While studying some Google Earth images earlier
this year, I discovered a geological anomaly that I later learned
was an extinct volcano in New Hampshire's Pawtuckaway State Park
area. This structure is specifically referred to as a "ring
dike" and as it turns out, is one of at least three in the
Granite State. This particular ring dike contains what was believed
to be an elevated pluton (volcano's plug). If you picture a cork in
a bottle, you are going to be climbing the cork. From the trailhead
coordinates, it is a one-mile hike up a moderately graded hill
(referred to as "Middle Mountain", and follows along a decent
trail. The trailhead starts off at about 200 ft. above sea level
and the summit stands at 800 ft. There is a road that loops off of
Reservation Road (Some maps refer to it as Mountain Rd. - junctions
at Gate 4, loops around the pluton and then comes out at Gate 5
back on Reservation Rd.). Note that as of 1/5/2011, Gate 5 is
closed but Gate 4 is open and the road is passable with something
of an offroad vehicle. I made it down in 2WD in my Tundra. There is
one questionable spot at N43° 06.193 W071° 11.198. Make it past
that and everything else is easier. During the snowy season, the
roads become snowmobile trails. If you choose to pursue this cache
during the Winter or Spring months, I would suggest that you park
at Gate 4 and hike in the .69 miles to the trailhead that takes you
to Middle Mountain's Summit. After tilting the view of the area in
Google Earth's software to get a better look at the complex's
elevated rims, the image (to me) looked like a large eye looking
back up at me as seen below, and hence the Dragon's Eye earth cache
got its name.
At the Posted coordinates, you will find a lone
boulder, measuring about 4 ft. high and 5 ft. wide. I
recommend that you bring a magnifying glass with you. I
would also suggest you bring a camera that is capable of taking
some close-up photos that you can study later. There is no
container to find and no physical log book to sign.
To get credit for this cache, you
must email me the answers to the following
questions:
1) Explain what a dike is
and how it is formed. Explain how a ring dike is
formed.
2) The
measurements of the length and width of the inclusion found in the
boulder at ground zero (found in the side that faces the trail).
What is the material that this inclusion is made of?
3) How did the
inclusion got into this rock.
4) What is the connection of this boulder to the ring
dike?
5) Detailed description of the boulder surrounding the
inclusion including color, pattern, material(s) contained therein,
whether it is course-grained, medium-grained or fine-grained. In
effect, describe in geological terms what this boulder is. You
might also want to throw in information about how it got to where
is is. After all, it's sitting out in the woods all by itself.
Where is the rest of its family?????????
My original
idea for this earth cache was to have you hike the outer rim of the
volcano and find various waypoints, where there would be examples
of volcanic activity or particular types of rock. This way you
would get the full experience of what the ring dike has to offer
for views and you would get a real feel for the geology involved.
You would then have to climb Middle Mountain to study the boulder
in question and put the information you gathered from the outer
ring to work in answering the questions. Middle Mountain does not
have an exposed summit, but as you are hiking to the boulder, you
will get peeks of North Mountain and South Mountain that will let
you see that you are inside the volcano. However, after considering
that many of the waypoints I found during my own exploration will
be buried by snow, I have decided instead to upload photos that I
took during several trips to the park or found through the internet
of rocks/minerals you might see or examples of volcanic activity.
In this way you can do the cache in the winter months and/or save
yourself a lot of legwork if you don't want to make the full trip
around the rim. Additionally, I am including links to various sites
of interest to give you a head start on your studies.
SIMPLIFIED VERSION OF HOW TO BUILD A RING DIKE
IN FIVE EASY STEPS:
ASSORTED DEFINITIONS, CONCEPTS AND
RELATED INFORMATION:
GEOLOGY 101:
Geology is
the study of the Earth; the materials of which it is made, the
structure of those materials, and the processes acting upon them.
Geologists use a dizzying array of terminology and complicated
theories to discuss and better understand these things, which I
will not expect you to master. Instead, I will put forward some
assorted definitions and concepts that I think are important in
helping you learn how Pawtuckaway's ring dike came to be what it is
today.
The very
fist thing you need to wrap your brain around is just how old the
earth itself is. Current estimates say that it is about 4.54
Billion years old - that's a really long diary and a long time to
allow for many, many changes to take place. So scientists have
broken up this timeline into smaller phases.
