general guide. Regína's account [S1] describes the 1935 drainage channel, its 1981 reopening for Hrafn Gunnlaugsson, and the end of soaping in 1992. The Nature Conservation Agency records Geysir's last known eruption in 2016: https://www.ust.is/english/visiting-iceland/protected-areas/south/geysir-area/nature-and-geology/ .
• [S3] Volcanic Springs: Geysir / Haukadalur Thermal Feature Inventory · Verified passages: Catalog and geochemistry of Blesi, Konungshver, Litli Geysir, Litli Strokkur, Smiður, Sóði, and Laugarfjall.
• [S4] International Parks: Geysir Geothermal Area Natural Monument Dossier · Verified passages: June 17, 2020 Natural Monument protection (1.2 km² within a 3 km² high-temperature zone), 364 mapped thermal spots, Laugarfjall lava dome, and 30-meter Strokkur eruptions.
• [S5] Eibl et al. (2021), Eruptive Cycle and Bubble Trap of Strokkur Geyser, Iceland, JGR Solid Earth, doi:10.1029/2020JB020769. Seismic source locations, conduit/pool dimensions, 1963 borehole, and eruption-cycle signals. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020JB020769
• [S6] (2021), Multifaceted incremental growth of a geyser discharge apron: Evidence from Geysir, Haukadalur, Iceland, Sedimentary Geology. Opal-A sinter, preserved microbial and plant material, and inferred activity since ~1150 CE. https://www.sciencedirect.com/science/article/pii/S0037073821000579
• [S7] US National Park Service, Thermophilic Communities. General background on temperature, chemistry, pigments, and photosynthetic life up to about 73°C; not a local species survey of Haukadalur. https://www.nps.gov/yell/learn/nature/thermophilic-communities.htm

Questions and answers

Does the Great Geysir in Iceland still erupt, or are visitors watching Strokkur? The Great Geysir, which gave English the word geyser, is currently dormant (last erupting naturally between the June 2000 earthquakes and 2016); the geyser that erupts every 4 to 10 minutes to heights of 15–30 meters is its neighbor Strokkur ("The Churn"), located 50 meters south.

Why does Strokkur form a glowing blue bubble right before it erupts? Rising steam pushes water into a transient dome before it bursts into a jet. Seismic work infers a bubble reservoir at 23.7 ± 4.4 meters depth, offset west of the conduit; it does not establish Blesi's colloidal-silica colour mechanism for Strokkur's dome [S5].

Why did Geysir form in Haukadalur instead of elsewhere in basaltic Iceland? Geyser activity requires heat, water, and suitable confining passages. Silica deposition helps shape Haukadalur's plumbing, and Laugarfjall's rhyolite is part of its volcanic setting; basalt is not intrinsically incapable of supplying dissolved silica.