Sunday, 21 September 2014

Day 17 - The Tuff vs The Bomb Puzzle

We are getting close to the end of our time here with only 3 more days in the field left. I at least will be very sad to go.

I have found with fieldwork in the past that as you draw to the end of a trip, regardless of how long that trip might be, the fieldwork tiredness starts to kick in. This trip I have been up early most days (around 6am) checking base maps, site maps, Google Earth (when the internet works!) and LandSat images in order to plan the day. This might sound somewhat last minute, but we have found on many occasions during this trip that the day’s plans can often change last minute. So I go to bed with an idea of the next day’s plan, but organise in the morning.

Our days in the field have varied from around 6 hours to close to 10 hours, with the shorter days usually being the hottest. It’s easy to forget how much the heat can take it out of you. It’s not like sunning yourself on a great beach. It’s hot and very dusty, with a heavy field bag on occasion and walking several kilometres most days, often up steep hills. Don’t get me wrong, I’m not complaining, I genuinely love what I do and hope others may be inspired too.

After each day in the field, we get back, get our things ready for the next day, shower (though our nails get cleaned in the caldera using the high pressure water from the pipes!), work more (blog writing, emails, in my case go over what has been covered on that day), dinner and work a bit more. The day usually ends around 11pm. So as you can imagine, doing that for (currently) 17 days straight and by the time we finish here 20 days, plus 2 days travel, it catches up with you.

Cleaning the fingernails!

So on to today. The plan was to try and locate evidence that indicates that the Solai TVA cuts in to the caldera along the eastern edge. Satellite images show there to be something. But after yesterday, we know this may be inaccurate or a trick of the sunlight. We found a round that would take us between the plantations of maize, corn and bananas. After driving as far as we could we walked a little further until we could see the caldera security gate. Immediately south we could see two outcrops in the cliff face. We could have used an old farmers path to walk across, however, the advise from our driver was that we shouldn’t, because we hadn’t asked for permission. In the UK we have ‘right to roam’ or you would simply ask the land owner. Here you have to ask community elders and often you would be accompanied by one. But it’s not the end of the world. It was still possible to record some approximate data. And should I feel this area needs looking at in more detail on my trip next year, I can make use of the many contacts and friends I have made on this visit, to arrange access next time.

We decided to go back in to the caldera, heading to an area already visited, the south. As we observed on Day 5, there are at least two faults that run north-south from Nakuru to the edge of the caldera. There are many articles available with schematic style maps of Menengai caldera showing these faults, then inferring their continuation in to the caldera. 

Referring once again to satellite imagery, it is possible to see where one of the fault traces might be observable. In the same locality there are two very large mounds that we wanted to check out. They sit on a north-south trend and there have been reports of young cinder cones on this trend also. These mounds however, looked to be the northern extent of one of the hummocky terrains. We also went back and observed the glassy scoria type bombs buried in tuff that we observed on Day 9.

For the fault we parked up and walked across about 100m of gently sloping compact tuff covered land to a steep sided gulley, the opposite side there was a dark blocky lava flow. This gulley was on the right trend and in line with what looks to be a fault trace in the distant caldera wall. As this gulley cuts through tuff, there were no slickenlines or other structures that we could measure and record. I know see why the faults with in this part of the caldera have remained inferred. I too would go as far as to infer myself.

The immediate fore-front of the image is the drop off in to the gulley that is a likely location for one of the faults inferred to run from Nakuru to the caldera and in to the caldera. 

Our last task of the day was to look at the bombs buried in tuff.

There are several models for caldera collapse: Plate/Piston Collapse – involves the collapse of a whole block in to a chamber along a ring fault; Piecemeal – the collapse of numerous floor blocks due to multiple magma chambers, tectonically controlled faults in the caldera floor or if the entire caldera is a mega breccia; Trapdoor – formed when subsidence occurs in an incomplete ring fault or where subsidence occurs assymetrically in a complete ring structure; Downsag – occurs where ring faults are absent and the rocks overlying the magma chamber deform without fracture; and, Funnel – formed as a result of piecemeal collapse or non-chaotic collapse with a deeper single collapse at the centre and a distinct v or funnel shape.

The general consensus is that Menengai is of the piecemeal style of collapse. The reason we wanted to return to the location where the tuff has buried glassy scoria type bombs is due to the development stages of Menengai caldera highlighted by Lipman (1984, 2000).

The author states the development of Menengai caldera occurred in four stages as follows:
Stage 1: Pre-collapse volcanism; this stage involves frequent surface volcanism that is sometimes explosive, magma accumulation and migration.
Stage 2: Caldera subsidence; collapse associated with large scale magmatic eruptions beginning with a central vent phase, proceeding to a ring vent phase coincident with caldera collapse.
Stage 3: Post-collapse magmatism and resurgence; volcanism is randomly scattered with in the caldera or localise along regional structural trends. A renewed rise of magma results in the uplift or doming of the central part of the caldera.
Stage 4: Hydrothermal activity and mineralisation; often occurring throughout the life of the caldera, dominating late in the cycle developing geothermal systems.

The ash flow tuffs are associated with syn-caldera collapse and the glassy lava flows are post-caldera collapse. Yet here we have evidence of apparently syn-collapse tuffs burying post-collapse glassy lava bombs. The wrong way round when compared to the collapse stages.

The pale rubble immediately above my notebook is a bomb. It's crust has been altered to a clay, but the inside remains a glassy bomb with very large vesicles

Large vesicles at the core of the bomb segments

Our thoughts behind this evidence are: (1) The tuffs within the caldera are not Menengai tuffs, but tuffs from other volcanic activity burying some of the post-collapse bombs of Menengai; or (2) Tuffs have been deposited after a section of the caldera has collapsed. That syn-collapse phase has subsided in to the post-collapse glassy explosive phase producing bombs. While elsewhere another collapse has generated the syn-collapse tuff deposits that are falling at the same time as the post-collapse glassy bombs from an earlier collapse location within the caldera.

Glassy 'scoria like' bombs in tuff

I have previously mentioned that the tuff deposits with in the caldera seem to be quite fine grained for the source to be the location of deposition, even more so as tuff deposits from the collapse have been identified 45km away. But saying that, based on the piecemeal model of collapse, (2) seems more likely.

Tomorrow we plan to visit a steep gulley observed from a distance a few days ago. Keep following to find out what it might be.

Lala Salama from Kenya!

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