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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