Glaciers, Volcanows & Jokulhlaups

What is glaciovolcanism?

Eruptions that interact with ice, snow, or meltwater; controlled by ice thickness and drainage efficiency.

What landforms result from glaciovolcanism?

Tuyas (flat-topped volcanoes under thick ice) and Tindars (elongate ridges from short fissure eruptions).

What controls eruption style under ice?

Ice thickness and pressure: thick ice = effusive pillows; decompression = explosive fragmentation.

Describe Stage 1 of a tuya eruption.

Dike reaches ice–bed interface; magma erupts effusively, melting cavity and forming pillow lavas.

Describe Stage 2 of a tuya eruption.

Pillow mound collapses; pressure drops; phreatomagmatic explosions create hyaloclastite breccia.

Describe Stage 3 of a tuya eruption.

Eruption breaches meltwater surface; foreset beds and lava delta form; subaerial lava cap builds; passage zone marks water level.

Describe Stage 4 of a tuya eruption.

Ice melts; tuya stands exposed with flat top; if eruption stopped earlier, a tindar ridge remains.

Give one example of a tuya.

Herðubreið, Iceland – classic 4-stage tuya formed under thick ice.

Give one example of a tindar.

Helgafell near Hafnarfjörður – subglacial mound that never breached the surface.

What happens when a stratovolcano erupts with snow or ice on its slopes?

Ice and snow melt rapidly, producing lahars or jökulhlaups.

Name three melting mechanisms on glaciated stratovolcanoes.

Lava contact, pyroclastic flows, and geothermal heating.

What are lahars?

Mudflows formed when volcanic debris mixes with meltwater; common on thinly glaciated stratovolcanoes.

What is a jökulhlaup?

A glacial outburst flood caused by pressure buildup and sudden drainage of subglacial meltwater.

Where are glaciated stratovolcanoes most common?

Alaska, the Andes, Cascades, and Iceland.

Give two examples of glaciated volcano floods.

Nevado del Ruiz 1985 (lahar disaster) and Katla 1918 (massive jökulhlaup).

How does ice thickness affect eruption style?

Thin ice → rapid surface melt → lahars; thick ice → trapped meltwater → delayed jökulhlaup.

What is an ice cauldron?

A depression on glacier surface caused by melting from geothermal or volcanic heat.

How do ice cauldrons relate to jökulhlaups?

They mark where meltwater collects beneath ice; when pressure exceeds overburden, drainage occurs as a jökulhlaup.

Describe the Nye Channel Model.

Slowly rising flood through subglacial tunnel melted open by friction; classic Grímsvötn-style flood.

Describe the Sheet Flow Model.

Rapid flood spreading beneath glacier as broad sheet; sudden onset and high discharge (e.g., Katla, Gjálp).

Name three monitoring methods for cauldrons and floods.

GPS (uplift/subsidence), radar RES (maps cavities), and river sensors (conductivity spikes).

Give two examples of monitored cauldrons.

Skaftá cauldrons and Grímsvötn, both in Vatnajökull.

Describe one hazard of jökulhlaups.

Extreme erosion and sediment deposition forming sandur plains; destruction of bridges (e.g., Skeiðará 1996).

What does Manning’s equation calculate?

Mean flow velocity and discharge of water in an open channel or jökulhlaup.

Write Manning’s equation for velocity.

V = (1/n) R^(2/3) S^(1/2)

What does Q = A×V represent?

Discharge (m³/s) = cross-sectional area × velocity.

Define each symbol: A, P, R, S, n.

A = area (d×h); P = wetted perimeter (d+2h); R = A/P; S = slope; n = roughness coefficient.

Typical roughness values?

n = 0.03 (clear water flow) or 0.1 (ice/sediment-rich flow).

Typical slope values?

Glacial valleys 0.015–0.03; gentle sandur 0.01–0.02.

Units of Q, V, R, and S.

Q in m³/s; V in m/s; R in m; S dimensionless.

Example of Manning calculation setup.

d=400 m, h=15 m, S=0.03, n=0.03 → Q≈2×10⁵ m³/s.

How does roughness (n) affect discharge?

Higher n = more friction → lower velocity and smaller Q.

Interpretation of Q results.

High slope and low n = faster, more erosive flood; low slope or high n = slower, depositional flood.