Around the planet there are a wide range of volcanic eruptions.
Some are effusive, consisting of runny lava flows, while other are highly explosive, sending
massive columns of volcanic gasses and ash tens of thousands of feet into the atmosphere.
Yet, despite their differences, more than 99 percent of the planet's eruptions produce
rock types that nicely fit on this diagram that shows composition with less explosive
eruptions generally originating from less silica rich magma and more silica rich magmas
aiding in the generating of more explosive eruptions due in part to their higher viscosities.
However, throughout the planet's history there have been some highly unusual volcanoes that
have produced compositions so strange that they do not fit anywhere on this diagram.
One of these differing composition volcanoes involves a rock type that is so rare it has
depending on the scientist you ask either not erupted or only erupted at one site on
the entire planet in the last 20 million years.
This strange rock type is generally involved in highly explosive eruptions that create
several hundred foot wide craters with a prominent crater rim instead of a volcanic cone.
But most importantly, the rock in question known as kimberlite is of great economic interest
as unlike it more silica rich counterparts it can often contain diamonds.
As a whole, kimberlites are quite strange for a number of reasons.
For example, they generally do not contain any of the two most common minerals on the
planet; feldspar and quartz.
Instead, they often have less than 30% by weight silica, which is far lower than of
most magma varieties.
Individual specimens often contain olivine which in its gem form is known as peridot,
garnets, and on rare occasions diamonds among other minerals.
Also, when looking at known kimberlite deposits you might notice something strange; they largely
occur in areas of the world have not produced any recent volcanic activity in locations
such as Brazil, eastern Canada, and most famously South Africa.
This is not merely a coincidence but rather a feature of kimberlite eruptions.
They generally only occur in highly stable segments of the Earth's crust known as cratons
which have uniformly old basement rock, typically more than 600 million years old.
Cratons generally have unusually thick segments of lithosphere, which aids in the generation
of diamonds.
There, at a depth of 150 to 200 kilometers, elemental carbon due to intense pressure and
heat slowly becomes diamond.
Kimberlite magma begins in the mantle, slowing rising shallower into the crust at unusually
high speeds due to their gas rich composition typically with a high level of carbon dioxide.
Along their path upwards, the magma sometimes grabs diamonds deep underneath cratons and
carries them upwards.
Moving through the crust, the speed of the kimberlite intrusion continually increases,
often surpassing a speed of 40 miles per hour, generating earthquakes along the way.
In a matter of hours, perhaps even timespans an order of magnitude shorter, the kimberlite
intrusion reaches the surface, exploding in a powerful and highly explosive eruption.
While a kimberlite eruption has never been witnessed in recorded history, it is speculated
that such an eruption would create a plinian like eruption column, possibly reaching far
above 50,000 feet in height.
As pyroclastic flows race across the landscape, the eruption continues.
After the eruption finally ends, what remains is a several hundred foot to 1 mile wide crater
in the ground that has a raised rim on its side being known as a tuff ring.
Although diamonds can be ejected onto the surface during these powerful eruptions, the
vast majority of deposits mined today represent highly eroded segments of a kimberlite pipe
where the modern surface level was originally located at a depth of several kilometers at
the time of the eruption.
However, just because a kimberlite pipe exists does not mean it contains diamonds, and even
if it does, the odds of containing gem grade diamonds are even lower.
The vast majority of kimberlite pipes do not contain any diamonds, much like the Buell
Park kimberlite pipe in Arizona.
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