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BASIC GEOLOGY – THE BIG IDEAS
BY LOOKING AT A ROCK, YOU CAN TELL HOW AND WHERE IT WAS FORMED AND SOMETHING ABOUT
WHAT HAS HAPPENED TO IT SINCE
Bedrock is the crust of the Earth. Sometimes we see
bedrock in a mountain or rock outcropping, but
bedrock is there even when we cannot see it, under
forests, houses, schoolyards, roads and oceans.
The rock type is determined by observing its
composition and texture (size of crystals or grains).
The minerals in a rock, their proportions, crystal size
or grain size, and whether minerals or grains are
arranged randomly or in layers are all taken into
account when interpreting how the rock was formed.
Rocks are always changing. Rocks may seem solid
and unchanging, but surface rocks are constantly
being weathered into smaller pieces, eventually into
soil. Bits of weathered rock are moved – eroded by
wind, and water. New rocks are constantly being
made from hot liquid rock deep under the crust of
the Earth and at its surface. New rock is also being
made from sediments that are now collecting in
oceans and lakes.
15,000 year ago water in the form of a glacier
covered this area. As the glacier flowed like a slowmoving river from Canada across New England, it
moved pebbles; rock and big boulders in front of it
like a bulldozer. It also carried rocks and sand on its
surface is like a conveyor belt. The glacier, with
sand and rocks embedded in its base, scraped like
sandpaper across bedrock it could not move,
smoothing and polishing the rock. Rocks and sand
from the melted glacier are scattered all over the
New England landscape.
Igneous Rocks (from Latin meaning fire)
1. Scientist tell us that rocks formed from
interlocking crystals are Igneous Rocks. Deep under
the Earth, heat and pressure can be so high that rocks
melt. This molten rock is called magna. As magma
cools, crystals form which grow together to become
solid igneous rock.
If the magma cools in pockets deep under the Earth
(perhaps several miles), the overlying rocks act as
insulation and magma cools slowly. This give
chemicals in the magma time to organize into
crystals that are large enough for us to see.
The longer the magma takes to cool, the bigger the
crystals. The deeper the magma is, the longer it takes
to cool. Therefore, the deeper the magma was when
it solidified, the larger the crystals. Rocks cooled
miles under the Earth's surface have medium to
large, easily visible crystals.
2. When magma reaches the surface through cracks
in the Earth, a volcano forms. The magma cools so
quickly that very small or even no crystals are
formed. Volcanic rocks usually have very small
crystals only visible with a magnifier. Magma
cooled at the surface is called lava.
The faster crystals are formed, the smaller
they are.
The closer to the surface when forming, the
smaller the crystals.
3. The kind of igneous rock depends on
a. the minerals present
b. how long it took to crystallize
Adapted from Bev Morrison's work with Fran
Ludwig, 2006
Do not try to identify worn or weathered rocks – you
cannot. Look for a fresh surface. Make a fresh
surface by wrapping a rock in an old cloth and
breaking it with a rock hammer. Be sure to use
goggles for all who are watching.
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4. Examples of igneous rock are:
Granite - minerals
are quartz, mica and feldspar
with small amounts of dark
minerals. Feldspar may be
either pink or white. Granite
is a light colored rock.
Crystals are visible.
Diorite – minerals are white
feldspar and black iron rich
minerals. Diorite looks like
salt and pepper all mixed
together. Diorite is
medium colored rock.
Crystals are visible.
Gabbro – minerals are
mostly black iron rich
minerals. Gabbro is a
dark rock with visible
crystals. Crystals are
visible but sometimes
hard to distinguish as
the minerals are all
about the same color.
All of these rocks crystallized deep underground
(evidence = medium to large crystals). The magma
pocket from which they formed might have been the
"roots" of an ancient volcano.
5. Sometimes liquid
magma flows into a
crack in a solid rock
and hardens. When
this happens we see
a vein of one rock
in the middle of a
different rock!
Often this happens
with quartz leaving
a white stripe or
vein in the middle
of a darker rock.
Some people call
pebbles with a stripe found on the beach "lucky
stones".
