Monday, 15 September 2014

Day 11 - Tuff Day

Today was another day mapping, we needed to finish off mapping the western side of the caldera. After security let us out of the caldera and we were happy they would let us back in (this security check is different to our normal one, where everyone knows us now), we drove to the western edge of the caldera.

Starting on the caldera shoulder, we found a unit about 30cm thick that was full of angular clasts of volcanic debris. Above this there was a unit that looked very much like a lahar. It contained small clasts from 3mm to 2cm in size in a muddy matrix. 

The lahar had a sharp contact with lava that in cross section had an undulating outline. Above this, there were many layers of tuff ranging in thickness from 4cm to around 30cm, alternating with layers of palaeosols. Each layer fines up with grains from approximately 500┬Ám to 5mm. The grains are still smaller than I would expect to see if the tuff was associated with the Menengai eruptions. Fines from a pyroclastic eruption are so small and so light they are ejected sometimes 10’s of kilometres in to the atmosphere. Once at such altitudes, the winds carry the fine particles many miles, sometimes around the world. Even particles of the sizes seen at Menengai would be carried great distances as a lot of the particles are very pumaceous making them very light weight. Overlying these tuff deposits are further tuff deposits. Each individual layer measures between 50cm and 1m. however, the thickness of the individual layers varies horizontally.

A handful of tuff!

In this area of the caldera, the walls are much lower than the south east area for example. Here they are about 100m high, with tuff deposits making up the last 15-20m. About 8-10m up in caldera wall, in stark contrast to the surrounding deposits, there is a white layer that does not appear to contain much in the way of debris. As the height means close observations of this deposit can not be made. The inference made about this unit is that it is a crystal tuff. This is based on the clay observed on a previous day that we believe to be a hydrothermally altered crystal tuff. 

Some of the tuff deposits observed in the caldera wall.

Today was a rare occasion where we managed to sit and enjoy lunch instead of eating on the go. We found a flat topped Acacia tree to shelter from the midday heat and enjoy. While eating we were lucky to see the golden breasted starlings flying near by. A very pretty bird with an orange underbelly and iridescent blue/purple wings.

Lunch time.

Unlike other areas of the caldera, in this region there are large areas covered by tuff. If like other areas there is ropey lava buried beneath, it is not seen here. I estimate that we walked 2kn through tuff before finding evidence of ropey lava. 

Characteristics not seen anywhere else so far, are many small mounds. These mounds are overlain by tuff that follow the topography. These mounds are often 20-30m high and sometimes elongate. These mounds remind me of the hummocks observed when on fieldwork in Sicily. These hummocks were associated with the 6000 year old collapse of the east flank of Mount Etna. Although here, the collapse mechanisms are not quite the same, there are similarities. I believe that these structures observed today are hummocks.

A hummock cut by the road. Volcanic debris in the middle of the image, covered by tuff. Me for scale

Heading along the road towards the west, although lava flows were yet to be seen, we started to observe volcanic bombs on top of the hummocks, the sizes of which decreased the further west we walked.  Finally we found evidence of ropey lava with flows towards the north-west. Unlike previous observations, here the ropey lava does not appear to be buried under tuff, then exposed due to erosional processes. The only way this might have been the case, is if the areas of tuff we observed, buried lava and we just didn’t see evidence for this.

The ropey lavas were once again very pumaceous in appearance and appeared to be of one flow exposed along about 500m of road length. The final 500m the lava flow was much more blocky and darker, but not as dark and scoria type, as seen in the youngest flows. It was easy to observe shallow fissures that became much deeper over the last 500m with the increase in elevation. The trend direction of these is similar to those observed on 2 previous days and are likely to be part of the same network of fissures. 

While I was having a much deserved shower, Jenny had a surprise when she looked out of the door to our bandanas to find a baboon sat on the fence post less than 5 meters away. Cheeky monkey ran away! We watched for a while as there were four big males, partly watching us, partly watching something else. We soon worked out they were standing guard, watching for the southern troop that were close to theirs, the northern troop's, territory. 

Tomorrow has the potential to be a bit different, I'm keeping my fingers crossed, it goes to plan. That is all I will say for now, 5.30am start.

Lala Salama from Kenya!

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