Our changing land : Stone Mountain State Park - Page 37 |
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Stone Mountain State Park, NC 3.1.8 October 2002
However, granodiorite contains
less potassium feldspar ( ortho -
clase) and more of the sodium -
rich plagioclase feldspar. Usu -
ally, granodiorite also has more
dark - colored minerals, such as
biotite or hornblende.
The Rock Cycle
The relationship between the
three rock classifications is de -
scribed as the rock cycle. Ge -
ologists believe that at one time
the earth was a ball of molten
magma and gases. As the earth
cooled, the outermost layer of
magma solidified into a crust
of igneous rock. Today, the
earth’s crust is 30 miles thick in
some places, yet in others it is
so thin that lava can spew up
through cracks.
Even as the first rocks
cooled, weathering began
breaking them down into sedi -
ments that were eroded away
by wind and water. Those first
sediments were deposited at
the edges of the continents in
the first oceans. The eroded
rock particles traveled more
quickly than they do today, as
there were no plants to stabilize
the loose soil. For over three
billion years the continents
were bare rock and sediments.
The sediments continued to
build up as the continents wore
down. The underlying sedi -
ments became rock again as
the pressure and heat cemented
the particles back together.
Often, as the continents eroded
they became lighter and rose
up, exposing the sedimentary
rocks to the air, where they
started to erode away once
again. Most of the earth’s
crust is made up of igneous
rock, but the most common
class of rock found on the
earth’s continental surface is
sedimentary, which lies on top
of the igneous crust.
The crust is divided into
many large crustal plates that
slowly drift about on top of the
denser “ plastic” mantle, which
lies beneath the crust. As the
plates move around, some -
times the crust at the leading
edge of one plate slides under -
neath the edge of an adjacent
plate. This pushes the crust
down far enough that it melts,
turning into magma. Volcanic
activity such as that shown by
Mt. St. Helen's can result.
When two continents col -
lide, the crust is folded into
mountain ranges. For ex -
ample, the Himilayas were
created when India collided
with the southern edge of
Asia. This collision caused
the rocks caught in it to be put
under tremendous heat and
pressure, so that they meta -
morphosed. Eventually this
metamorphic rock reaches the
earth’s surface where it is sub -
jected to weathering.
The continuing cycle of
rocks melting and cooling
into rocks again, or breaking
down and being pressed into
rocks again, or being put un -
der pressure and metamor -
phosing into new rocks has
happened many times on
earth. It's happening today
and will continue to occur on
our ever - changing planet.
Object Description
Description
| Title | Our changing land : Stone Mountain State Park - Page 37 |
| Full Text | Stone Mountain State Park, NC 3.1.8 October 2002 However, granodiorite contains less potassium feldspar ( ortho - clase) and more of the sodium - rich plagioclase feldspar. Usu - ally, granodiorite also has more dark - colored minerals, such as biotite or hornblende. The Rock Cycle The relationship between the three rock classifications is de - scribed as the rock cycle. Ge - ologists believe that at one time the earth was a ball of molten magma and gases. As the earth cooled, the outermost layer of magma solidified into a crust of igneous rock. Today, the earth’s crust is 30 miles thick in some places, yet in others it is so thin that lava can spew up through cracks. Even as the first rocks cooled, weathering began breaking them down into sedi - ments that were eroded away by wind and water. Those first sediments were deposited at the edges of the continents in the first oceans. The eroded rock particles traveled more quickly than they do today, as there were no plants to stabilize the loose soil. For over three billion years the continents were bare rock and sediments. The sediments continued to build up as the continents wore down. The underlying sedi - ments became rock again as the pressure and heat cemented the particles back together. Often, as the continents eroded they became lighter and rose up, exposing the sedimentary rocks to the air, where they started to erode away once again. Most of the earth’s crust is made up of igneous rock, but the most common class of rock found on the earth’s continental surface is sedimentary, which lies on top of the igneous crust. The crust is divided into many large crustal plates that slowly drift about on top of the denser “ plastic” mantle, which lies beneath the crust. As the plates move around, some - times the crust at the leading edge of one plate slides under - neath the edge of an adjacent plate. This pushes the crust down far enough that it melts, turning into magma. Volcanic activity such as that shown by Mt. St. Helen's can result. When two continents col - lide, the crust is folded into mountain ranges. For ex - ample, the Himilayas were created when India collided with the southern edge of Asia. This collision caused the rocks caught in it to be put under tremendous heat and pressure, so that they meta - morphosed. Eventually this metamorphic rock reaches the earth’s surface where it is sub - jected to weathering. The continuing cycle of rocks melting and cooling into rocks again, or breaking down and being pressed into rocks again, or being put un - der pressure and metamor - phosing into new rocks has happened many times on earth. It's happening today and will continue to occur on our ever - changing planet. |