Crystal
Size
Color
Light
Formed
Small
near
(Almost
Rhyolite
surface…
too tiny
Lava
to see)
flows
Formed
deep
under
surface…
forming
large
crystals
Large
Granite
Medium
Dark
Andesite
Basalt
Diorite
Gabbro
6. Sedimentary Rocks
Rocks at the surface of our planet are weathered and
eroded by rain, ice, wind, plants and other
environmental factors. Slowly but continually,
rocks are broken down into small pieces and
eventually into tiny particles – pebbles, grains of
sand and silt or mud. Most of these rock particles are
carried by water and deposited as sediments in lake
or ocean bottoms. The particles are worn down and
rounded by water. The faster the stream, the larger
the pebbles it carries. As the stream slows, larger
pebbles will drop to the bottom. Finally, far from
shore, the fine mud and silt particles will settle out
of the slowly moving current. Materials will be
continually added on top, especially during spring
floods, until many layers are formed.
The layers are always laid down flat and parallel.
The youngest layers will be on top. These
sedimentary layers may be thousands of feet think.
In ocean trenches near the edge of continents they
may even be seven or eight miles thick!
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Pressure from other layers, water, and gravity plus
heat and pressure from inside the Earth compress the
lower layers into sedimentary rocks. Therefore,
sedimentary rocks are always layered, parallel to the
surface and to each other, made from particles of
other rocks – usually worn down to a single mineral,
and with a grainy not smooth texture.
The grains or particles in most sedimentary rock are
the same size, but sometimes water worn pebbles or
rocks were mixed in with the sand or clay forming a
conglomerate rock looking somewhat like concrete,
i.e. a sandstone or shale with rounded larger rocks
imbedded in it. Sometimes other things fall into a
layer, are buried by other layers, and become part of
the rock. Plants, animals, footprints, shells, all may
leave their mark in sedimentary rocks and become
fossils.
Igneous and Sedimentary Rocks
Granite as seen through a hand lens.
•
•
•
Feldspar
(pink or
white) has
two sides
(cleavages) at nearly right angles.
Mica (shiny, clear or black) has one
cleavage and forms very thin plates.
Quartz has no cleavage. It is the last mineral to
cool and fills the spaces leftover, forming
irregular shapes.
•
A sedimentary
conglomerate has
rounded grains cemented
together by smaller grains
of sand, silt, or clay.
Sedimentary rocks found in Wellesley, for example,
are sandstone containing mostly quartz grains, shale
containing feldspar and mica grains and
conglomerate – a sandstone or shale containing
rounded water worn pebbles. Clay deposits are a
layer not yet compressed enough to form shale.
Other common sedimentary rocks include limestone
– made from marine shells, chalk, a fine form of
limestone, and coal, plants laid down in layers in
swamps. Since sedimentary rocks are always laid
down in flat layers, a rock with layers tilted or
twisted must have been moved after it became a
rock.
An igneous rock has
interlocking mineral grains.
A sandstone or shale
has rounded grains all
the same size
cemented together.
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7. Metamorphic Rocks
Rocks that have been changed by intense heat and
pressure, but have not melted, are called
metamorphic rocks. Examples of metamorphic
rocks are gneiss (pronounced "nice") and schist.
Gneiss is usually banded with black and white or
black and pink stripes. Some gneiss was originally
granite. Schist has thinner layers than gneiss and
with high proportion of mica, has a shiny
appearance. Schist may have originated as
sedimentary rock. The rocks along Route 2 in Acton
are mostly metamorphic with some veins of granite.
formed in an ancient ocean millions of years ago.
Fossils of trilobites from this ocean bottom were
found when excavating the foundation of the
Prudential Center. Sedimentary rock is not as
common in Massachusetts as igneous and
metamorphic rocks are.
As igneous, sedimentary and metamorphic rocks are
buried or thrust deep under the earth by movement
of continental plates, they melt and the rock cycle
starts all over again.
The Stories Rocks Tell
By looking at a rock you may be able to tell how and
where it was formed and something about what has
happened to it since. What is the story each rock
tells us?
•
Rocks with visible crystals such as diorite,
granite, and gabbro were all formed from
molten magna many miles under the surface.
•
Surface rocks are exposed to wind and rain,
temperature extremes and plant roots. Iron
stains on rocks are evidence that the
minerals are reacting with air and water to
form rust. Sometimes earthquakes occur and
cracks form, making it easier for water and
air to get into a rock. This weathering breaks
up a rock into smaller pieces and makes it
soft.