Three "eras" have been designated with each era broken down to
"periods". For example, when you read something referred to as
coming from the Devonian period, it
means that it occurred somewhere between 410 mya and 360 mya (note
that geologists use the abbreviation "mya" to mean "million years
ago" or "kya" for "thousand years ago". Sometimes they shorten even
this to "ma". Also, be aware that the actual date ranges may change
slightly from one source to another). The original rock that was
laid down throughout the area came from the Silurian or Devonian
period. Imagine this as the foundation from which our ring dike
would later grow. Other periods are; Carboniferous (360 mya
to 286 mya), Permian (286 mya to 245
mya), Triassic
(245 mya to 208 mya), Jurassic (208 mya to 146
mya) and Cretaceous (146 mya to 65
mya).
Locally,
three
layers (called formations) of materials (combinations of muds,
sediments, and shales) were laid out and are known as the Elliot, Berwick and
Littleton Formations. They are shown in the illustration
above. There is also an excellent map of New Hampshire's
Geologic Bedrock make up located here. Once we have this nice
3-layer foundation laid out, we can start the process of making a
ring dike. Now, when we think about volcanoes, we usually think
about the classic paper-mache' school project displaying a
deep-throated mountain with an open core; spewing "lava" (baking
soda and vinegar) out all over its sides. A ring dike on the other
hand is different. Fred Langa described in his (slightly
paraphrased) August 4,
2008 blog:
"Ring dikes are formed when a dome of molten magma rises in a
slow-motion bubble from deep within the earth (think of a lava
lamp). The solid rocks surrounding and above the rising bubble are
stretched and broken/fractured; making room for the ascending mass.
Magma can then flow upward through these cracks and form dikes.
Because the rising magma bubble is roughly circular, the field of
dikes surrounding the bubble will also be roughly circular.
Eventually, the magma bubble cools down. Erosion sets in and lowers
the surface down to the level of the previously-buried dikes.
Remember that these dikes are intrusions of one kind of rock rising
through a different, older rock. Because the rocks are
different, they erode at different rates, which eventually creates
and exposes the structure that we call a ring dike."
It is
important to understand that a ring dike is not a "bubble that
popped", but rather a process in which an expanding dome of
underground activity fractured some of the surrounding bedrock and
backfilled it so the thickness of the crust in this area is became
flared with additional material; plus the dome itself cooled
slightly to become a solidified plug (pluton). Lava would
sometimes flow past the pluton as it escaped to the surface, adding
additional material around what became the ring's footprint. During
my explorations of the complex, I found several examples of smaller
dikes everywhere. In some instances, these rocks remain part of the
bedrock while other examples are found as loose stone. There is a
general principle that says coarser grained stone tends to have
cooled more slowly, allowing grains or crystals to grow larger
before they become solid. Lots of dike material tends to be finer
grained material that backfilled older stone, which was split
during the ring dike formation process. The finer grained materials
also tend to be considered as the more recent additions. The outer
rim of the complex is a coarse grained Monzonite but there are
pockets of fine grained Monzonite that chemically is the same as
the coarser stuff but cooled faster to make finer grains. Note that
there are pockets of fine grained Monzonite on either side of the
coarse grained Monzonite that makes up most of Middle Mountain.
Unique to
Pawtuckaway's complex is what some believe to be a second crater -
a ring dike within a ring dike. This possibility was raised in G.
Nelson Eby's 1984 study,
"Mount Pawtuckaway Ring Dike Complex".
Click here to view a slide show of various images and maps showing
ring(s) of the Pawtuckaway complex and illustrate both the
original outer ring and evidence to support the theory that the
"pluton" (Middle Mountain - where ground zero is located) is
actually part of a second, smaller ring formed after the outer one
solidified. It bears mentioning that the processes involved in
creating this complex would have taken place over millions of years
and not as a quick, violent, single eruption or event. It is
estimated that the magma would only have been ejected at a rate of
six feet per year over a period of perhaps some seven to ten
million years. Consider these three points on this issue:
1) A pocket
of Felsite (light colored stone and probably some of the most
recent rock to be introduced to the complex) exists around the
center of what would be the inner ring, suggesting an
intrusion of new material after the pre-existing Monzonite
(outer ring) layer was emplaced.
