This week, Volcanoes and their Hazards students @GeologyHull were conducting experiments to understand more about magma movement using candle wax, sand and water... but what can this simple set up tell us about volcanoes? ...pic.twitter.com/x1H0dHsyY8
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In the basic experiment, the wax = the Earth’s mantle (hot and SOLID- but can undergo partial melting in the right conditions- such as at a Mid Ocean Ridge or a subduction zone). The sand and water = rocks of the Earth’s crust (colder and rigid)pic.twitter.com/Jlh1GYQn0s
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So we apply heat, and start melting the wax (mantle). When the mantle melts, it produces magma. This magma rises through the “crust” because of its lower density.pic.twitter.com/8IjTiK2Nqs
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You can also think of this on a slightly more local scale, as a reservoir of magma (the wax) beneath a volcanic cone (the sand and water)
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You can think of it in terms of feeder systems of dykes leading to eruptions of lava (wax “erupts” at waters surface) or intrusions of sills (wax “sill” is emplaced at “planar stratigraphic contact” between water and air! One experiment, many thought experiments!pic.twitter.com/C5ebUZvayN
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Before the experiment, we got the students to hypothesise what they would expect to see. What amount of partial melting would be needed to start movement? How would gas bubbles play a role? Where/how would the magma rise? What kind of shapes may be produced at the water surface?
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Many students thought they would see magma rising in balloon “diarir” shapes, like a lava lamp- when this didn’t happen, we discussed the controls on this- if the water had been a more viscous substance (like in my desk toy here) we may have seen diapirs ..pic.twitter.com/dixKPUWl73
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In the video, you can see how the wax melts but can’t break out from the sand at first... air pockets and gas bubbles build up- at one point in the video, you can the pressure building to the point where the wax is inflating and “jacking up” the sand layer above...
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... this is similar to what happens at real volcanoes, where magma activity can cause deformation of the surface. Volcanologists can measure how the surface of the ground changes and bulges, to help them understand what the volcano is up to.
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We also saw long thin tubes of wax form, which transported “magma” to the surface. When we cut these open, we could see that they had a hard wax edge but a more fluid centre. This is really similar to the real world!...pic.twitter.com/dbAlY7Ubfd
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The outsides of magma intrusions commonly have ‘chilled margins’ because they cool very quickly when in contact with the crust around them. Inside, the magma can still flow. Same thing happens on surface- lava flows may form tubes with chilled crust, but flow continues inside!pic.twitter.com/HEq5rXAuQN
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We also saw evidence of “assimilation” in our experiments- the wax had scavenged sand particles from the crust on its way to the surface- see little grains at centre of the tube, and in the “lava flow” formed at the surface of the water ...pic.twitter.com/TVtUKDiikj
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...this also happens in nature- magma can steal bits of the crust as it moves through it. These bits may melt and alter the overall magma composition, or stay pretty solid and be erupted as “xenoliths” (foreign rocks!) Here’s a nice (green) peridotite xenolith in a basalt lavapic.twitter.com/fPLQaQs6cn
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...and there’s more! When we took the top “lava flows” of wax off and examined them, we saw some lovely structures... ropey, lobate shapes...pic.twitter.com/iNy6JV9v1S
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... these shapes can be seen in real life lava flows! This pic of Pahoehoe lava (from Wiktionary) shows ropes and lobe-type shapes. These types of lavas are thought to form during low rates of lava eruption, where the skin formed by cooling isn’t disturbed too much by new lava.pic.twitter.com/yOPKx31NDm
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Other things we discussed included how the lava took a path of least resistance (at the beakers edge)...but that’s enough for now! Moral of this story is- everyday household objects can teach you a lot about science, the natural world, and volcanic hazards. Happy weekend all!
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