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Summer Ash: Have you ever looked at a city skyline and wondered why it looks the way
it does?
Why buildings of a certain size, or shape, or height are built where they are?
New York City's famous skyline may have been predetermined well before humans ever arrived on the scene.
Karin Block: With respect to studying geology
in the city, it is important to understand
what is the foundation of New York. Even though
much of the bedrock is covered in New York City,
you can still see portions of it in
Central Park, and Inwood Park, and
it is extremely relevant to the infrastructure
of New York.
You may notice that where the clusters of
skyscrapers are located in New York is directly
related to where the bedrock is closest to
the surface. Those rocks happen to be extremely
hard and ancient rocks that can actually support very, very tall buildings.
Summer Ash: Then there's a lack of it below
Midtown, and in the Financial District?
Is there more bedrock than down there?
Karin Block: Between Midtown and the Financial
District, the same rocks exist, but they exist
a little bit deeper. They are not as close
to the surface, so there's a bit more of a
sediment cover, which makes a little bit more
challenging to build tall buildings in those
areas.
Summer Ash: Geologists study the past in order
to better understand the present and the future
of our planet. In this video, we'll talk to
geologists who work in the field and in the
lab to answer big questions like, "How did
this rock get here? What can it tell us about
the history of where it's found? And how can
we uncover the geologic history of our planet
from right here in New York City?"
But first, let's go over some basics.
Rocks can be divided into three different categories, based on how they formed:
We can find all three types
of rocks right here in New York City.
Alan Benimoff: Well, you've got almost every
rock type. You have igneous rocks, which come
from the melt. You have sedimentary rocks,
which are products of the erosion of other
rocks, and you have metamorphic rocks, which
would change from some other rocks.
You also have effects of the ice when it was
here. Remember, ice was here about 22,000
years ago, and when it went through the area,
rocks in the ice scratched the bedrock
and so forth, and so we have evidence of that.
So, we have a great deal here.
For Staten Island, we have almost every rock
type here. It's a small island, 13 miles by
7 miles, and we have igneous rocks, we have
metamorphic rocks, and we have sediments,
we have beach deposits…
Summer Ash: We have all these great rocks
to study, but how can they tell us about what
the city was like millions of years ago? Geologists
have formulated a series of guiding laws based
on what we can observe today. When we talk
about scientific laws, we are talking about
statements that describe phenomena based on
repeated observations or experimental results.
They are reliable descriptions of the world,
given specific conditions.
Karin Block: Geology has, in fact, a set of
guiding laws. The first and foremost is the
concept of uniformitarianism, which is the
idea that the physics and chemistry that operate
right now have always operated, that that
has never changed. It was first vocalized
about 1,000 years ago by a Persian philosopher
and natural scientist named Ibn Sina, popularized
by James Hutton.
What he said was that, as he observed the processes
that were involved in forming geology, or making
geologic formations, that there was no vestige
of a beginning, and no prospect of an end.
That’s where the concept of Deep Time was
really formulated.
The other pivotal figure in geology is Nicholas
Steno. Steno's laws are still used to interpret
the structures that we see in the field, and
they involve the law of horizontality; if
sediment or particles are deposited in a lake
or in the ocean, they will lie flat, and that's
the original way in which they were deposited.
The idea of superposition, meaning that if
you have a series of rocks or layers of sediment
that are being deposited, the layer that is
at the bottom is the oldest. The law of lateral
continuity, which is the idea that rocks will
remain of a particular time until there is
a change in environment that will then curtail
the formation of that particular rock.
And the law of cross cutting relationships,
which indicates that if you have a series
of rocks that are cut across or faulted, the
fault or the rock body that is cutting across
has to be younger than the rocks that already
existed.
Summer Ash:We can use these laws to make sense
of the patterns we see in rocks around New
York City. In geology, the present is the
key to the past. What can we observe now?
What does that tell us about how the city's
bedrock was formed, and how it changed over
time?
