Again using the LandSat data, a further 3 locations for lava
sampling were identified. The source of the young lavas are believed to be the cone
near well MW-12, almost dead centre to the caldera and the highest temperature
values recorded in the geophysics. Yesterday’s samples were collected from the
east side of the cone. Today the aim is to sample from the same flows on the
west side of the cone.
Venting Well MW-10
The first location selected was behind well MW-10, this is
the one that has been venting since before our arrival. On arriving at the well we discovered that scientists from Geothermal Development Company were just
setting up to collect samples from the well. They were happy for us to observe
and take pictures and came over for a chat. I was able to find out more
information regarding the temperature of the well, the pressure it is venting
at, the pH and what is in the pipes. This is important information when
preparing for a sampling trip myself next year.
On leaving the well site we walked a few tens of meters to
the south of the well as my field assist Jenny queried if something she had
seen was a fire or a surface emanation. Time to investigate.
Crossing the ground scattered with spatter and volcanic bomb
deposits, we soon realized that every bomb or large spatter deposits was
steaming, some more than others. Around 20 had reasonable amounts of steam
emanating from them with others just a whisp, a total of almost 100. Some of
them were lovely and warm, just like your bath at home or a sauna, the
temperature changes seem to pulse. Some were too hot to leave your hand in
place for more than a second!
Fascinating to see, the best was yet to come. Suddenly Jenny
and I turned in to big kids. A 3m long, 1m wide fissure venting steam, was a
short scramble up about 5m of altered lava flows and spatter deposits – a
fumarole! The depth could not be determined, but it was possible to climb in
safely to almost head height and experience this natural phenomena. Even in the
morning air temperatures of around 25°C, the fumarole was warm. So warm in
fact, the temperature contrast caused a collection of condensation on my skin
and my sunglasses.
A natural sauna. And it's free!
Trying to sample surface emanation gases from within lavas
is usually very difficult because of the hardness of the lithology. However,
the steam has resulted in alteration so that that the lavas feel very spongy
under foot. A perfect location for surface gas emanation sampling on my next
visit.
Back to being a little more serious! The fissure has a NW-SE
trend as does a larger, but not venting fissure a few meters away. This trends
matches that of the region of high temperatures within the caldera, as
identified by the geophysics; the trend of the recent fissure eruptions
mentioned on Figure 9 of Mariita (2013); and the Aswa Lineament. A mylonite
zone that runs from central Sudan all the way to the Indian Ocean, an estimated
distance of over 2000km. It has widths ranging between 5km and 30km and is
believed to be associated with the Pan-African Orogeny, a mountain building
event that occurred about 600 million years ago.
Initially, I had planned to sample lave from this site, but
due to the alteration, the plans changed, as is often the case in fieldwork. So
I moved about 40 meters to the south. Here the lavas demonstrating nothing more
than alteration on the surface and a much fresher sample could be collected
with the aid of my trusty rock hammer.
The NW-SE trending fissures identified at the previous
location were not the only ones. Further investigation provided clear evidence
of a network of fissures. Further fissures with both NE-SW tends and N-S trends
were identified, with the NW-SE being dominant! The fissures are within the
young lavas.
Below the lava flows and fissure locations, evidence of
pahoehoe lava flows were observed. The ropes were quite large, though had to be
hunted for due to the overlying grass and scattered bombs.
When sampling any type of rock for analysis, it is important
to try and sample from in situ
locations. This then guarantees you are sampling what you are aiming for.
However on occasion this is not possible. The final location of the day proved
to be just like this. The sample site was that of the young lavas to the north
of the crater, the source of which is a flank eruption centre from prior to the
caldera collapse. The collapse of the caldera also resulted in the collapse of
a small section of the crater along its south-east edge. The lavas flowed in an
easterly direction and cooled with a 40m high wall of very large blocks, some
of which had fallen away. Climbing to an in
situ sample site would not only have been impossible, but dangerous, and
not just because of loose rocks. By the time I made it to this location, the
temperature had begun to drop. This means that any snakes that had spent their
day basking on a warm rock, would have now found a warm space with in the
rocks.
I chose to sample from a block of lava that had clearly
fallen from the side of the cooled flow. The block was over 2m in size and had
come to rest just a meter away from the base of the flow. It’s certainly not
ideal, but the best that could be done in this situation.
Tomorrow? Mapping begins.
Lala Salama From Kenya.
No comments:
Post a Comment