•
Erosion carries away the bits of rock. Wind,
l water, and ice move weathered rock.
30,000 years ago a glacier carried rocks and
sand from further north to Massachusetts. It
polished and smoothed bedrock in its path.
If this weathered rock is deposited, possibly in
layers, and over a long period of time, the grains are
cemented together by minerals in water, sedimentary
rock is formed. In Boston, such sedimentary rock
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How the Hills Were Made
(and also mountains and valleys and the bays and the sea and even Cape Cod)
by Helga Burre
If you're headed to Cape Cod this summer, or if
you're thinking of buying some land there, or,
worse yet, if you happen to live there, you might
keep in mind that the Cape is a temporary
phenomenon. It is the sea's great sandbox, a loosely
piled arm of sand and gravel, a great place to play,
but not a good long-term real estate investment.
On the other hand, compared to bedrock areas in the
center of the United Sates, you might say the same
thing about all of New England. The whole
Northeast is a relatively young region geologically.
Over a billion years ago, sediments deposited in the
ocean on the eastern edge of the continent were
pushed up into a tremendous mountain chain by the
approach of a drifting continent. After these
continents separated, a new ocean was formed, and
erosion wore down the mountain chain to create the
present-day Berkshires.
About 500 million years ago, the new ocean started
to close, and in the process, an island chain was
formed, not unlike the Japanese island chain. This
chain eventually collided with North America (435
million years ago), crumpled the ocean floor
sediments, and pushed them up into folds (like
pushing a flat tablecloth with your hands), thereby
creating the Taconic Mountains. Some of the folds
actually broke loose, and one was pushed up and
over the Berkshires to form Mount Greylock.
As the ocean continued to close, two other slivers of
land – one carrying the present site of Boston –
were crushed into North America, creating another
mountain range in central New England, which has
now totally eroded away. The collision compressed
sediments into hard rock that was able to withstand
erosion and eventually became the summits of
Mount Monadnock, Mount Washington and the
Presidential Range. Magma melted deep in the
Earth and welled up to form the Mount Cardigan
Dome in New Hampshire and Maine's Cadillac
Mountain and other Down East hills and islands.
In this period, a visit to Africa would have been a
relatively short land journey – just a stroll through
Rhode Island; in fact, 250 million years ago, a land
journey to any of the continents would have been
possible because the continents had all joined into
one super continent called Pangaea.
Pangaea soon began to separate (245 to 150 million
years ago), putting tremendous stress on the Earth.
The stretching of the crust of the Earth caused
earthquakes and faults, which caused the
Connecticut Valley, the Newark Basin in New
Jersey and the Bay of Fundy. These basins filled
with reddish brown sediments. Wandering
dinosaurs crossed the basins periodically – leaving
their tracks, which can still be seen in some areas.
The separation also caused upwelling of dark basalt
magmas, which oozed up through cracks in the
Earth to flood the surface of the Earth. The exposed
edges of these (now tilted upward) sheets form the
Palisades on the Hudson River, Mount Tom and the
Holyoke Range. To the north, reaction to stress
resulted in thick basaltic flows of lava, and, when
the outpouring of lava left a hollow underground,
the Earth collapsed into a caldera and the Ossippee
Mountains in New Hampshire began to form.
Upwelling of lighter colored granitic magmas also
added to the White Mountains and formed the face
of the Old Man of the Mountains.
Lexington Gabbro formed over 400,000,000 years
ago.
http://www.platetectonics.com/
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Then, 150 million years ago, the Atlantic Ocean
started to open. Erosion moved sediments onto the
new continental shelf and deposited the clays that
now form Gay Head on Martha's Vineyard. Erosion
continued to lower mountains and fill in valleys to
form a flat peneplain, and then, 65 million year ago,
the land arched up, and erosion increased, leaving
the hardest rocks exposed as our present-day New
England mountains.
lakes, and moving loose rocks, sand, and gravel
south to form the place that today is known as Cape
Cod.
Helen Burre is Massachusetts Audubon's regional
director for programs in the Education Department.
Reprinted with permission from the Massachusetts
Audubon Society.
One million years ago, a series of ice ages scoured
the new land, carving cirques in mountains such as
Tuckerman's Ravine, gouging out lowlands to form
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