Looking around the planet, there are a number of medium sized volcanic cones, the most common
of which are cinder cones.
Individual volcanoes of which there are many in the southwestern United States can have
dozens or even hundreds of these features, which generally produce moderately explosive
eruptions.
For example, the San Francisco volcanic field in Arizona which produced its last eruption
in 1085 contains an estimated 600 cinder cones per the U.S. Geological Survey across an area
of 1,800 square miles.
However, even when closely spaced, cinder cones tend to be at closest 800 to 1600 meters
from one another.
This makes the following location in Iceland all the more unusual.
Here, at Lake Myvatn in the northern section of Iceland`, there are more than 1,400 closely
spaced volcanic cones which dot the landscape being as close to one another as 10 to 50
meters apart.
For example, while the most number of volcanic cones I could fit in a 300 by 300 meter square
in the San Francisco volcanic field was 2.
But, adjacent to Lake Myvatn, an example patch of ground of the same size has 27 volcanic
cones.
This means that in some areas the density of volcanic cones is stunning; measuring in
at 300 volcanic cones per square kilometer!
However, not everything is what it seems.
Although these volcanic cones are natural, you might have noticed that they are smaller
than typical cinder cones.
And, while being about the same size as some spatter cones these structures are not spatter
cones either.
In fact they are not true vents where magma travelled hundreds or even thousands of meters
from an underground magma chamber to the surface.
Instead, these are all referred to as "rootless cones" which are best described as volcanic
cones constructed not directly through a magmatic intrusion but rather due to the movement and
interaction of lava flows with the surrounding landscape on the surface.
The various volcanoes in Iceland including the ones in the central northern half of the
country exist due to two factors.
Here, the crust is spreading apart to the east and west as the Eurasian Plate is spreading
apart from the North American plate.
This has created a series of weak points in the crust, and due to a mantle hotspot underlying
the country, it sends large volumes of molten rock into the crust.
Since the divergent plate boundary represent a path of least resistance, the intrusions
sometimes subsequently erupt onto the surface, which over time has formed a chain of volcanoes.
Adjacent to Lake Myvatn are two active volcanic complexes; Krafla and Heiðarsporðar.
Approximately 2,300 years ago, an body of magma advanced upwards, generating a series
of earthquakes as it intruded into the crust.
This magma eventually trended into a north to south direction, involving what eventually
became a slightly more than 7200 meter long intrusion of magma which was long and thin.
Eventually, this magma reached the surface, ejecting lava into the air, and creating a
series of lava fountains along a several kilometer chain of activity.
As material fell back to the ground, it create dozens of spatter cones.
Due to the region's topography, this lava which had a relatively low viscosity for molten
rock flowed towards the west.
When the lava came into contact with the present lake, it generated a series of littoral explosions.
Eventually, a brittle lava bench was constructed, and active lava channels moved on top of it.
However, this ground soon became saturated with water.
This water due to the heat above it soon flashed to steam and exploded upwards in sections,
cutting a hole in the lava tube.
This ejected material in a mostly circular radius.
This process then repeated several times in the same location until a small volcanic cone
known as a rootless cone had formed.
Lava flows at one point likely created a bridge which flowed across the lake, but this has
most likely since been eroded.
By the time the eruption had ended, 220 square kilometers or 85 square miles had been covered
in a layer of volcanic rock.
Such abundances of littoral cones were only possible due to the unusually large area the
lava covered, abundant groundwater, the low viscosity of the lava, largely flat topography,
and the large lake which lava encountered.
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