Pyroclastic flows, pyroclastic density currents (PDCs), or pyroclastic clouds are dense, ground-hugging, hot gases laden with volcanic debris racing down the slopes of a volcano.
These high-density fluidized mixtures of hot gases, entrapped air, ash, pumice, blocks, and other fragments are extremely hot. Also, they flow at high speeds under gravity, like avalanches, and can travel great distances.
The high speed, heat, and density make PDCs the most devastating volcanic hazards that cause severe destruction and deaths. They have historically killed many people and burnt and buried villages and towns.
A closely related term is Nuées ardentes. Nuées ardentes is a French word for glowing clouds. It refers to incandescent pyroclastic flows that glow red in the dark or at night. Such have incandescent pyroclasts.
Another term you should know is pyroclast. It originates from two Greek words: pýr = fire, and klastós = broken in pieces. It refers to the rock debris or fragments broken during an explosive volcanic eruption.
These fragments can be magmatic, i.e., from erupting lava, or lithic if from country rock. They include ash, lapilli, volcanic bombs, and blocks, depending on their size.
Lastly, the moon and Mars have an equivalent of pyroclastic flow.
Facts and characteristics of pyroclastic flows
Some of the characteristics of pyroclastic density currents include the following:
- Speeds: PDCs are highly mobile, and most travel at an average speed of at least 100km/hr. (28m/s) . But can reach 700 km/hr. (194m/s). Speed depends on slope gradient, volcanic output rate, and density of the flow or current.
- Temperatures: Pyroclastic flows are extremely hot, with temperatures of 200 °C (392 °F) and 700°C (1292°F. However, they can reach up to 1000 °C (1832 °F).
- Size: Most are relatively small, with volumes of about 1-9 km3 (0.25-2.25 cubic miles). Nonetheless, they can be large, 1000Km3 (240 cubic miles), but such haven’t occurred over the last millennium.
- Distances: They travel tens of kilometers, most going 10-15 km (6-9 mi). However, some can travel as far as 100 km.
- Appearance: PDCs appear as thick, greyish, or blackish turbulent clouds, with the lower part of the ground-hugging and hurtling down the volcano slope and spreading laterally under gravity and upper turbulently rising.
- Flow: Most flow in topographically low areas and valleys, but dilute ones or those at high speeds can surmount topographical barriers or go some distance if they are fast enough. Also, they can gain momentum and speed during flow.
- Erosion: They can erode low topographical areas like river valleys or create new ones as they spread radially downslope from the vent. The eroded materials are remobilized into the flow. Their erosion power depends on speed, density, temperature, size, and surface characteristics.
Type of pyroclastic density currents
There are two types of PDCs: pyroclastic flow and pyroclastic surges. They can develop together, or one can evolve into the other. Basal surge is only a type of pyroclastic surge.
1. Pyroclastic flow
Pyroclastic flows are high-density currents of hot gases, ash, pumice, blocks, and other fragments moving fast down the slopes of a volcano.
They are denser than pyroclastic surges due to a high debris-to-gas ratio. Some can have as much as 80% debris.
Usually, they flow primarily in valleys and topographically low areas following drainage patterns. This makes them predictable. However, at high velocity, they climb hills and surmount ridges.
When flowing over oceans, the larger debris will fall into the water, causing it to boil and producing steam explosions that propel lighter flows faster.
An example was filmed in Soufriere Hills volcano in Montserrat, traveling 1 km (0.5 nmi). In Stromboli, it flowed hundreds of km above the sea.
While flowing, pyroclastic flows can create surges that can flow ahead or away.
Lastly, pyroclastic flow deposits are poorly sorted, concentrate mainly in low topographical areas, and are less than a meter to over 200 meters thick.
2. Pyroclastic surges
Pyroclastic surges or fully diluted pyroclastic flows are fast-moving, low-density currents of hot gases, ash, crystals, and pumice.
However, cold pyroclastic surges can form below 250°C (482 °F). Such may have steam, water, or rocks, often resulting from eruptions under a shallow river or lake vent.
Due to finer and lower particle concentrations, they are lighter or less dense and flow turbulently. Therefore, they have enough energy to surmount topographical barriers or obstacles like ridges and extend over mountains and hills.
Also, they can move over water like seas, rivers, or oceans. For instance, surges from the 1883 Krakatoa eruption reached Sumatran, 48 kilometers (26 nautical miles) from the volcano.
These flow behaviors make their flow less predictable and dangerous.
Don’t mistake their lower momentum due to high gas content and shorter distances they travel; pyroclastic surges are very hazardous. What killed 30,000 at Saint Pierre from Mount Pelée 1902 eruption pyroclastic surge.
Lastly, their deposits are better sorted and show flow features like low-angle cross-bedding. Also, these deposits cover smaller geographical areas, are thinner, and have primarily crystals and lithic or rock fragments.
3. Base surge
Basal surges are a type of pyroclastic surge associated with phreatomagmatic eruption. These eruptions occur when hot magma interacts with shallow, surface, ground, or permafrost.
The resulting explosion will send lateral moving ground-hugging surges of gas, steam, and pyroclasts from the base of the eruption column.
Usually, basal surges rarely exceed 5 to 6 kilometers and are associated with maars and tuff cones.
How do pyroclastic flows form?
They form from 1) eruption column or fountain collapse, 2) collapse of lava domes or steep fronts of thick lava flow, 3) lateral blast or eruption, and 4) direct frothing or boiling over the vent.
Eruption column or fountain collapse
Explosive eruptions produce high-speed jets of gas and debris (ash, lava, and other debris). They will incorporate heat air that expands, causing convective currents to form an eruption column that rises turbulently.
However, the eruption column may lack enough momentum or upward thrust/buoyancy.
