Over the past 15 years one of my more pleasant distractions has been the gradual exploration of the Mediterranean coastline in my much loved little yacht Rebellion II. By summer of 2004 I had reached the Bay of Naples and over the winter months had left Rebellion out of the water adjacent to the charming little port of Chiaiolella on the Isola di Procida about 10 miles west of Naples. If you have not visited Procida I would strongly recommend putting it on your list of places to go; it has all the geographic charm of its better known and larger neighbours of Capri and Ischia, but has the great advantage of having been largely overlooked by a tourist trade which really has come to blight its larger neighbours and especially Capri - but perhaps more about the delights of Procida and the rest of the Italian coastline in later postings. Its significance for the present is how it is relevant to the next theme of my blogs?
It was while flying down to Naples that the flight path from London had us crossing the Alps which even in late spring were still heavily shrouded in snow and ice. From my 35,000 ft vantage point on a perfectly clear day and while I was mulling over the little paper I intended to prepare for the upcoming IUTAM Symposium in Warsaw, I started thinking about the movement of glaciers, some wonderful examples of which could be clearly seen below. I had not really had a great deal of cause to think about glacial motion in the recent past but was aware that the driving mechanism was considered to be the gravitational components of force dragging the visco-plastic solid down through glacial valleys which were consequently gouged out by the massive forces being developed. And yet this did not seem to square with some of the features I was observing. At the heads of the glaciers there were large accumulation zones over which the lack of any shadows made it clear the ice surface was largely horizontal. (Being a bit of a skinflint I always tended to catch the very early but cheaper flights since anyone in their right mind would prefer to travel at more sociable hours. But this did mean there were still long early morning shadows highlighting the relief of the alpine topography below). Ice was evidently being shoved out of these accumulation zones over ice falls from which the valley glacier flowed. I say shoved because it was clear from the highly fissured and creviced outfalls from the accumulation zones that any possibility of tensile pull from the weight of the ice below could be dragging the ice from the accumulation zones was effectively ruled out. I had done enough geology during my engineering training to know that these accumulation zones would when the glaciers receded and eventually disappeared leave the distinctive features known as cirques. Now cirques tend to be gouged-out rock bowls surrounded by rock walls but usually with a lip leading out to the relic glacial valley. How I wondered did the ice in these horizontal bowls get extruded out over the lips to start what would become the downward flow of the alpine glacier? The Matterhorn was also clearly visible. Like a lot of the horns it was clear that this had been formed by the gouging-out of the rock from the back walls of at least 3 cirques that had eventually overlapped, with the steep sides of their back rock walls coalescing to form the characteristic horn shape. But where on earth did all the force required to chisel out such a shape come from? Again, it was pretty evident that gravity could not be providing this force. Having not resolved these and a few other question on arrival in Naples, I then spent any moments of mental calm, for sailing can often be a demanding activity in which there is not a lot of spare mental space, thinking about the questions of how all that ice is shoved out of the accumulation bowl to make its way down the steeper inclines of the glacial valleys. Could whatever was this motivating force also account for the tremendous erosion accompanying this glacial motion? And of course, given my own preoccupation at the time, I particularly wondered whether the ratchet model I was invoking to explain the upward distortions of permafrost to form pingos, might just provide at least some of the energy required to move this mass of glacial ice.
The result was a little paper entitled “The Movement of Glaciers”, which upon return to London was submitted to the J of Glaciology, I think sometime in late 2004. I say think since it too has not yet seen the light of day and until now has languished on a memory stick along with those thousands of photographs that also never seem to see the light of day.
But more on this and where it all led in later blogs.
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Interested in your aerial observations T-J and wonder if they could be extended to account for some of my own observations during a recent flight from Hawaii? A beautifully clear morning and a great circle flight path had the airbus passing over Northern Canada and Greenland. Having a window seat I frequently risked the wroth of my fellow passengers by lifting the blind to take in the quite breathtaking vistas of the ice covering Greenland. Massive glaciers making their way down from the more central regions often appeared to be covered with regular wavy patterns in the form of convex downward arcs. But what intrigued me perhaps more than these caterpillar like forms crawling down towards the sea were some fascinating patterns on the surface of what appeared to be otherwise massive flat sheets of ice. These patterns often looked like the waves rippling out from a stone dropped into the otherwise tranquil surface of a lake. Have you seen similar forms and are there simple explanations as to how they develop?
ReplyDeleteOh yes, and I should have added that sometimes these wavy ripples were interrupted with massive ice free rocky peaks poking through the ice sheet. The wavy ice ripples showed patterns very like those you would expect were a lake ripple to pass around a small stone breaking the surface of the lake. Fascinating, but again I found these forms difficult to explain.
ReplyDeleteYou make some very interesting observations and I believe raise some important but largely unresolved issues sottovoce1. In the context of alpine glaciers – those moving down valleys – these wavy forms have sometimes been referred to as “Forbes bands” to mark the important contributions made by Forbes to the understanding of glaciers (see some of my earlier postings) although some prefer the alternative name of “ogives”. It is widely recognised that these ogives are produced by cyclic changes in the form of ice occurring over the annual seasonal cycles. But just how is in my view not very clearly understood. For the similar bands on what you describe as continental sized ice sheets the origins and forms of similarly banded structures appears to be even less well documented and understood. Rather than elaborate on my thoughts as to their origins, and how these origins may have something to do with the annual thermal cycles, in this short reply (and not be able to embed photos) I would prefer to respond to your perceptive observations more fully in a new posting later today.
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