Volcanic explosions are one of nature’s most extreme displays of power. A common and devastating result of volcanic eruptions is the fast movement of hot gas and rock that flows away from the volcano. The Greeks called them πῦρκλαστός (pronounced pyr klastós) which loosely translates to ‘broken fire’. Nuée ardente is another name used to describe these events which is French for ‘burning cloud’. In the science world today, they are referred to as pyroclastic flows.
Here is a link to a pyroclastic flow video. Some flows, like this one, are created by structural collapse of the volcano.
Pyroclastic flows normally hug the ground and travel downslope. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope. Pyroclastic flows can reach speeds up to 700 km/h (450 mph) and temperatures of about 1,000 °C (1,830 °F) (3). With this information in mind, it is easy to imagine how these flows can be so exceedingly destructive and dangerous.
Here in Santorini our class spent quite a bit of time studying the different phases of the Minoan eruption. The eruption took place in four phases, and scientists agree that the pyroclastic flows occurred during the third and fourth (1). For more information on the different phases read Carly Stefano’s blog called A Geologic Lesson for the Little Ones.
The third and fourth phases are not found everywhere on Santorini, which means that the flows did not cover the entire island. In fact, the deposition of the flows can fluctuate from place to place if they are found at all (1). Pyroclastic flows can vary in temperature and fluidity based on the composition of the magma. Water can invade the magma chamber altering the entire eruption by lowering the temperature (it is still very hot), as well as making the composition much wetter. This is what happened during the Minoan eruption on Santorini and it is called a phreatomagmatic eruption. Phreatomagmatic eruptions are much more explosive and lead to colder pyroclastic flows being deposited.
Our class found two different kinds of flows on Santorini. We found hot pyroclastic flows on the southern parts of the island and colder flows on the northern end. This change in flow type is due to the shifting levels of sea water throughout the eruption. First the cold pyroclastic flows were deposited due to cracks in the volcanic vent allowing water to mix with the magma chamber. Then the vent became water tight again leading to hotter flows which are what destroyed what was left of ancient Akrotiri (2). Scientists disagree as to how the vent became closed off again, but one theory is that a tuff ring formed around the vent cutting it off from the seawater. A tuff ring is a type of volcano that is common in phreatomagmatic eruptions that is formed when the vent is filled with water and surrounded by a rim of ejected material that was probably formed by explosive interaction of magma and groundwater.
One way to distinguish the two types of flows is by observing the lithic fragments in the deposit (the bits and pieces of pre-existing volcanic rock) found mixed in with the volcanic material. The colder pyroclastic flows are from a more explosive eruption and have a higher percentage of lithic fragments than the hot flows (Figure 1). This is because of the higher level of destruction of the adjacent country rock at the vent location A collection of basement rock (older underlying rock that predates the volcanic activity) was also found in the pyroclastic flow which was brought up by the eruption from places like the sea floor (Figure 3). The lithic fragments of the hotter pyroclastic flows originate from a much shallower depth due to a calmer eruption style (Figure 2). Using these observations our class identified and studied both these types of pyroclastic flows.
Being able to see the remains of such powerful and destructive events was a memorable experience that doesn’t even compare to pictures in a book. Studying them was another experience entirely, as it tested my knowledge in ways that no classroom setting could offer. Seeing the different layers first hand gave me a new perspective on pyroclastic flows allowing me to comprehend just how mighty they can be. Imagine an earth shattering explosion ejecting massive loads of rock and ash, which is then followed by a huge pyroclastic flow filled with broken fragments of searing rock that incinerates everything in its path. Surely nothing can stand in the way of this deadly mechanism. Broken fire is truly a fitting name for such events.
- Druitt, T.H. 2014, New Insights into the Initiation and Venting of the Bronze-Age Eruption of Santorini (Greece), from component analysis, Bull Volcanology, 76:794
- Friedrich W.L., 2009, Santorini: Volcano, Natural History, & Mythology, Denmark: Aarhus University Press, 312 P.
- Sidell, K.M. 2008, Pyroclastic Flows and Their Efffects: http://volcanoes.usgs.gov/hazards/pyroclasticflow/