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The Rock cycle
Igneous rocks
•
A rock "sight-seeing" tour around the world
might well begin in the Hawaiian Islands
with a visit to the world's most active and
best-studied volcano, Kilauea. This
famous volcano, like others across the
globe, is the birthplace of thousands of
tons of rocks.
• The rocks are "born" as
volcanic eruptions eject
massive amounts of
magma onto Earth's
surface. Remember that
magma is nothing more
than a large body of hot
liquid rock and minerals.
As the magma cools into
a solid rock around the
base of the volcano, it
can be collected as
samples of rocks such as
basalt, pumice, or
obsidian.
Mount Rushmore is made of
Granite
Metamorphic rock
• What does it mean to be "rock
solid?" The phrase is used to
describe something that
cannot easily be changed. But
metamorphic rocks are rocks
that have been changed within
the Earth. Because rocks are
not easily changed,
metamorphic rocks must
develop in environments
where heat and pressure are
intense and extreme. These
extreme conditions are only
present deep inside the Earth.
Contact Metamorphism
• CONTACT METAMORPHISM
occurs when a rock is exposed
to hot magma inside the Earth.
The intense heat of the magma
alters the rock, often causing
its minerals to recrystallize.
Thus, the new rock has new or
larger mineral crystals than the
older rock. Sometimes, the hot
magma will even introduce
new minerals and modify the
entire chemical composition of
the original rock. The area of
rock affected by contact
metamorphism is appropriately
known as the baked zone.
Regional Metamorphism
• REGIONAL
METAMORPHISM, on the
other hand, occurs during the
formation of mountain ranges.
As tectonic plates collide and
converge, intense pressure
deforms and alters
sedimentary and igneous
rocks already buried in the
Earth. The mountain ranges of
east Greenland are an
example of where this has
taken place. Often, folds or
curves in the rocks indicate the
direction of the intense
pressure.
Sedimentary Rock
• Traveling east to Zion National Park in Utah,
tourists would find the well-known Navajo
Sandstone Formation. It looks like a towering
sand dune that has been permanently solidified
against the landscape. The sandstone rocks that
make up the Navajo Sandstone Formation
began millions of years ago as tiny grains of
sand in a prehistoric desert. Over time, winds
carried the sand and deposited it into giant sand
dunes. The pressure of each new sand deposit
caused the sand underneath to compact and
cement together into solid rock.
Sedimentary Rock
shale
• A common clastic
sedimentary rock is
shale, the most abundant
of all sedimentary rocks.
Shale forms when mud
and clay harden.
Because the clay
sediments are extremely
small, they settle out
slowly. In fact, shale
formations can take about
5 million years to form.
limestone
• The most common chemical sedimentary rock is
limestone. Most limestone rocks are organic —
that is, they develop from the remains of
organisms. Much limestone is made up of
marine animals and contains pieces of shells,
corals, and mollusks. Another chemical
sedimentary rock is coal, which is often found in
layers with other sedimentary rocks. As plant
remains are deposited in layers and slowly alter
into carbon over millions of years, they form
coal.
Limestone cliffs , limestone with
fossils
• Although these sandstone rocks began as
a collection of individual grains of sand,
the sand grains themselves had to come
from somewhere and something else.
Where might these sand grains may have
originated?
• Geologists theorize that the sand grains may
have broken off a larger rock formed earlier by a
volcanic eruption. Or they may have broken off
older sandstone rocks. Because the Earth is so
dynamic, rocks are always changing. Minerals
that make up rocks are constantly moved and
acted upon by the environment. Environmental
agents change one type of rock into an entirely
new rock. The web of environmental processes
that forms and changes rocks is known as the
ROCK CYCLE.
The Rock Cycle
Another view of the Rock Cycle
Rock cycle
• To gain an idea of how the rock cycle works, the
world rock tour that began in Hawaii might end
in east Greenland. Millions of years ago there,
magma deep inside the Earth forced its way into
another rock and solidified. This created an
igneous rock. Then, intense pressure in the
Earth caused the rock to up-heave, fold, and
crumple, until it became an entirely new rock: a
metamorphic rock. That new rock now makes up
a mountain formation in Greenland.
Activity
• The rock cycle never ends.
•
I, Rock: Sandy Stone said to her son,
"Rocky, I remember when you were just a
cute little grain of sand..."
Tell the story of a rock's progression
through the rock cycle.