Hello my name is William and I am a third
year joint honours Geology and Geography undergraduate
at the University of Birmingham. My object
of the month is this volcanic rock specimen
from Mt Meru in Tanzania and may have been
from the most recent eruption in 1910. Mt
Meru is the 2nd highest mountain in Tanzania
after Kilamanjaro and as you can see this
particular rock specimen has a ropey appearance.
The name for this specimen is translated from
Hawaiian and is known as pahoehoe.
Pahoehoe forms where cooling basaltic lava
will spread out as it erupts from the volcano.
A thin elastic skin will congeal on the surface
of the lava. As molten liquid lava continues
to flow beneath the elastic skin because this
lava has not cooled as much it will influence
the top surface giving us the ropey appearance
that we see. Once the lava cools completely,
this shape will be retained.
If I turn this specimen around, you can see
that there are actually vesicles on the side
here and flow can be indicated by these. The
vesicles align themselves at the path of flow
and are formed where the gas escapes once
the lava cools.
Pahoehoe forms from mainly basaltic lava flows
which are rich in iron, silica, magnesium
and calcium. This basalt rock has a low silica
content and affects how viscous a lava is.
In terms of pahoehoe, low silica means the
flow of the lava would have been less viscous
therefore fast flowing. Basalt can form many
different types of geological features such
as sills, dykes and pillow lavas. These impact
the surrounding environment where erupted
material can completely swamp the land making
it inhospitable for humans and other organisms
to live.
People living close to active volcanoes such
as Meru are at risk of their eruptions and
their subsequent lava flows and the nature
of those flows is dependent upon the chemical
make-up of the lava. This object demonstrates
the variable features of lava and the processes
required for igneous rock formation that makes
up the Earth's surface.