This will collapse the column, sending the denser ejected material cascading down the volcano flanks, forming pyroclastic flows.
2. Collapse of lava domes or spines and fronts of lava flow
Collapse or disintegration of forming lava domes, spines, or fronts of thick lava flow can release an avalanche of lava debris and sometimes gas downslope under gravity.
These avalanches will further fragment as they tumble downslope. Also, they will entrain and heat gas, causing it to expand above and in front of it and grow. This can form a pyroclastic flow, often called block and ash flow.
3. Direction or lateral blast or eruption
Pyroclastic density currents can also form from lateral explosive eruptions due to 1) landslides or 2) sector collapse that destroys part of the volcano.
These eruptions propel fast-moving jets or hot gases and fragments ejected sideways, not vertically. As they move away from the volcano, they will transform into gravity-driven PDCs like during the May 18, 1980, Mt. St. Helens eruption.
4. Frothing or boiling over
Frothing happens when some volcanoes erupt explosively without forming high plumes. Instead, a dense froth of hot gases and debris will pour over the vent lip and rapidly move downslope under gravity, as seen in Novarupta in 1912.
Parts of pyroclastic flows
Pyroclastic clouds or density currents have two parts, i.e., stratified into two: the denser basal flow or avalanche and the overriding ash plume.
1. Basal flow
The basal flow is the dense, ground-hugging basal avalanche with laminar flow. It is close to the surface and has coarser boulders and some rock fragments rolling on the ground.
Basalt flows can erode river valleys or create new ones. Also, it can overcome smaller ridges with its deposits chaotic.
2. Ash plume, phoenix cloud, or coignimbrite plume
Ash plumes are hot, thick ash and gas clouds turbulently rising above the descending basal flow.
They form when basal flow incorporates and heats cold air that expands and rises convectively. The rising gas will buoyantly carry a dense cloud of volcanic ash and small fragments.
This turbulence suspension of volcanic ash can rise kilometers high and is less constrained by topography. Later, it will fall, covering a large area downwind and forming thin deposits.
Pyroclastic flow hazards and examples
Pyroclastic flows and surges are the most hazardous and deadly volcanic hazards because of their high speeds, kinetic energy, temperatures, and great distances. Also, they are unexpected or sudden and almost silent.
PDCs, including small ones going a few kilometers, will:
- They destroy, bury, carry, or even burn anything paths. This includes destroying farmlands and burning forests.
- Incinerate, carbonize, and kill people via burns, asphyxiation from ash, and toxic gases or trauma. People living on flanks are the most vulnerable.
- Melting snow or ice causes deadly volcanic mudflows or lahars.
- Dam rivers create lakes that later breach the dam, causing flooding and lahars.
Notable examples of pyroclastic flow and surges examples that caused deaths are:
- The 1902 eruption of Mount Pelée eruption pyroclastic flows and surges killed about 30,000 people, destroying St. Pierre within a few minutes.
- In 1985, they triggered lahars at Nevado del Ruiz volcano in Colombia, destroying 5000 homes and killing about 23,000.
- 79 AD Eruption of Mount Vesuvius generated ashfall and PDCs that engulfed Pompeii and Herculaneum, Italy, killing many people.
- March-April 1982: El Chichon volcano’s PDCs destroyed villages and killed 2000 people in Mexico.
- Pyroclastic density currents on June 3, 1991, on Mt. Unzen in Japan killed 43 people, including volcanologists Harry Glicken, Katia Krafft, and Maurice Krafft.
- In 1997, the Soufrière Hills volcano dome collapsed, causing PDCs that killed 19 people.
Pyroclastic flow and surge deposits
Pyroclastic flows and surge deposits are less than a meter to several hundred meters thick. For instance, during the Mount Pinatubo 1991 eruption in the Philippines, PDCs buried Marella River Valley 50-200 meters.
Deposits include ash (glass shards, crystals, and pieces of rocks), lapilli, pumice, blocks, and bombs.
They can be sorted or poorly sorted, show cross bedding or no bedding/internal stratification. Some are duned.
These deposits include ignimbrite or ash-flow tuffs and block-and-ash flow. They can be welded or unwelded depending on emplacement temperatures.
Some deposit sizes are small and concentrated in valleys and other topographically low areas. Others are large, blanketing hundreds of square kilometers, especially around the caldera collapse in plateaus.
Frequently Asked Questions (FAQs)
Pyroclastic flows are expected in explosive eruptions like Vulcanian and Plinian, often involving silicic magmas like rhyolite or dacite. However, mafic, i.e., basaltic pyroclastic density currents, can occur in phreatomagmatic eruptions.
Pyroclastic flows form mostly on composite volcanoes. Also, most resurgent calderas or huge caldera collapses cause massive PDCs covering vast areas inside and on areas surrounding the caldera.
PDCs have a mixture of volcanic 1) debris generated from explosive eruptions that include (pulverized rock, crystals, glass), pumice, scoria, volcanic blocks, and bombs, 2) exsolved gases, and 3) trapped air. Gases include superheated steam, carbon dioxide, sulfur, hydrogen sulfide, and air.
They travel fast and far because of 1) hot and expanding exsolving magmatic gases and 2) expanding and heated air incorporated cause buoyancy and fluidization. Fluidization minimizes friction and collision, favoring almost frictionless motion under gravity and power to destroy.
What are the main differences between a Nuée ardente and pyroclastic flow? Are these two the same or do they have differences? If they are different, what are these differences?
Nuée ardente is a French term that translates to glowing or burning cloud. It describes the red glowing pyroclastic flow seen at night.
Pyroclastic surges glow because they are ejected at very high temperatures, especially those close to the vent and ground.
Therefore, there is no difference between pyroclastic flow or nuées ardentes, except that the latter appear red at night.