Lava Flow - Amazing Geology at Kilimanjaro
All lava is a stream of molten rock, that erupts from volcanic vents.The same name is also given to already solidified rock bodies that formed as molten or semi-molten flows of rocky material. Lava flows are the most common volcanic feature on Earth.
Inside the Earth, the heat is so intense that the rocks and gases that make up the mantle are melting. Our planet has a core of lava. This core is covered by crust and layers of hard rock. This molten material that forms is magma, and when it's pushed to the surface of the earth we call it: lava. Although the two layers of crust and rock are different, in reality both are constantly changing: solidified rock becomes liquid and vice versa. When magma seeps through the earth's crust and reaches the surface, it turns into lava.
For this reason we call lava the magma material that has erupted from the earth's crust and thus spread to the surface. The temperature of lava is very high, ranging from 700°C to 1200°C. Unlike magma, which can cool down quickly, lava is denser and therefore takes longer to cool down. This is one of the reasons why it is very dangerous to approach the site of a volcanic eruption, even after a few days.
The color of lava as it exits is red, sometimes grey. After cooling, the color changes to dark grey, brown or black. Carbonatitic lava is very rare (e.g. Ol Doinyo Lengai volcano in Tanzania), which is dark when it emerges, but turns beige to almost white in a relatively short time.
When we speak of lava, we are actually referring to lava flows. There are many factors that affect the type of ejected (erupted) lava, such as composition, which affects how it looks when it hardens.
The classification is based on their surface morphology and depends largely on their composition and viscosity. Depending on the outlet pressure, temperature and composition (gas content), lava can have a high to low viscosity. The range of flowability ranges from fast-flowing lava flows to oozing out lumpy to dough-like masses.
The Lava flow is most commonly divided into distinct types: Pahoehoe lava flow, Aa lava flow, Blocky lava flow, and also Pillow lava flow. All of these lava flows are found on the ground.
Pahoehoe-Lava:
Pāhoehoe lava (Hawaiian pāhoehoe) is the name for a thin (i.e. low-viscosity) basaltic lava with high temperature and flow rate and low silica content, which has a relatively smooth surface after solidification.
The very hot lava flows beneath a horizontal solidifying crust that frequently breaks up into individual floes, throws up into lava ridges, or slides apart, leaving lava pillows. While the lava flows are still active, the already solidified lava cover can grow up to 50 cm thick. This allows lava tubes to form.
In contrast to the viscous ʻAʻā lava, the surface is quite smooth, so that it is easy to walk on after it has cooled.
Alternative names are flat lava, clod lava, beaded lava, crusted lava, rope lava or „knitted lava“. The name "knitted lava" originates from the outer shape of the lava, whose surface is structured like a knitted pattern. This is caused by the fact that when the lava slowly flows away, a kind of skin forms on its surface, under which the thin lava flows further and shifts the skin.
Pictures: Geo-Link
Aa-Lava
Of the various types of lava, ʻAʻā lava, or chunk lava, is the most viscous form. The name comes from the Hawaiian language and means burning, fiery, stony. As the melt solidifies and partially crystallizes, its crust fractures, leaving a surface interspersed with sharp-edged, irregularly shaped, jagged chunks and clods.
Compared to the Pāhoehoe lava, the ʻAʻā lava flows more slowly and is cooler; it is often found in the lower part of low-viscosity lava flows, because the viscosity increases through outgassing and cooling. At the end of the stream it forms a steep front.
Pahoehoe and aa flows from the same erupting vent are usually identical in chemical composition. In fact, it is common for a flow that leaves the vent as pahoehoe to change to aa as it progresses downslope. The greater the viscosity and the greater the stirring of the liquid (as by rapid flow down a steep slope), the greater the tendency for the material to change from pahoehoe to aa. The reverse change rarely occurs.
Picturce: Geo-Link
Block-Lava
Block lava flows have surfaces that consist of large angular blocks of lava. Blocky lava flows usually have andesitic or basaltic-andesitic compositions and are more viscous than basaltic lava flows that form pāhoehoe or ʻa‘ā. The blocks in block lavas are smoother than the rough clinkery ones in ʻa‘ā. The blocky tops of these flows generally grade into a massive unbroken lava interior, with a brecciated layer at the base of the flow. Block lavas are also typically glassy.
Compared to basaltic lava flows, block lavas are thicker and have steep flow fronts. They also tend to not travel as far or as fast as basaltic flows.
Pictures: sandatlas.org
Pillow-Lava
Pillow lava is lava that has solidified under water and has a pillow-like shape and is therefore considered to be a sure sign of subaquatic volcanism. Pillow lavas also form when subglacial volcanoes erupt.
The solidified lava owes its glazed consistency to the extremely rapid cooling of the hot molten rock when it comes into contact with water, a medium that can absorb large amounts of heat. A tough, plastic mass of rock glass forms on the quickly cooled surface. Inside the pillow, the lava cools much more slowly, resulting in the formation of a crystalline mass. In most cases, radially arranged rings of gas bubbles are formed just below the crust, since the lava experiences massive pressure relief when escaping from the rock and thus leads to degassing of the melt. If more lava flows, the shell tears open at one point and more cushions are created.
Pictures: Wikipedia
Sources:
sandatlas.org
wikipedia
mineralienatlas.de
So lets take a closer look of the geologic formation at the coordinates.
To log this Eartcache, go to the given coordinates and answer the following questions. This via email or the message center to me:
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Look to the wall in north-west direction. You can identify different layers of lava, as in the listing described.
a) What 2 different lava flow-types can you identify?
b) Which one is older?
c) What do you think is the reason for these two lava-types at the same place?
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Go to waypoint 1:
a) What kind of lava flow can you identify here?
b) Touch and describe the surface and the structure of the lava?
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Go to waypoint 2:
a) What kind of lava flow can you identify here?
b) Touch and describe the surface and the structure of the lava?
c) Please guess and argue, which lava is older. That on waypoint 1 or waypoint 2
Addionally
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Take a photo of yourself and/or a personal item (e.g. your GPS) and attach it to your log!