Into the Air: a Sky Filled with Ash, Gas, and Glass

Growing up in the mountains of Colorado I appreciate fresh, clean air. As I inhale, I greatly enjoy the crisp aromic air of Santorini.  In general we take for granted the air we breathe in, we know that it’s oxygen, nitrogen with a little too much carbon dioxide sometimes, and some places are more humid than others. Otherwise most places we go we don’t have to worry about blocks of hot volcanic rock the size of small cars zooming through the air or breathing in high amounts of methane, carbon dioxide, liquid glass, and hot ash.  During the cataclysmic Minoan eruption in 1613±13 BC the air here resembled the latter. Almost instantaneously the air went from out typical Santorini weather to being suffused with acidic gases, ash, pumice, and various rocks filling the sky above. As we went to various outcrops on the island we saw evidence of the massive amount of material that filled the sky’s that day 3600 years ago.

The Minoan eruption may have lasted only one day but the complexity of the explosion can be represented by the hours one can spend arduously examining, describing, and interpreting each stratigraphic layer; some only a centemeter thick and representing its own distinct event.  In general the entire eruption is studied in five phases starting with phase 0, the warning phase, each with a distinctive look and type of event and each changing the content of the air.

Phase 0

Although it is only an average of 4cm thick, it tells us a story of the initiation of the eruption.  We first studied this phase near Vlychada (a hard to pronounce beach on the south coast of Thera) where the meters and meters of material above owe its production to this small distinguished layer.

Figure 1: The outcrop at near Vlychada beach shows the here 4cm warning layer followed by phase 1,2, and 4. Note: phase 3 does not appear at this outcrop although there is a scour channel and an erosional contact. This is evidence from some sort of stream that also appears in other locations between phase 3 and 4.

On a day 3600 years ago ash and pumice were suddenly ejected into the Aegean air. We know that the smaller the material the more efficient or violent, the eruption was. We also know that when water enters a volcanic chamber the results are even more violent; imagine pouring cold water into boiling oil. The water instantaneously flashes to steam and expands, causing rapid expansion of gasses and in this case results in the surrounding rocks to shatter and causes an array of hydrothermally altered red, yellow, and black rock fragments to be shot into the air.  Although only visible through my handlens, these rock fragments tell of an sky full of shattered volcanic rock and steam.  The northern winds of Santorini that still exist today carried these fragments to the southern part of the island and mark only the beginning to a sky of stones and gas.

Phase 1

This phase is a little more noticeable in the stratigraphy.  One of the first outcrops we visited, and only a 5 minute walking distance from our hotel, the Fira Quarry is a great example of the massive amounts of material that erupted in the sky lying down 4.41m (13.6ft) of ash and pumice across the island. A purely magmatic (caused by the rapid depressurization of the chamber and conduit with no water interference) sub-plinian eruption, pumice would have been erupting and reaching heights just bellow the stratosphere at 11 miles in altitude.  Extremely hot liquid glass rapidly cooling and containing bubbles of gas expanding, stretching, and crystallizing almost instantaneously as they are thrown and blanket the landscape like snowfall. Although continuous, the eruption would have an almost sputtering behavior, changing in violence and height.

Figure 2: Although this bed is not a part of the Minoan eruption, this view from our boat on boat day is a great example of the even, snow-like blanketing that pumice fall creates.

Phase 2 

The first time I saw this deposit I was confused. I didn’t know what could have caused these huge crossbeds that I had only seen in correlation with massive sand dunes. Lisa let us ponder the processes that could create such bizarre beds while we took a couple hours to make stratigraphic columns of the outcrop. After meticulously drawing in all the details and descriptions I have the skill to observe the explanation was welcome.

The opening of the vent during the first phase would naturally leave the volcano susceptible to the introduction of water of the surrounding Aegean sea. As mentioned before, the addition of water causes phreatomagmatic eruption (caused when water and magma interact) that tend to be more violent and the introduction to a seemingly endless water supply front he Aegean lead to an  violent explosion.

As sea water entered the volcano and immediately flash vaporizes, massive amounts of volcanic material was shot laterally from the vent.  In this case it created what is known as a pyroclastic surge. A vast cloud of gas, steam, ash, and energy created a turbulent surge of material that rushed across the landscape and ocean, rounding the pumice and creating crossbeds as the material builds upon itself.  Pyroclastic surges are known to have velocities up to 200 km/hr.

Water droplets form as the vapor condenses and ash accumulates around the droplets and small peices of pumice creating small balls called accretionary lapillli. Like a snowball, these 5mm balls of ash build themselves, creating small onion layers as they fly and tumult through the air.

Figure 3: Like a little snowball formed from wet ash accreting in the air, accretionary lapilli are very fragile and when cut open have rings similar to an onion.

As sea water continues to enter the vent, it widens and the earth surrounding the vent fragments and  gets incorporated in the surge.  Near the end of the phase larger and large pieces of rock are ejected in the air, some the size of smart cars.  These ballistic projectiles are shot into the air and land as far as 8km (5 miles) from the vent.

Figure 4: On the left this particularaly large 3m block was a ballistic projectile that was ejected and thrown 5km front the vent and plunged meters deep into the wet ash with me for scale. On the right is two large blocks in ancient Akrotiri (8km from the vent) with Natalie for scale.

