14. Pyroclastic flows and surges.Dan Barker March 2009

S coast of Pantelleria

Eruption clouds with high concentrations of pyroclasticmaterial can form in several ways. 1) The mass fraction of particles is so high that the bulkdensity exceeds that of air at the same height2) The magma discharge rate decreases, so there is notenough hot material to heat the entrapped air and makeit buoyant3) The ejected material becomes coarser or cooler, so it heats less air4) Probably a corollary of 1), a lava dome or volcanoflank collapses, launching a dense cloud downslope.

In any of these events, the cloud accelerates rapidlydownward until it reaches the surface and spreads asa high-velocity pyroclastic density flow. There are twokinds, surges and pyroclastic flows.

Surges are more dilute suspensions than flows, and have densities that are higher than that of air but lessthan that of liquid water. Forward movement is unsteady, producing stratification as material settles on the surface.Cross bedding, dunes, and antidunes are common; so are accretionary lapilli if the surge carried condensing steam.Blocks and bombs may produce impact sags if they cameinto the surge by "ballistic" parabolic trajectories, but manyother blocks and bombs have no impact sags because theywere carried laterally by the surge, along with smaller particles. Some surges are dense enough and fast enoughto erode the ground over which they travel.

Surges from the maar at Kilbourne Hole, NM, show cross bedding.

Surges in Haroharo caldera (NZ) show dune structures.

These surges are on the rim of Cerro Colorado tuffring, Pinacate volcanic field, Sonora.

This block has an impact sag; the source vent for it andthe surge was close by. Crater Elegante, Sonora.

Look Ma, no sags! Blocks carried laterally by a surge, Burro Mesa, Big Bend National Park, TX

Surges with rock fragments, near Cannetto, Lipari

79 AD surges over buildingfoundation, Herculaneum

79 AD surges on amosaic floor, Herculaneum

Surges at Boatman's Harbour, South Island, NZ

These surges were deposited on a steepslope, and slumped downhill. Boatman'sHarbour, NZ.

Soft mud lump carried by surge, Wingertsberg,Laacher See, Germany. K. Bell for scale.

Surges over pahoehoe, Santiago Island, Galapagos

Surges, Tent Rocks National Monument, NM

Surges, Tent Rocks, NM. K. Kempter for scale

Antidunes (crests moved toward the source, to the right) in surges, Pulvermaar, W. Eifel district, Germany

Mt St Helens: May 18, 1980 surges banked against tree trunksthat had already been felled by the lateral blast.

In 1790 there was an unusual eruption at Kilauea that produced a muddy surge that killed many Hawaiians inthe Kau desert. These are eroded dune forms.

Here are some poorly preserved footprintsalong the crest of a dune in the 1790 surge.

79 AD surges (dark) alternated with airfalls until a pyroclastic flow (top) ended the sequence. Oplontis, Italy. These all buried and preserved a Roman villanow being excavated. Exposure is about 2 m high.

airfall

surge

airfallsurgeairfall

flow

This is the cast of a tree trunk that was in the garden of thevilla and stood through airfalls and surges, but then was torn off at the base of the pyroclastic flow.

These doors were ripped from their frames by a surge,which then stacked them at the rear of the villa.

Pyroclastic flows have bulk densities greater than 1g/cm3 and more commonly erode their substrate than do surges.Movement is steadier than in surges, and stratification is rarely present. Particles may be graded by size and density,so that the larger and more dense rock fragments areconcentrated near the base, and pumice toward the top.The higher concentration of particles in pyroclastic flowsallows heat to be retained longer while moving. If the temperature is high enough (at least 600°C) and the flow is thick enough, the individual glass and pumice particles may begin to weld (flatten and stick together) while rock fragments remain undeformed. Welding is common in pyroclastic flow deposits, less so in surges, and rare in airfalls (usuallythose that have fallen on hot lava or pyroclastic flows, orhave rapidly been buried by them). Flattened and collapsedpumice fragments are common in pyroclastic flows. Theyare called fiamme (Italian for "flames") for their shapes.

Pumice lumps are notdeformed in this nonweldedpart of the Battleship RockTuff, Valles Caldera, NM.

This is a pyroclastic flow that filled a canyon.

Deeper in the canyon filling, retained heat and pressure of the overlying flow welded the material, collapsing the pumice lumps but leaving the rock fragments undeformed.

By the way, an older term for a pyroclastic flow deposit is ignimbrite ("fire cloud rock").

Bedding plane view

Cross section

The upper, less welded Bishop Tuff still hasrecognizable pumice lumps. Owens River Gorge, CA.

Lower, the Bishop Tuff is a hard, densely welded rock.

This coffin lid of an Etruscan nobleman (~200BC)does not memorialize a skin condition. The grayblotches are fiamme (collapsed pumice). Partly welded tuffs are favorites for carving, because they can be shaped with woodworking tools.

Here an entire Roman amphitheatre was carved outof an outcrop of a pyroclastic flow deposit. Sutri, Italy.

This is densely welded Gomez Tuffin Madera Canyon,Davis Mtns, TX.The fiamme havebeen pressed flat,then wrinkled intosmall folds.

Here, in the same outcrop in Madera Canyon, the foldsare larger but a rock fragment is not deformed.

Pale fiamme in the Rhyolite Canyon Tuff, ChiricahuaNational Monument , AZ

The mold of this small tree trunk shows that the tree stood throughseveral pyroclastic surges. Salt Lake Crater, Oahu.

But just above, a horizontal tree trunk mold (arrow)in a pyroclastic flow shows that this tree was downed.G.P.L. Walker for scale.

At the far end of their travel, two pyroclastic flows(arrows) pinch out at a topographic obstacle, butairfalls drape right over it. Haroharo caldera, NZ.

Pyroclastic flows can be erosive. Here the SantanaTuff has picked up fragments of the underlying lavaas it moved from right to left. Lajitas, TX.

The most extreme example of erosion by a pyroclasticflow is in this cut on Kanuka Rd, North Island, NZ. Theflow moved from left to right and not only beheaded asmall ridge but overturned the beds.

The 186 AD Taupo eruption, NZ, first deposited airfall and surges in the little valley, then carried large glassy blobs (probably from a lava dome) in a pyroclastic flow. The vertical scale in this image is about 25 m.

The chips in thispyroclastic floware spalled lavafragments froma collapsed lavadome. Burro Mesa Pouroff,Big Bend N P

Lava dome collapse forms a specific kind of pyroclasticflow, a block-and-ash flow, with large blocks in afine-grained matrix. Keyhole Falls, BC

Block-and-ash flow, Puy de la Tache, Mont Dore,France.

Pyroclastic flows can travel tens of kilometers fromtheir source. Bandelier Tuff caps this dissectedplateau east of the Valles Caldera, NM.

Pyroclastic flow delta, Augustine volcano, AK