Several million years ago, a towering gray stratovolcano on the border of Arizona and
Nevada was producing an explosive eruption.
During this eruption, intermittent vulcanian explosions occurred, ejecting bombs of lava
and producing small pyroclastic flows.
Simultaneously, a viscous lava flow of andesite rock slowly flowed down the mountain, forming
a lobe of fresh volcanic rock.
This eruptive process repeated for several hundred thousand years, until the volcano
in question became extinct.
Upon its completion, this volcano, known as Hamblin Mountain stood at 4,000 feet or 1,220
meters tall and 8 miles wide.
Thus, you might be wondering where this volcano is today as much of its edifice would be expected
to remain relatively intact.
The answer is on the northern shoreline of Lake Mead.
The mass of rock outlined on screen is one remnant of the ancient Hamblin Mountain.
Several miles to the west is another large chunk of this volcano.
And, 7 miles to the west is a third mass of material which represents where the modern
Hamblin Mountain's summit is today.
This volcano was strangely perfectly split in two right down the middle, and went on
to be divided once again, forming three distinct segments.
These segments are spread apart by a vast area, indicating an unusual geologic setting.
The reason why the Hamblin mountain volcano formed relates to its location in the "basin
and range province".
East of the Cascade Mountains and west of the Colorado plateau, the crust is slowly
stretching and spreading apart.
This stretching has created thin zones in the crust which allows magma to travel upwards
and on occasion erupt onto the surface.
While this extension is still ongoing today, producing relatively young volcanoes such
as Soda Lakes in Nevada, it was occurring at a much greater rate between 10 and 16 million
years ago.
Approximately 13 million years ago, a volume of low silica basaltic composition magma intruded
into the crust.
Over time, this magma leached material from surrounding rock and experienced fractional
crystallization, causing the underlying magma to switch to a basaltic andesite and then
andesite composition.
Then, this magma advanced towards the surface before erupting in an explosive manner.
Soon a gray volcanic cone was constructed, which subsequently continued to get taller
and taller over time.
Then, after hundreds of thousands of years of intermittent eruptions, the volcano became
extinct.
At the time, Hamblin Mountain had a large summit crater, layers of ash, dikes on its
edifice, and andesite lava lobes.
More importantly, a very large strike slip fault just happened to perfectly split the
volcano right down the middle, which was known as the Hamblin Bay Fault.
Every year due to this fault, the southern half of the volcano moved several hundredths
of an inch to the east, while the northern half moved to the west.
After more than 5 million years of fault movement, the two halves of the volcano were miles apart.
Then, a large north-northeast trending fault activated in the southern block of the volcano.
This caused the northwest corner of the block to sheer off, creating a third lobe of material.
This lobe was then slightly stretched, forming the modern appearance of Hamblin Mountain.
What remained of the rest of the southern section is today called the Cleopatra Lobe,
which can be easily spotted due to its darker coloration from satellite.
While millions of years of erosion has destroyed the vast majority of what remained on each
lobe, one prominent feature has remained.
Most notably on the Cleopatra Lobe are dozens of raised masses of solidified rock known
as dikes.
If you map the orientation of these dikes, you will note that they all point towards
what was formerly the center of the overall volcano.
Each of these dikes represents vertical intrusions of magma which for the most part never reached
the surface.
Dikes which point towards the center of a stratovolcano or at depth magma body are a
relatively common occurrence, but it is nevertheless impressive to see the interior of a large
volcano.
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