Landslides are an incredibly common geologic hazard which occur worldwide for a number
of reasons.
They can occur in response to earthquakes, heavy rainfall, and even volcanic activity.
For example, in 1772 at the Papandayan volcano in Indonesia, 140 million cubic meters of
rock slid up to 13.5 kilometers distant in response to a phreatic eruption.
While the landslides shown so far have all been fairly large, they represent merely a
fraction of the size of one of the planet's largest non-submarine landslides.
Within Utah is a massive complex of what was originally thought to be a series of ancient
landslide deposits.
However, this area seemingly contained a highly unusual rock type called pseudotachylyte.
Pseudotachylyte is a rock type often associated with incredibly voluminous landslide events
that create so much friction that they melt some of the involved rock.
Due to the appearance of this rock and the mysterious landforms in the area, it soon
became clear that the area did not contain numerous landslides but rather 1 supermassive
landslide.
Known as the Markagunt megaslide, this ancient landslide covered an area of 1,390 square
miles or 3,600 square kilometers, which is slightly larger than the entire state of Rhode
Island.
However, this megaslide was not a completely isolated event.
To the east and west are two other megaslides which also formed between 18 and 25 million
years ago.
Of these 3, the Markagunt slide is by far the most voluminous, as it involved 2,000
cubic kilometers of rock.
So, why was an area the size of Rhode Island involved in a catastrophic collapse?
To understand this we need to jump back to 23 million years ago.
At the time, central and western Utah were the site of numerous large volcanic fields
which produced fairly frequent eruptions.
One of these volcanic fields was known as the Maryvale volcanic field, containing cinder
cones, lava domes, stratovolcanoes, and large calderas.
In a manner of speaking, the area was somewhat similar to the modern Newberry volcano and
its surroundings in Oregon.
As time progressed and eruptions continued throughout the volcanic field, a specific
area began to be the site of highly focused activity.
This resulted in the creation of either a towering lava dome complex or a stratovolcano.
Magma slowly built up underneath this volcano until a magma chamber with a volume of more
than 100 cubic kilometers was present.
Eventually, the pressure underlying the volcano became too great and a catastrophic volcanic
eruption was triggered.
Due to the massive amount of erupted material, the underlying magma chamber was greatly drained.
This caused the ground overlying it to collapse downwards, forming the massive Big John caldera
which may have at one point looked similar to the caldera seen at Aniakchak in Alaska,
albeit with less snow cover.
In the millions of years following this eruption, the large silica rich volcanic center which
resulted in the Big John caldera moved northwards.
It then slowly began building a new volcanic center called Mount Belknap.
Since such a large volume of magma was involved, and more material kept intruding into the
shallow crust, this volcanic center was slowly pushed upwards by perhaps hundreds of feet
if not more.
This caused the strutural stability of the prior largely basaltic lava fields which acted
as a sort of homogeneous layer to be placed on a significant tilt.
Eventually, this tilt became too great, and a large section of the uplifted older lava
field collapsed to the south in an enormous landslide.
This landslide soon propagated to take adjacent rock with it, eventually cascading to include
2,000 cubic kilometers of rock.
Racing southwards at a speed of several hundred miles per hour, the landslide raced over mountains
and natural topographic highs, forming fold like features which are in reality large tilted
blocks.
By the time the landslide had settled, some material had travelled well beyond 40 miles
distant.
Although this landslide was truly massive, it was still smaller than the Nuʻuanu Slide
around 1.3 million years ago when the northern half of the island of Oahu collapsed, transporting
5,000 cubic kilometers of rock.
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