Lava bombs – Amazing Geology at Kilimanjaro
A volcano erupts when hot magma from beneath the surface of the Earth breaks through the crust. Some of it comes out as lava, hot ash, gasses, pyroclastic flows, and even chunks of rock that rain down around the volcano. Any chunk of rock spewed out of a volcano that’s larger than 65 mm in diameter (2.5 inches) is considered to be a volcanic bomb, or lava bomb.
Because volcanic bombs cool after they leave the volcano, they are extrusive igneous rocks. Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight. Bombs can be extremely large; the 1935 eruption of Mount Asama in Japan expelled bombs measuring 5–6 m (16-20 ft) in diameter up to 600 m (2,000 ft) from the vent.
| 1. Magma Chamber |
8. Vent / Main Pipe |
| 2. Country Rock |
9. Crater |
| 3. Ash Layer from earlier eruption |
10. Lava Bombs (> 64mm) |
| 4. Lava Layer from earlier eruption |
11. Ash Steam and Gas |
| 5. Sill |
12. Lava Flow / Pyroclastic Flow |
| 6. Secondary Vent / Side Vent / Dike |
13. Lapili (2 -64mm) |
| 7. Secondary Cone / Parasitic Cone |
|
Lava Bomb Types
Bombs are named according to their shape, which is determined by the fluidity of the magma from which they are formed.
Ribbon or cylindrical bombs form from highly to moderately fluid magma, ejected as irregular strings and blobs. The strings break up into small segments which fall to the ground intact and look like ribbons. Hence, the name "ribbon bombs". These bombs are circular or flattened in cross section, are fluted along their length, and have tabular vesicles.
Spherical bombs also form from high to moderately fluid magma. In the case of spherical bombs, surface tension plays a major role in pulling the ejecta into spheres.
Spindle, fusiform, or almond/rotational bombs are formed by the same processes as spherical bombs, though the major difference being the partial nature of the spherical shape. Spinning during flight leaves these bombs looking elongated or almond shaped; the spinning theory behind these bombs' development has also given them the name 'fusiform bombs'. Spindle bombs are characterised by longitudinal fluting, one side slightly smoother and broader than the other. This smooth side represents the underside of the bomb as it fell through the air.
Cow pie bombs are formed when highly fluid magma falls from moderate height, so the bombs do not solidify before impact (they are still liquid when they strike the ground). They consequently flatten or splash and form irregular roundish discs, which resemble cow dung.
Bread-crust bombs are formed if the outside of the lava bombs solidifies during their flights. They may develop cracked outer surfaces as the interiors continue to expand.
Cored bombs are bombs that have rinds of lava enclosing a core of previously consolidated lava. The core consists of accessory fragments of an earlier eruption, accidental fragments of country rock (such as xenoliths) or, in rare cases, bits of lava formed earlier during the same eruption.
To log this Eartcache, go to the given coordinates and answer the following questions via email or the message center
- When you enter the coordinates, you will see several lava bombs. How many lava bombs do you see? Do they look all the same or can you identify different types of lava bombs regarding the described types?
If yes, what are the differences and different types ?
So lets take a closer look on the big lava bomb at the coordinates.
- Describe the surface, the structure and the colour of that lava bomb? To which type does this one belong?
- Measure or estimate the size (width x height x depth).
Regarding the size, what do you think, to which eruption does this lava bomb belong (Shira, Kibo or Mawenzi) and why?
Additionally
- Take a photo of yourself and/or a personal item (e.g. your GPS) and attach it to your log!
Source
Wikipedia