Post by fernwood on Jul 19, 2016 20:43:33 GMT -5
I was asked to create a thread about Volcano/Lava Bombs, so here it is. Local photos included.
Lava bombs are formed when molten lava is ejected with extreme force into the air in clumps. These clumps often partially cool before reaching the Earth. Volcanic bombs are larger than 64 mm in diameter. Some are huge boulders. Smaller pyroclasts are called lapilli. They may contain other elements, including lava from previous eruptions, crystals including diamonds/opals and particles the picked up after landing and rolling to their final resting site. There are several types of lava bombs whose names refer to the conditions they were formed under.
They are fragments of fluid or partially fluid lava or rock. Bombs are ejected during the eruption in a semi-solid, or partial molten, condition. Volcanic bombs undergo widely varying degrees of aerodynamic and/or impact shaping, depending on their fluidity, during the flight through the atmosphere and subsequent impact with the ground. Based on their shapes after they hit the ground, bombs are variously described, in the following graphic terms:
Bread-crust Bombs - These are bombs that have had their surface fractured, without breaking the bomb apart. These fractures form in one of two ways. In both cases, the exterior shell of the bomb cools and solidifies, while the interior remains molten or partially molten. Impact with the surface can cause the bomb to flex, thus fracturing the solid surface. Once on the ground, gases in the molten interior begin to expand, causing the solid surface to fracture and expand. These are still occurring in parts of the world where there are active volcanoes and can be a huge hazard.
Cow-Pie Bombs - Also called cow-dung bombs. These are very fluid bombs that are greatly deformed during impact. The fluid nature of the bomb causes the lava to flow outward from the center of the impact, forming a roughly circular, pancake-shaped bomb.
Fusiform bombs - Also called spindle or almond bombs. These bombs have an elongated form which tapers down at each end, with a relatively smooth surface. Many have an almond shape.
Irregular Bombs - These are bombs that have no distinguishing shape.
Ribbon bombs - Bombs that have a long flat, ribbon-like form.
Spherical Bombs - As the name implies, these are bombs with a spherical or ball-like shape.
The ones I am finding are attributed to the Milladore volcanic complex which was located about 25 miles from here. They arrived in my fields via the Glacial Lake Wisconsin. This volcanic complex was active during the Proterozoic Eon extended from 2.5 billion to 541 million years ago.
Below are some photos of the lava bombs I have found. These were identified by a local geologist/UW Geology Professor and Museum Director.
I have found some with opalized areas and unknown crystals. Included a close up of the largest one, a close up of crystals on some, and one that was cut in half by manofglass.
Last ditch effort. Wiped a lot of programs off computer and was able to use Flickr without it crashing. Am unable to use half the feature there, so do not know if this will work or not.
Posting thumbs here, as that is all I can do. Click, if you dare, for larger photos.
I doubt that these would tumble well, as areas of them exhibit properties of pumice, ie they absorb water. Overall weight is a lot heavier than pumice. But they are exciting to many local rock collectors and children. A polished face specimen would look great.
Let me know if there are any questions, as I have done a lot of research on these.
Lava bombs are formed when molten lava is ejected with extreme force into the air in clumps. These clumps often partially cool before reaching the Earth. Volcanic bombs are larger than 64 mm in diameter. Some are huge boulders. Smaller pyroclasts are called lapilli. They may contain other elements, including lava from previous eruptions, crystals including diamonds/opals and particles the picked up after landing and rolling to their final resting site. There are several types of lava bombs whose names refer to the conditions they were formed under.
They are fragments of fluid or partially fluid lava or rock. Bombs are ejected during the eruption in a semi-solid, or partial molten, condition. Volcanic bombs undergo widely varying degrees of aerodynamic and/or impact shaping, depending on their fluidity, during the flight through the atmosphere and subsequent impact with the ground. Based on their shapes after they hit the ground, bombs are variously described, in the following graphic terms:
Bread-crust Bombs - These are bombs that have had their surface fractured, without breaking the bomb apart. These fractures form in one of two ways. In both cases, the exterior shell of the bomb cools and solidifies, while the interior remains molten or partially molten. Impact with the surface can cause the bomb to flex, thus fracturing the solid surface. Once on the ground, gases in the molten interior begin to expand, causing the solid surface to fracture and expand. These are still occurring in parts of the world where there are active volcanoes and can be a huge hazard.
Cow-Pie Bombs - Also called cow-dung bombs. These are very fluid bombs that are greatly deformed during impact. The fluid nature of the bomb causes the lava to flow outward from the center of the impact, forming a roughly circular, pancake-shaped bomb.
Fusiform bombs - Also called spindle or almond bombs. These bombs have an elongated form which tapers down at each end, with a relatively smooth surface. Many have an almond shape.
Irregular Bombs - These are bombs that have no distinguishing shape.
Ribbon bombs - Bombs that have a long flat, ribbon-like form.
Spherical Bombs - As the name implies, these are bombs with a spherical or ball-like shape.
The ones I am finding are attributed to the Milladore volcanic complex which was located about 25 miles from here. They arrived in my fields via the Glacial Lake Wisconsin. This volcanic complex was active during the Proterozoic Eon extended from 2.5 billion to 541 million years ago.
Below are some photos of the lava bombs I have found. These were identified by a local geologist/UW Geology Professor and Museum Director.
I have found some with opalized areas and unknown crystals. Included a close up of the largest one, a close up of crystals on some, and one that was cut in half by manofglass.
Last ditch effort. Wiped a lot of programs off computer and was able to use Flickr without it crashing. Am unable to use half the feature there, so do not know if this will work or not.
Posting thumbs here, as that is all I can do. Click, if you dare, for larger photos.
I doubt that these would tumble well, as areas of them exhibit properties of pumice, ie they absorb water. Overall weight is a lot heavier than pumice. But they are exciting to many local rock collectors and children. A polished face specimen would look great.
Let me know if there are any questions, as I have done a lot of research on these.