2) A large
wedge of the original outer dike presents as being forced upward
and outward from the ring by several hundred feet. It appears to be
broken free from this original position and not "stretched" away;
indicating that the outer ring would first have had to be solid
rock. Note the locations of adjacent fault lines that lead directly
to this displaced wedge. They cross,
forming a large "X". The map places a "U" at the "uplifted"
section for clarification. The wedge occupies these uplifted
sections, while the opposite wedge of the X, shows the land to have
dropped down compared to the adjacent fault land. Would the
uplifting of the wedge and a later de-pressurization of the magma
chamber have created a void into which that opposite side of
the X dropped?
3) Where the
inner ring contacts the outer ring, it should be pointed out that
this happens to be located very near North Mountain's summit; the
highest of the three summits in the complex. Looking at the
topographic map on the outside wall where the word "Park"
appears (as in State Park Boundaries), it appears that there is a
stress break, as if caused by the intrusion of the second ring into
the first, which perhaps added the additional height to the
intersecting point with the additional activity??????
Now let's
discuss the material that we will be working with. When you talk
about rocks and minerals, you might imagine soups. Go to the
supermarket and there are a lot of soups to choose from. Just as
each soup has a different combination of ingredients; when you
mention granite, you are talking about a rock that is made up of a
certain combination of minerals such as feldspar, quartz and
biotite that make it "Granite" and not "Gabbro". Even if your
chicken noodle has a little cilantro thrown in when you get it
home, it's basically still chicken noodle soup. More on soups and
their ingredients in a moment, but let's go over a few definitions
that you are likely to see in some of the information presented
before going further and then we can get into the nitty-gritty of
rock and mineral examples.
Arcuate: curved - in the shape of a bow.
Aphanite: Used as an
adjective (aphanitic) to mean a super fine grained igneous
rock.
Caldera: The center area of a volcano, like a bowl, when the
bottom of the volcano collapses into the magma chamber.
Dike: A crack in
the earth or in the bedrock, which is later filled with magma/lava.
Often the new layer of material is a different rock or
mineral or consistency and you end up with something that
looks like a stripe, or like the grouted line between two ceramic
tiles in your bathroom (keep in mind that this stripe is sometimes
actually a wide sheet of material and you are in fact only seeing
its edge view). Again, note that in general, faster cooling lava
creates finer material (smaller crystals or grain sizes). Lava
solidifying at the earth's surface, tends to cool fastest and can
chill to a fine enough grained material that grain size is not
easily seen without magnification. Conversely a dike filled with a
coarse grained material was likely formed deep below the earth's
crust.
Fault:
A crack in the earth where land masses have shifted. Sometimes they
slide over one another like plates and sometimes the shift side to
side.
Felsic: Adjective used to
describe lighter-colored stone. Opposite of mafic. Common felsic
minerals include quartz, muscovite, orthoclase, and the
sodium-rich plagioclase
feldspars. The most
common felsic rock is granite.
Igneous: refers
to rock that was made from magma or lava (it's magma if underground
and lava if above ground). Granite and Diorite are two
examples.
Intrusion: Pretty much
a dike on a smaller scale. More often used when talking about a
particular boulder, rather than a long trench running in the
ground. This will sometimes have the "striped" look of a dike but
other times will appear as blotches of one material that sort of
melted into another - one material "intruded" into another.
Mafic: Adjective used to
describe dark-colored rock. Opposite of Felsic. Such rock usually
has high concentrations of magnesium and
iron;
the term was derived by contracting "magnesium" and "ferric".
Think dark brown, black or green, and sometimes rusty looking.
Phenocryst: Crystals
imbedded into rock, whose grain is substantially finer than the
phenocrysts themselves.
