Day 17 - A game of 'guess and seek' (plus rifting margins and TVA's explained)

Today we were looking for faults. And finding them was certainly a bit of a guessing game. We knew roughly where the faults were…how to get to them was a case of ‘lets take this road and see if leads us there!’

Pretending to be the fault I found! A small fault within the tuff - it's not often you find that. The first fault we were looking for was part of the Solai tectono-volcanic axis (TVA). A TVA is a tectonically active region made up of faults and volcanoes, where you get a chain of small volcanoes following the trend of the faults. The Menengai caldera is thought to be intersected by two TVA’s, the Solai TVA, and the Molo TVA. The Molo TVA is the more volcanically active of the two, while the Solai TVA is more seismically active. The Solai TVA has a roughly N-S trend and is inferred to cut into the caldera at the north-eastern side. And so we went off looking for some N-S trending faults on the north-east edge of the caldera. As much of this land is privately owned farmland we were limited as to where we could go to look.One of the small roads we took yielded success. It brought us to the top of a steep escarpment, a good 2kms long, that led to, and then became the caldera wall. This escarpment was on the right trend to be a fault on the Solai TVA, and dipping at about 60 degrees. From where we were stood it also looked as though this fault could be trace on the other side of the caldera, though any evidence of it in the caldera has been covered by lava flows.

Looking SSW across the caldera from the top of the fault scarp. Highlighted by the red dots is where this fault can possibly be traced to continue on the opposited side of the caldera. We then headed into the caldera in search of more faults. On day 5 we observed two normal faults dipping in opposite directions forming a small horst structure.

A cartoon image of horst and graben structures - formed in an extensional regime, where you get normal faults dipping in opposite directions. We were looking for evidence of these faults observed on the flanks of the volcano continuing into the caldera. These faults again trend roughly N-S. We didn’t find evidence of either of these two faults, but just to the east of where we expected them to be we did find what looked to be a fault. Another escarpment (turned gully due to a lava flow) which extended as far as we could see in both directions. It had the same dip direction as the western fault observed at the flanks, but it was too far to the east to be this fault. We think this a third fault as opposed to being one of the two we were looking for. Meaning that the horst structure observed on day 5 is in fact a horst and graben structure. As we followed the trend of this fault south to the caldera walls there was a prominent gully in the wall which had the same trend. We didn’t observe this on day 5 as we didn’t manage to explore that far to the east. So at this stage we can only infer it to be one large fault. It certainly looks to be one.

Looking south across the caldera from the top of the fault scarp -of the third fault. You can see a gully filled with trees on the caldera wall that follows the same trend as the fault we are stood on. Vegetation can be a very good indication of the presence of structures. Here, the only big trees in the area are growing along the fault trace. This is because the fault allows plants to get their roots deeper, enabling them to grow bigger and taller. On our way back we stopped to look at one of the nice tuff exposures we have looked at before. It contains bombs of scoria (basaltic pumice) however there is no deformation of the underlying tuffs, so we wanted to look in more detail and see if we could explain this.

The scoria-like volcanic bomb within the tuff. We were perplexed as to what these could be until we hammered into them as they were so altered. Only on hammering into them did the hard, white clay-like deposits reveal themselves to be bombs! What was even more impressive though, was the structure of these bombs:

The 'vesicles' were so large in places that you could see how the bombs are made of very fine glassy sting. Pretty if nothing else.

My theory is that the tuffs at the base are part of an earlier tuff deposit and were consolidated before the occurrence of a later eruption with bombs. So the scoria bombs were deposited as part of a later tuff deposit, they were likely some of the first material to fall (due to their size) and then the tuff filled in around the bombs. What was more exciting to find in this area was the small fault I discovered within the tuff. This is reasonably rare to find because tuff is very loose and doesn’t easily preserve structures. We are sure it is a small fault as it can be seen on either side of the road, on the same trend and with the same dip. We expect this fault is very young as it has been preserved in tuff.

The small fault within the tuff. The left hand side has moved down relative to the right hand side. The first picture of this post shows this same fault on the other side of the road from this picture, and me showing the orientation of the fault. As we ended today it dawned on us that we have just three days left in the field. Only two of these will be in the caldera. It also dawned on me that I was going to explain rifting margins in a bit more detail.... Rifting margins occur where tectonic plates move apart. This usually occurs under the sea at ocean spreading centres, such as the Mid-Atlantic Ridge. Iceland sits above the mid-Atlantic ridge and so is one of the few places where rifting can be observed on land. The only other example of rifting occurring on land is here in east Africa along the East African Rift. Rifting can be active or passive. In active rifting a mantle plume rises near to the surface causing uplift and thinning of the crust. The stress caused by the uplift leads to the land splitting (through normal faulting) and eventually it moves apart.

A cartoon image showing ACTIVE rifting. A mantle plume (very hot magma rising from deep within the mantle) rising to the crust causing the crust to be uplifted and thin. A and B show active rifting on land. C and D show active rifting in the sea, leading to the formation of a mid-ocea ridge. In passive rifting the thinning of the crust occurs first. This thinning is due to extensional stress, most often a result of a process known as ridge-push slab-pull; this process is thought to drive plate tectonics and is where ocean spreading pushes tectonic plates and subduction pulls tectonic plates. Passive rifting – due to extensional stresses- will occur where subduction at one of the plate boundaries is dominant (the rate of subduction is fast) or where a plate is sandwiched between two subduction zones – thus pulled apart.The thinning of the crust allows for magma to rise up as well as producing faulting. This eventually causes the land to move apart. The East African rift is an example of active rifting. Along the eastern edge of Africa there are two plumes that have caused doming (uplift) and have developed to form the rift system we see today. The rifting here is relatively young beginning around 12 million years ago. The break-up of a continent by rifting is a process which takes tens of millions of years so there is no need to worry about Africa breaking up in our lifetimes.