Friday, 12 September 2014

Day 7 - Perfect Day for a Sauna

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