Porphyritic: A
mixture of different grain sizes in a rock due to a difference in
cooling history; cooling slowly at first (making larger grain or
crystal sizes), followed by a period of faster cooling (creates
smaller grains/crystals)
ROCKS YOU MIGHT SEE AT PAWTUCKAWAY:
Basalt:
example slide show: most widely distributed of all volcanic
rocks. Fine grained and always dark, giving the appearance of
burned sand. Often found as dike material and consists of Olivine,
Plagioclase Feldspar, Pyroxene, and sometimes has accessory
quantities of Biotite, Ilmenite, Hematite, Apatite, Quartz,
Magnetite, and Amphibole (Hornblende)
Diorite:
example slide show: Only slightly different from Gabbro
(Diorite being slightly darker). Typically speckled black/white in
equal proportions and consists of Amphibole (black) and a white
variety of Plagioclase Feldspar. Typically has very little
quartz.
Granite: example
slide show: Granite is composed primarily of Feldspar, Quartz
and one or more dark minerals, which may be mica, pyroxene, or
amphibole. It is essentially solidified magma and comes in a
variety of colors.
Gabbro:
example slide show: Basic volcanic rock such as basalt, usually very
dark in color and high in Magnesium
and Iron.
Syenite:
example slide show: Similar to Granite but without excess
Silica and little or no Quartz. Generally, coarse-grained.
Lighter materials are feldspars and darker minerals could be
Biotite, Hornblende, Acmite, or Riebeckite. Accessory minerals are
Muscovite, Apatite, Titanite & Corundum. Syenites containing
significant amounts of plagioclase Feldspar are called
Monzonites.
Monzonite:
example slideshow: Course grained rock of the Granite type, but
difficult to distinguish from Granite. Alkali and Orthoclase
Feldspars are present in approximately equal amounts. Typically
only a minor contribution of Quartz.
Pegmatite:
example
slide show: A coarse-grained, igneous rock, but irregular in
texture, and similar to a granite in composition; sometimes occurs
in dikes or veins and sometimes contains valuable minerals.
MINERALS YOU MIGHT SEE IN PAWTUCKAWAY'S
ROCKS:
Amphibole: 3D wall
group: Generally, a dark colored rock-forming material,
comprised most often of Hornblende but also
Anthophyllite,
Cummingtonite and
other minerals such as Tremolite.
Anorthite:
example slide show: A rare compositional variety of
plagioclase, occurring in mafic igneous rock.
Augite:
example slide show: Mineral shows as dark and has an appearance
of iron.
Biotite:
example slide show: A dark colored (usually black) mica. Very
common and often found in Granite. Often shows as
piles of flakes (called books).
Feldspar: 3D wall
group:
example slide show: Most common mineral world-wide. Makes
up about 60% of the earth's crust, and is one of the components of
Granite.
Lamprophyre:
example slide show: Modern science treats lamprophyres as a
grab-bag term for ultrapotassic (silica-depleted), mafic
(dark-colored) igneous rocks
which have primary mineralogy consisting
of amphibole or biotite,
and with feldspar in the groundmass.
Magnetite:
example slide show: dark colored, as iron. magnetic.
Muscovite:
example slide show: Light-colored mica, typically silvery-clear
but can come in other colors.
Olivine:
example slide show: Typically in shades of green but running
from very dark green to light.
Pyroxene: 3D wall
group:
example slide show: a grouping of dark-colored (typically
Brown-green, black, green-black, brown, purplish brown) family of
minerals, the most common of which in the Pawtuckaway area is
called
Augite.
Quartz
example slide show: Second most abundant mineral in the world;
easily spotted minerals in New Hampshire. Comes in a variety of
colors, but often as clear, smoky or in rose colors.
Click here for a NH
Mineral Species List from MINDAT.ORG
LINKS TO ADDITIONAL
READING:
Mount Pawtuckaway Ring-Dike Complex by G. Nelson Eby (1984)
Igneous
Lab: White Mountain Magma Series NH and Cape Ann Igneous Suite, MA
(1986)
Ring Dikes and Plutons: A Deeper View of Calderas as Illustrated by
the White Mountain Igneous Province, New Hampshire by John W Creasy
and G. Nelson Eby (1993)
Third Hutton Symposium on Granites and Related Rocks by G. Nelson
Eby (1995)
Map of Pawtuckaway State Park hiking trails, Mineral Data for Mount
Pawtuckaway
Volcano and Ring Dike
Animation
YouTube Video - Basic
Rock Identification: Geology
YouTube Video -
Identifying Rocks : Identifying Igneous Rocks