Due to the amount of water in the eruption the ash has almost plastic-like qualities and as the blocks pellet the ash it bends and creates block sags. Imagine throwing a large rock into thick mud and the mud bending around the projectile.

Figure 5: A block sag at Fira quarry shows the bending of the wet ash under the impact of the block.

Phase 3

 This phase shows that the excavation of the vent continued as the sky became full of volcanic gasses such as carbon dioxide, sulfer dioxide, hydrogen sulfide, methane, carbon monoxide, and of course water vapor.  As the air became more acidic the eruption moved north to a new vent that ejected new material as well as fragmenting old volcanic rocks and creating a plume that builds and mushrooms in the air until gravity and convection drives the whole cloud to flow back to earth and creates anywhere from 3-50m of pumice, ash, varying sizes of rocks scattered throughout.

A small pause. The accumulation of massive amounts of gasses and water vapor pour into the atmosphere and even reach Plinian heights in the stratosphere where gasses and ash get incorporated into the jetstream and travel around the world.  These gasses don’t easily leave the stratosphere and create a reflective surface that the sun’s rays can’t penetrate causing global cooling.  Locally, these gasses condense and the heat from the eruption condenses even the moisture previously in the air.

Figure 6: Taken from the international space station the eruption of Srychev volcano in Russia shows how a Plinian eruption condenses the moisture in the air (the white part of the plume)  on the ash and gas (the grey/tan part of the plume). Source: NASA

This slight gap in the eruption gives this weather an opportunity to create streams and erosion on the recently created topography of the island. The extremely acidic, putrid air is in stark contrast to the air I breath in now.

Phase 4

This phase is split in two different parts, 4a and 4b, and we traveled to two different beaches on different sides of the island to observe this final event. The water supply that drove the previous phases is cut off and as a result this phase is both much hotter and also completely magmatic.

For 4a we took a whindy road βόρεια (North) to Cape Mavropetra where the beach contained every type of rock we have seen on this island, from the basement limestone and marble, basalt, hydrothermaly altered rock, scoria, marine conglomerate, and schist. The excavation of the chamber ejected these fragments as the almost empty chamber started to give way. For mor information on this phase read Moving Megatons: The Excavational Eruptions of Calderas.

Phase 4b is a massive bed (homogenous in composition and containing no layering or order) that is more than 50m (164ft.) in some places.  This event in the eruption would have been a massive column being fed by four vents 11miles wide forming a plume of ash, gas, and lithic fragments.  The pyroclastic flows created from the collapse of the plume creates the mounting cliffs seen from the inside of the caldera. The lighter material would have stayed suspended in the air and eventually settle.

Figure 7:  At Vlychada beach the pyroclastic surge material of phase 2 lies under 50+ meters of phase 4 phyroclastic flow material with Ben for scale.

Although the air I breathe today is filled with the aroma of ocean and gyros, on that day 3600 years ago the space above me looked and composed of much different matter. First with ash, extreamly fragmented rock, and rapidly expanding hot glass followed by massive amounts of steam and turbulent phyroclastic surges. Next, the pyroclastic flows follow and car sized ballistic projectiles are ejected and shot kilometers from the vent and pellet the fresh surface. Volcanic weather created in hours would have produced a storm of acidic rain and poisonous air. The final excavation of the chamber and last pyroclastic flow ejected every rock this island has to offer and filled the air up to 36km  (25miles) in altitude with gas, ash and lithic fragments. The aftermath of this cataclysmic eruption may have changed the  surface but the atmosphere is quicker to return to its normal state. With only two more days left on this island it’s nice to be able to appreciate the violent skies of the past under the clean, blue skies of the present.

8 thoughts on “Into the Air: a Sky Filled with Ash, Gas, and Glass

  1. Hi Emily,

    Both your introduction and conclusion to your entry depicted the overall topic for your post, and they represented what was most important to you while creating it. You separated each phase you were explaining into separate sections making it impossible to confuse one for another. The organization was well thought out and you wrapped everything with a nice little bow. Your images portrayed what you were explaining and were a useful visual aid whilst reading your entry; you know what you want to put down, how you want to present it, and do it very well. They are detailed with writings showing what phase is represented where on the structure and then your explanations tell us what each phase represents, allowing us to see what you see when looking at the structures. You ensured your time on the island was not wasted and it is shown through the professional way you wrote this blog.

  2. Well done. This post was excellently organized and your figures are great (particularly your annotations). Figure 6 correction: That is a Plinian eruption column (no phreatomagmatic). The moisture in the atmosphere will condense on the ash cloud because of the temperature differences (hot cloud, cold atmosphere).

  3. Your post is very well explained by carefully marked up pictures. Each phase is clearly explained in a scientific yet interesting a flashy way. You did a real good job of making this explanation have some flair. The best part was definitely mixing in a first person perspective into something that is almost entirely scientific. Good job!

  4. I really enjoyed your post. You explained each phase very clearly and it was easy to understand. I also really like pd how you added in the smell of gyros.

  5. I really enjoyed this blog. You did a good job explaining the phases and showing good figures along with them. You fit in a lot of information and did it in a way that made a lot of sense to people who didn’t already know about the eruption. I also really liked your opening paragraph about how you are used to clean fresh air and the differences in how air can be. I did find a few typos but besides that it was a great blog.

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