Friday, 19 September 2014

Day 15 - Chilly Lavas

Geology days are always very varied. Many days you may find the weird and wonderful or unexpected. Other days, like today, they are much easier. We are very close to the end of our time here, so identifying individual lava flows has become a little easier. Today there were no real surprises, the only characteristics observed that were new, were gulleys that look like lava gulleys. This is where lava would have once flowed through already cooled lava, carving a path through and smoothing the gulley edges. There were small hints of flow bands on the gulley sides.

A gulley, likely to have been carved and smoothed by a lava flow.

So today was the final big mapping day, covering the eastern area of the caldera. We had several ropey lava flows to distinguish one form the other, the scoria type lava overlying the ropeys and a very impressive chilled margin.

Checking out a great example of a chilled margin

Now, using colour as a way of identifying something in geology is always advised against. But occasionally it does help. From a distance the base of this lava flow appears to grade from dark to light. Knowing I was looking at multiple lava flows, one over the other, and recalling what I have seen on fieldwork in places such as Cyprus and Cornwall, the graded colouration got me wondering, Is it a chilled margin?

The left of the image shows a dark grey, almost black at the base of a flow, getting paler upwards.

The base of the chilled margin is almost glassy, grading to fine grained (can just see with a hand lens) to the smaller end of medium grained. This image does not do it justice. 

How can this 'on the ground' work be used? Well seeing it allows a geologist to get a much more accurate impression of the area. Images and data are great, but I've said it before and I'll say it again, there is nothing like being in the field. 

What we have been able to establish is that the lower down the stratigraphy the narrower the ropes are. Something you wouldn't get from data or satellite images. The scoria type lavas that overlie the ropeys in all examples. are likely to have been a high viscosity flow and the blocky appearance is due to the cooled flow being brecciated by the energy of the flow behind it. 

We can establish the sequence of flows and chemical analysis of samples from different flows will allow geochemists to at least being to unravel what is going on in the subsurface based on the composition of the different flows. And where there is enough feldspar, dating the flows can also be accomplished. 

Tomorrow we are off to look at the affects of horizontal displacement on a volcano.

Lala Salama from Kenya!

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