Today we're standing in Central Park, in New
York City, and we're surrounded by these really
cool rocks. What's so special about them?
(Merry) Yue Cai: Well, the first thing is
they are called Manhattan Schist. They used
to be sediments on the seafloor between America
and Europe at the time. When plate tectonics
carried an island called Avalon that collided
into Manhattan, the sediment got squeezed
and uplifted.
They were part of a mountain the size of the
Himalayas that was called the Taconic Mountain.
This mountain forming event, or like geologists
like to call it, “orogeny,” this is what
we see today.
The entire mountain has been eroded down to
what's behind us, but we can still see the
folding, the violent colliding action that
happened back 400, 500 million years ago.
But we're here today, specifically because
this piece of rock called the Umpire Rock
preserves one of the best features, one of
the best examples of the glaciation that happened
recently, about 10 to 20 thousand years ago.
Summer Ash: That's recent for geologists.
(Merry) Yue Cai: Recent for geologists, exactly.
The glacier shaped the rock, made the rock
really smooth, created these beautiful undulations,
the wavy features on top of the rock.
Summer Ash: How does ice actually do that?
Karin Block: Well for one, the ice was really
thick. If you look around, the ice was probably
as tall as that building there. Imagine that
much ice, and it basically spreads because
it's so heavy. As it spreading, it's freezing
up any little piece of rock underneath, and
carrying these little loose rocks with it.
These rocks are very effective in sending
down the basement underneath.
When you carry a rock on top of that rock,
you make these glacier grooves and glacier
striations. You see, they all go in one direction,
and they don't care about the bending of the
rock underneath.
In geology, we often look at this cross cutting
relationship. If Line B cross cuts Line A,
the event that generated Line B must have
happened later. In this case, the glacier
striae cuts everything, so this must have
been the most recent geologic event.
Summer Ash: All of this great geologic history
is exposed all over the city. How do scientists
examine it in detail?
Karin Block: The way I study those rocks entails
mostly going out into the field and sampling
rocks at a regular interval, or at least as
regular as the outcrop will allow.
I bring them back to the lab, where the rocks
are then sub sampled in many different ways,
by crushing the rocks, separating the rocks
into the mineral components, slicing the rocks
into thin sections that are 30 micrometers
thin, so that then we can study the optical
properties under a microscope or do electron
microscopy.
I love to break rocks. When you look at a
rock intersection under plain polarized light,
or cross polarized light, the information
that you get is an easier way of identifying
the minerals involving the range of colors
that you see. Sometimes, when you spin the
stage, you get a different set of colors that
show up.
All of those features are inherent to the
mineral crystals, so it's a little bit of
a process of elimination, but the actual combinations
of minerals that exist in that thin section
give you the specific pressure and temperature
conditions at which those minerals must be
stable, in conjunction with the others.
Summer Ash: Even in this urban and human altered
landscape, geologists can use the tools they
have to decipher our planet's past. In fact,
sometimes those landscape altering activities
give us better access to the underlying rock
formations. Think about this the next time
you're on the subway, or driving by a road
cut.
Karin Block: Many geoscientists practice geology
along roadcuts, because that's where the rocks
are very visible, and where you have artificial
outcrops that give us a lot of information.
We do a lot of roadside geology.
Summer Ash: Almost, the highways of America
are laboratories of geologists.
Karin Block: Exactly. The highways of America
are indeed a natural laboratory.
Summer Ash: What do you find most exciting
about your research?
Karin Block: The thing that excites me the
most about my field, and about science in
general, is that to me, everything is interesting.
Understanding how the world works involves
a lot of the minutia, and the details, and
the observations that science has equipped
me to do.
The fact that I have the ability to have a
better understanding of the world around us
is an incredible privilege, and for me, there's
just no shortage of the things that I want
to study; just a shortage of time, really.
Summer Ash: For someone who works on billion
year timescales, that's saying something.
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