Pahoehoe lava - Object of the Month

William, a third year Geology and Geography student, talks about his choice of the Object of the Month.

Title: Object of the Month - Pahoehoe lava - Lapworth Museum of Geology

Duration: 2.12 mins

Speaker Names (if given): S1 William, Geology and Geography student

S1  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.

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