within the state of arizona are a number
of fascinating geologic features these
range from the impressive grand canyon
to the towering pillars of ash
representing an ancient super volcano in
chirakahua national monument to the
well-preserved meteor crater impact
crater near flagstaff another one of
these geologicalities is located in the
northeast part of the state on the
navajo nation zooming in from satellite
you might think that this feature term
build park is also an impact crater
however this is not the case instead it
is volcanic in origin but does not
purely represent only a caldera
rather this circular area represents the
largest kimberlite pipe on the planet
kimberlites are the main source of
industrial and gem grade diamonds and
this individual vent has produced a
number of beautiful gemstones
the vast majority of kimberlite pipes
around the planet are quite small
measuring between 75 and 300 meters
across
the most famous of these known as the
big hole measures 463 meters across and
produce 14 and a half million carats of
diamonds
these pipes are named due to their
rather round shape and abundance of the
rare volcanic rock known as kimberlite
kimberlite is an unusual gray volcanic
rock which has a low silica content and
originates from molten rock deep within
the mantle between a depth of 150 and
450 kilometers as a result kimberlites
contain seemingly exotic minerals that
are otherwise not commonly found due to
the depths they originate from formation
of this kimberlite pipe began
approximately 25 million years ago when
an unusual series of volcanics referred
to as the navajo volcanic field were
erupting across parts of arizona new
mexico utah and colorado at the time the
farallon tectonic plate was subducting
underneath the north american plate at a
very shallow angle melted material from
this collision then migrated upwards
before erupting on the surface thus
forming this chain of volcanoes much of
the magma which erupted into these
scattered vents had an unusually low
silica content as it originated deep in
the mantle
eventually a volume of magma unusually
rich in carbon dioxide was quickly
propelled upwards into the crust on the
surface a series of increasingly shallow
earthquakes continued then less than two
hours after the entire sequence began
the magma had reached the surface this
magma quickly interacted with the volume
of groundwater causing it to rapidly
flash the steam this steam then expanded
in volume until a massive explosion
occurred as a tall plume of ash rose
into the atmosphere pyroclastic flows
raced across the surrounding landscape
the eruption which had just occurred
from the four and a half kilometer or
2.8 mile wide volcanic crater called a
tough ring then out of this crater a
large volume of magma erupted onto the
surface carrying with it an abundance of
kimberlite lava once this solidified a
different composition magma intrude into
the crust forming two large intrusions
on the overall kimberlite pipe this
fresh batch of magma created another
large volume explosive eruption once
again sending a plume of ash high into
the atmosphere then due to the amount of
magma erupted the underlying magma
chamber was nearing on empty this caused
a section of ground to collapse
downwards forming a caldera
the rain dike shown here merely
represents solidified magma at the edge
of that caldera which has since been
exposed due to erosion
sadly this kimberlite pipe did not bring
any diamonds to the surface however it
did bring a number of other wonderful
gemstones to the surface the most
abundant gemstone which can be today
found at the site is peridot which also
originates in the upper mantle if a rock
with a reddish black crystal is spotted
you have found a pyrope garnet many of
the black crystals found are diopside
while oranges brown crystals are
clenohumite all of these can be used in
jewelry presuming you have permission to
collect there as it is navajo land i
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