Vibe-Fall-2024 - Flipbook - Page 64
A look at a topography map of New Hampshire shows a clear indication of the
remnant Ossipee ring dike. The geology of modern mountains can be hard to
see because mountains are usually covered in trees, but hikes throughout the
Ossipee Mountains provide a glimpse at the geologic landscape. Connor Pond
and Dan Hole Pond on the eastern side of the complex are thought to be the
remnants of ancient gas vents.
local geologic name for the syenites that comprise the dike itself
is the “Albany porphyritic quartz syenite.” Syenite is a feldspar-rich
igneous rock. Porphyritic refers to the large, dispersed crystal
chunks of syenite and quartz found within this rock.
The collapsed caldera and its fractures sometimes sank to a
low enough position in the crust, with subsequent burial by sediments, to preserve it from erosive forces for millions of years until
the ring dikes and inner layers of the caldera were later exposed
to the earth’s surface and the forces of weathering.
ring dike provide evidence that volcanoes were here, but the
Ossipee Mountains are not an extinct volcano, rather the Ossipee
Mountains are comprised of what came out of volcanos: crystallized magma, ash, lava flows, and chunks of rock that spewed
forth at times from explosive eruptions.
What happened to most of these lava and ash deposits if
they used to cover much of New Hampshire? The same force that
continues to wear down all of our mountains over eons: erosion.
Most of the Moat volcanic rocks have been eroded from the
In addition to the Ossipee Mountains, there are other remnant ring dikes throughout the
White Mountains to the north. Thick deposits of volcanic rock are found on Moat Mountain.
When the rocks beneath your feet look black or have rusted to a brownish color, you are
stepping on the remains of volcanic eruptions that took place right here in New Hampshire.
Although the Albany syenite forms the dike, the most notable
sign of past volcanic activity is the presence here and there of
the Moat volcanic rock. This rock is the remains of ash and lava
that flowed from fractures in the earth’s crust as it stretched out
and apart preceding the eventual breakup of the supercontinent
Pangea. This rock is around 170 million years old, a bit older than
our local granites, which formed as a consequence of the same
stretching of the earth’s crust. This was during the geologic era
known as the Jurassic, while dinosaurs roamed the landscape.
During this era, Moat volcanic rocks were subsequently buried by
more lava and ash flows and sediment deposition over millions of
years. Thus, the Moat volcanic rock and the Ossipee Mountain’s
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surface, despite the fact they were once several miles thick, in the
process exposing slightly younger aged granites and much older
schist rock beneath. The Presidential Range rocks, for example, are
composed of this older schist, the metamorphosed remnants of
oceanic clays and shoreline sands that became mica and quartzrich rock, known as the Littleton Schist by geologists.
Moat volcanic rocks outside of the White Mountains and
Lakes regions are generally not preserved, possibly because they
were not dropped down into ring dikes and were exposed to
surface erosion for a much longer period of time as a result. If
you stand tall, erosion will get you! These unique volcanic rocks
have been wiped off the face of most of New Hampshire forever,
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