Our initial task of today was to map the along the last road in the western section, the only one we didn’t manage to complete yesterday. This road heads right out of the caldera and in doing so passes beside (and through) the caldera walls. If you remember back to earlier posts, our main objective out here was the structural mapping of the caldera walls which we quickly realised wasn’t going to be possible due to there being no safe access to the walls. There are however two places we are still hopeful to get to the walls and today we were heading towards one of them. We decided we would drive to the very edge of the caldera, do some structural mapping of the walls then walk back along the road mapping. When we got to the wall however we instantly knew that we wouldn’t be mapping any structures on the walls today. The lava of the walls here is highly altered and very weathered, so weathered in fact that we could not tell if any of the lineations were structural or just down to weathering. Above the weathered lavas there was lots and lots of tuff. Tuff made up the top 25m or so of the 100m high caldera walls.
A handful of tuff. Due to construction lots of the tuff along the road has become unconsolidated, allowing me to pick it up like this. All the clasts in this tuff are pumice.
I mention tuff a lot in this blog – and that is because there is a lot of tuff here. I will take this opportunity to discuss just what tuff is. During an explosive eruption tonnes of material is ejected into the atmosphere. Much of this is small fragments of rock or pumice (scoria if basaltic), along with fine ash and many smaller crystal fragments. It depends on the density contrast between the material ejected and the atmosphere whether this material forms a pyroclastic flow or if it remains in the atmosphere for some time and then settles – producing fall out. When the ejected material is less dense than the atmosphere it will remain in the atmosphere and produce a fall out deposit. This fall out deposit is known as tuff.
One of the tuffs we observed today on the flanks of the volcano (just outside the caldera walls). In this tuff you can see some darker palaeosols - ancient soils - between some of the layers of tuff.
Tuff can be further classified depending on clast (fragment) size and type of clast. The tuffs we have been observing here in Menengai vary somewhat. They are predominantly pumaceous tuffs containing medium (10-50mm) size clasts of pumice. Though in some places there are finer pumaceous layers and some crystal tuffs – a tuff composed mostly of small crystals- mixed in with this. Having taken some detailed notes on the lava and tuff deposits exposed along the road as it climbed out of the caldera we stopped for lunch.
A bit of a change from the usual eating luch on the go, we found this lovely spot to sit and enjoy lunch.
We chose a rather idyllic spot for lunch today, sheltering in the shade of a flat-topped acacia tree – again referring to the Lion King, these are the trees which are seen across the Pride lands. We were treated to a small flock of the beautiful golden-breasted starlings flying about. These small birds are stunning with a golden orange underside and iridescent blue wings and back.
The beautiful golden-breasted starling. Some of these guys made friends with us as we sat and enjoyed lunch. I didn't manage to capture a good photo of them so have taken this picture from the web (antpitta.com).
After lunch we walked back along the road, mapping as before. It was pretty straightforward with only two different flows - the ropey lava and some fissures cutting through it. The lava’s only occurred in the last 1km of the ~3km road. The first 2km or so, from the caldera walls in towards the centre was all tuff, tuff, and more tuff. Unlike anywhere else however the tuff deposits formed a rather ‘hummocky’ local topography. It seemed to form lots of small hillocks with steep valleys between them. Helen has seen similar before - she saw in Sicily (Mt Etna) how flank collapse results in a hummocky terrain- and so her interpretation of this hummocky tuff I quite like, and agree with. We think that the hummocky terrain isn’t formed by the tuff, but that it was in fact created during caldera collapse (as volcanic debris-flow avalanches) and that the tuff has just settled on top of this existing topograhphy. Over the past few days Helen and I have been discussing the origin of the tuffs we are observing here in the caldera. As I mentioned above, tuffs are fall out deposits- material that has been ejected high into the atmosphere then fallen back to the surface. As a result you tend not to find tuff deposits at the location of the eruption, but some distance away, as once it gets into the atmosphere winds carry it over huge distances. When it does fall out you will get thick, coarse tuff deposits nearest the eruption, these become finer and thinner as you travel further and further away. As we are only finding medium to fine grained tuffs here in the caldera, and the layers -at their largest- are only around 1m thick we find ourselves questioning if the tuffs are actually related to the caldera collapse or subsequent eruptions. We no longer think they are. Instead we think the tuffs we are observing are deposits from eruptions of some of the many volcanoes in this region. I had something of a surprise this evening back at the camp.As I was in our tent I heard some noises outside, I first though that it was one of the staff coming to say hi – they do that quite often. But when I shouted ‘hello’ and got no response I knew it was the baboons. I wandered onto the balcony to check it out, only to see a baboon on top of the wooden posts of the electric fence. I think the noise I’d heard was him getting a wee shock from the fence. He jumped off the fence on seeing me, but this cheeky fella wasn’t going to give up easily... he sat about 5m from the fence for a good 10 minutes, every now and a again glancing over in my direction – chancing his luck I think. Anyway we have a VERY early start tomorrow for a rather exciting experiment. I will share the results tomorrow… for now I will share a picture of me trying to demonstrate 'fall out' earlier today.
Lala Salama