Archive for May 2013

The Charm of Columns   2 comments

Devil’s Postpile (US), Fingal’s Cave (UK), Giant’s Causeway (Ireland) – these are nature’s beauties because of their spectacular columns. But how do columns form? Well this illustrated and short blog post is a selective stravaig through the charming world of columns, with a focus on lava-ice and lava-water interactions. It also provides a bit of insight into the science behind column formation, as well as hinting at unexplored areas and discoveries yet to be made.

Terminology

The massive interiors of lava flows often show different zones, and where a zone consists of well-developed columns it is given the term ‘colonnade‘, and the simplest of lavas has an upper colonnade and a lower colonnade which meet (usually imperfectly) towards the  middle of the flow. ‘Entablature‘ is the name given to a zone displaying irregular columns oriented in various directions, or a zone comprising much smaller blocks (known as ‘cub-jointed’ lava or ‘kubbaberg’). Entablature is interpreted by all workers as representing penetration of additional coolant (water/steam) into a lava flow, and thus is a useful environmental indicator.

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When a basalt lava flows down an Icelandic river valley. Lower colonnade demonstrates that water did not interact with the base of the flow, but the upper entablature zone indicates that river water flowed across the top of the flow and down into the still hot lava.

Background

One of the plus points of being an academic and having a bit of time to do research is that you can get a good PhD student working on something that really fascinates you. For many years I have looked at columns and other fractures in lavas, especially those involving volcano-ice interactions, and I knew there was some good science to do on them. So I managed to persuade a couple of colleagues that a PhD project entitled ‘Lavas in Glacial Settings’ would be a winner, and of course you cannot study these lavas without being surrounded by columns. So part of the project became an in-depth look at columns and fracture formation in lavas, and we had some nice surprises and made some new discoveries.

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In the land of rhyolite columns. Entire rhyolite domes hundreds if metres high have columns throughout, indicating effective penetration of coolant into the interiors of domes and their constituent lobes. Iceland.

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A dacite lava lobe that flowed in an ice tunnel – this is the side of the lava lobe where it was moulded against the ice wall. Villarrica volcano in background. Chile.

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Way-up indicator! The pipes form vertically as they propagate upwards. So the columns formed in an inclined orientation and afterwards. Basalt lava, Iceland.

I am going to show some examples of fractures that illustrate a number of points. But first, some basics about columns. For those who want further information I have added ‘geek notes’ but you really don’t have to read these to understand and enjoy the images.

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Small-medium columns in a basalt intrusion into a moist fragmental host (volcaniclastics). Iceland.

The basics

  1. Columns form due to thermal contraction of cooling lava.
  2. They form at right angles to the cooling surface, so a horizontal lava body (e.g. lava flow or sill) will have vertical columns, whereas a vertical intrusion (e.g. dyke) will have horizontal columns. [Geek note: Columns propagate in the direction of the thermal gradient defined by the isotherms in the cooling lava body. In a horizontal lava body the isotherms will be horizontal, and therefore the columns will propagate vertically.]

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    Lava flows with vertical columns at their base, forming a ‘colonnade’. The upper zone of more irregular columns oriented in various directions is ‘entablature’. Iceland. Look carefully at the basal colonnade and you will see that the early formed columns are small (rapid cooling) and that they merge upwards to form larger columns reflecting a more stable and slower cooling regime.

  3. Column diameter is related to cooling rate, so for example a faster cooling rate will produce smaller columns. [Geek note: Column diameter is controlled by the viscoelastic response of lava to cooling, so faster cooling leads to smaller-diameter columns. There is as yet no widely-accepted model which relates column diameter to cooling rate. There’s a PhD project for you…!]
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    Large vertical columns in a basalt lava flow in west Iceland. Note the horizontal ‘chisel marks’.

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    Small columns in a rapidly-cooled rhyolite lava. Rule is 1 metre. Iceland.

  4. Columns form in lavas of all compositions, though basalt columns are best studied. [Geek note: Rheological factors mean that basalt columns best approach the ‘equilibrium’ condition which is the formation of hexagonal columns with equal sides. Rhyolite columns get nowhere near this ‘equilibrium’ condition.]
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    Large rhyolite columns at the top of a subglacial dome.

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    Rhyolite columns around 0.5 metres in diameter. How many hexagons with equal sides can you identify?

  5. Columns do not form smoothly – they form in distinct steps. This process is reflected in the ‘steps’ that can be seen on many columns, and on vertical columns these ‘steps’ are horizontal. These have various names, such as  ‘chisel marks’, ‘striae’, ‘chatter marks’ and ‘step-wise advance cracks’. I’ll call them ‘chisel marks’ in this blog.  [Geek note: These form parallel to the long column axis. They are beautiful examples of cyclic fracturing in a uniform stress environment where fracture propagation is retarded until further cooling and contraction enables the tensile strength of the lava to be overcome. One it has been overcome the fracture is initiated and propagates from cool into hotter lava, but stops as the fracture encounters lava too hot to fracture in a brittle manner. Then the process starts again.]

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    Chisel marks (horizontal) on near-vertical columns, with inclined flow banding, in a rhyolite lava. Iceland.

  6. Small but subtle features at the chisel marks enable the direction of formation of the columns to be determined. [Geek note: these ‘hackle’ marks and plumose structures identify the point of fracture initiation in the cooler part of the lava and the direction of fracture propagation into the warmer part of the lava.]

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    The little ‘hackles’ starting at the chisel mark at ’22’ on the left side of the rule and terminating upwards, indicate that cooling was from the base upwards. Dacite lava, Chile.

  7. In a horizontal lava body such as a lava flow or sill, cooling occurs both at the top and that the bottom, so columns grow upwards from the bottom and downwards from the top. They meet in the middle, often surprisingly well, though an accommodation zone of imperfect joins is usually present. [Geek note: The upper columns are usually longer than the basal columns, suggesting that cooling from above is more effective than cooling from below. This is not surprising, as convective removal of heat from the top is generally more efficient than conductive removal of heat from the base.]
  8. Not all lava bodies have columns, and the reason why they don’t is probably another PhD project….
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Upper and lower colonnades with well-formed but short vertical columns. Interior zone comprises most of the flow and consists of entablature, which I informally call ‘chevron’ entablature. This is interpreted as a lava flow which dammed a river, and whilst the river was rising behind the dam the lava was cooling from above and below. When the dam broke the released water flowed across the lava top, percolated down into the flow, and the more rapid cooling produced the thick entablature zone. Iceland.

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The broader context of the previous image. This lava flow, c.6,500 years old, has flowed into a river valley. The edge of the flow is exposed left of the waterfall, and the lava thickens to the right of the waterfall into the palaeovalley. Entablature is not present at the edge of the flow – it is only present in the thicker and lower-lying parts of the flow.

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Lower colonnade with upper entablature zone, with this entablature zone being of what I informally call ‘cube-jointed’ entablature (equivalent to kubbaberg). Iceland.

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The transition zone where an upward growing lower colonnade meets cube-jointed entablature propagating downwards. Iceland.

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The sub-horizontal columns on the right-hand side of this glassy dacite lava in Chile indicate a vertical cooling surface to the right. Given the the lava is thin on the ridge crest (top left) and thickens down to the right, the interpretation is that when the lava flowed off the ridge crest it encountered ice.

 

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The author examining a remarkably smooth surface within a subglacial rhyolite lobe. Iceland.

Posted May 18, 2013 by davemcgarvie in Uncategorized