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METAMORPHIC ROCKS- WHY ROCKS BEND AND FOLD- PART
3: COMPLIMENTARY EVIDENCE
JOE REILLY: So Britt, I think I understand how colliding plates can
cause mountains to form. But if we take a look at this map and find
the Green Mountains, right about here, theyʼre nowhere near a plate
boundary. I donʼt get it. Are we sure that this is how these mountains
formed?
BRITT ARGOW: Well, Joe, scientists believe that at the time these
mountains were forming, Clay Point was near a plate boundary.
Geologists have determined that the age of Clay Point rocks is about
500 million years old. Using data from rocks around the world,
scientists can model what the surface of the Earth might have looked
like back then. It turns out that half a billion years ago, the African
plate slammed into North America. When the two slabs of continental
material collided, it drove the Green Mountains skyward. Scientists
like Keith Klepeis believe that although Clay Point is relatively far
away from where the plates collided, it still was affected by the
enormous pressure of that collusion. The rocks folded and started to
break, as seen in this v-shaped fracture. Ten miles east of Clay Point
is another outcrop, where folding, faulting, and breaking of the rock
occurred. This outcrop is called Lone Rock. Here we can see
evidence of even greater pressures from the plate collusion, where
one rock was thrust up on top of another.
KEITH KLEPEIS: When you walk up the beach at Lone Rock, the first
thing that strikes your eye is that youʼve got a cliff face that has two
different colored rocks. Thereʼs a black rock in the bottom part of the
cliff, and thereʼs a yellow rock at the top part. And in between is a
large crack that separates the two. Now, geologists have been able to
determine that the yellow rock on top of the cliff is actually older than
the rocks located at the bottom of the cliff. And thatʼs a very unusual
configuration. And the reason why we know itʼs unusual is that most
often, the oldest rocks are on the bottom and the youngest rocks on
the top--the very opposite situation. You can imagine, if we have a
rock layer, and the rock layers are made up of sediment, and those
sediments are percolating down through the air, through the water,
the first layers to form are down here at the bottom. Then as those
sediments accumulate, the youngest ones start to form at the top. So
we have this normal sequence, oldest on the bottom, youngest on the
top. Thatʼs not what we see at Lone Rock. At Lone Rock, we have the
opposite situation, where the oldest rocks are on top, and the
youngest rocks are on the bottom.
BRITT ARGOW: The pressures at Lone Rock were powerful enough
to fault and thrust huge sections of rock on top of each other. We can
even see evidence of this movement in the crack between the two
rock faces.
KLEPEIS: When you go up into that crack and look in the bottom, you
see little grooves, little track marks, little scrapes along that bottom
layer.
ARGOW: Imagine Lone Rock before the mountain building event.
Here, the layers on the bottom are the oldest, and the layers on the
top are the youngest, as we would expect. But as the plates collided,
enormous pressure folded parts of the rock. At some point, the rock
faulted. The continued pressure thrust one section of rock on top of
another, doubling the rock sequence. After years of weathering, the
top layer eroded away, leaving a cliff face with older rocks on top of
younger rocks. And if we look carefully at the bottom layer at Lone
Rock, small folds provide evidence of extreme pressure and
metamorphism. Further east, in the heart of the Green Mountains,
more evidence of this enormous pressure exists. But most of it is
buried. Some of the most intensely metamorphosed rock occurs within
the central cores of mountains.
KLEPEIS: Now, what this is telling us, the reason why metamorphic
rocks are important is because they tell us about immense forces that
have occurred sometime in the past that have changed rocks. Theyʼve
changed them away from their original configuration, just like we saw
at Clay Point, where rocks that were originally horizontal or flatlying
became folded and steeply dipping. Metamorphic rocks tell us about
these forces, these changes that have happened in the past. It is
amazing. I mean, what this is telling us is intense pressure, and the
rocks respond to that pressure, and they create features that we can
read today and piece together that story. Itʼs telling us about an
ancient mountain building event that created the western part of
Vermont, and has created some of these fantastic exposures that are
now along the shores of Lake Champlain.