Volcanic Bombs
Museum Specimens
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USGS Western Region Geology and Geophysics Science Center
The URL is http://pubs.usgs.gov/of/2004/1007/volcanic.html
Page Contact Information: USGS Publications Team
Volcanic bomb is a pyroclastic formation which was semi-molten (viscous) while ejected from a volcanic vent and is
therefore shaped while in flight. The term “Volcanic Bomb” is a term often mistakenly used by non-volcanologists to
describe all sizes of formations that fall to earth already hardened. By definition volcanic bombs are larger than 64 mm in
diameter. Smaller pyroclastic bombs are officially called Lapilli (plural) or Lapillus (singular).

Lapilli are a size classification term for Tephra, which is material that falls out of the air during a volcanic eruption or
during some meteorite impacts. Lapilli mean "little stones" in Latin. By definition lapilli range from 2 mm to 64 mm in
diameter. A pyroclastic particle greater than 64 mm in diameter is known as a volcanic bomb when molten, or a volcanic
block when solid. Pyroclastic material with particles less than 2 mm in diameter is referred to as volcanic ash.

Lapilli are spheroid, teardrop, dumbbell or button-shaped droplets of molten or semi-molten lava ejected from a volcanic
eruption that fall to earth hardened, or may be partially molten. These formations are the direct result of liquid rock cooling
as it travels through the air. The Teardrop or almond shape occurs when a blob of lava is ejected with a spin which pulls
the lava into a shape which cools and hardens before falling to the surface. We shall call these volcanic rocks “Lava
Bombs” based to how they were formed. They are the smaller brothers and sisters to volcanic bombs.
Specimens listed here are natural as they were found. They were only rinsed off with
pure water to remove the loose dust and dirt.  Photos taken in natural sunlight with the
volcanic rock dry.
This is a set of six (6) small "lava bombs" (Lapilli) from near the Amboy Cinder Cone in Mojave Desert in
California. This set has pieces from 1.3 to 2.1" (33 to 54mm) in length and from 0.4 to 1.1" (11 to 28mm) in
width and 0.4 to 0.7" (10 to 19mm) in depth. This set weighs from top left to top right;  0.12oz (3.5g), 0.28oz
(8g), 0.13oz (4.5g), then from bottom left to bottom right; 0.34oz (9.5g), 0.96oz (27g), 0.58oz (17.4g).
Item# Set 1

Volcanic Bombs from
near Amboy Cinder
Cone, Mojave Desert,
California

$15.00
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photo to see a
larger version
Amboy Crater, San Bernadino, California
Amboy Crater is one of a chain of cinder cones extending
along old US Route 66 "The Mother Road" from Amboy on
the east to Pisgah Crater just west of Ludlow. Volcanic
activity at Amboy occurred in the last 6000 years. The
cinder cone is actually four coaxially nested cones and is
surrounded by 24 square miles of basalt lave flows.
"Bombs" are formed when a blob of lava is ejected with a
spin which pulls the lava into an almond or tear drop form
which hardens before falling to the surface. These bombs
were collected from the surface of the surrounding lava flow.
These small bombs are typical. The were collected over 20
years ago from the area near Amboy Crater in the Mojave
Desert. This cinder cone volcano is now a National Natural
Monument. Larger bombs had been picked up by
generations of rockhounds and geologists. They missed
these smaller ones.
Cinder cones are composed of random blocks of lava,
bombs and smaller loose cinder. You will recognize cinder
as maroon flower bed rock. Basalt is normally black, but
since it is iron rich, it can oxidize to a reddish hue during
cooling.  
photos courtesy of USGS, BLM and Google
This is a set of six (6) small "lava bombs" (Lapilli) from near the Amboy Cinder Cone in Mojave Desert in
California. This set has pieces from 0.87 to 1.99" (22 to 50mm) in length and from 0.5 to 1.8" (13 to 46mm) in
width and 0.48 to 0.8" (12 to 22mm) in depth. This set weighs from top left to top right;  1.3oz (36.5g), 1.2oz (35g),
0.38oz (11g), then from bottom left to bottom right; 0.28oz (8g), 0.16oz (4.5g), 0.36oz (10g).
Item# Set 2

Volcanic Bombs from
near Amboy Cinder
Cone, Mojave Desert,
California

$16.00
This is a set of six (6) small "lava bombs" (Lapilli) from near the Amboy Cinder Cone in Mojave Desert in
California. This set has pieces from 0.8 to 3.1" (22 to 80mm) in length and from 0.39 to 1.1" (9.9 to 28mm) in
width and 0.28 to 0.86" (7 to 21mm) in depth. This set weighs from top left to top right;  0.82oz (23g), 0.58oz
(16g), 0.28oz (8g), 2.5oz (71g),then from bottom left to bottom right; 0.04oz (1.5g), 0.28oz (7.5g).
Item# Set 3

Volcanic Bombs from
near Amboy Cinder
Cone, Mojave Desert,
California

$17.50
this sheet included with set
this sheet included with set
this sheet included with set
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