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Rocks
There are three main groups of rocks
1.) Igneous
2.) Sedimentary
3.) Metamorphic
Igneous Rocks
• Igneous rocks are classified by their
• Origin
• Texture
• Mineral composition
Origin
• Igneous rock may form on or beneath
Earth’s surface.
• Extrusive rock is igneous rock formed from
lava that erupted onto Earth’s surface and
cooled.
• Intrusive rock is igneous rock that formed
when magma hardened beneath the surface
of the Earth.
Texture
• The texture of an igneous rock
depends on the size and shape of its
mineral crystals.
• Rapidly cooling lava forms finegrained igneous rock with small
crystals or no crystals at all.
• Slowly cooling magma forms coarsegrained rocks with large crystals.
Granite
Rhyolite
Mineral composition
• Lava (on Earth’s surface) that
is low in the mineral silica
usually forms dark-colored
rocks such as basalt.
• Magma (beneath Earth’s
surface) that is high in silica
usually forms light-colored
rocks such as granite.
Sedimentary rocks
• Sedimentary rocks are made up of sediment, which is small,
solid pieces of material that come from rocks or living things.
• Most sedimentary rocks are formed through a sequence of
processes:
• Weathering: the force by which rocks on Earth’s surface
are constantly broken up by forces such as freezing and
thawing, plant roots, and acid.
• Erosion: the process by which running water, wind, or ice
carry away bits of broken-up rock
Most sedimentary rocks are formed through a
sequence of processes:
• Deposition: the process by
which sediment settles out of
the water or wind carrying it.
• Compaction: the process that
presses sediments together
• Cementation: the process in
which dissolved minerals
crystallize and glue particles of
sediment together
What are the three major types of
sedimentary rock?
1.) Clastic rocks
2.) Organic rocks
3.) Chemical rocks
Clastic rocks
• A clastic rock is a sedimentary rock
formed when rock fragments are
squeezed together.
• Clastic rocks are grouped by the size of
the rock fragments, or particles, of
which they are made.
Clastic rocks
• Shale forms from tiny particles
of clay.
• Sandstone forms from the sand
on beaches, the ocean floor,
riverbeds, and sand dunes.
• Conglomerate has rock
fragments with rounded edges.
• Breccia has rock fragments with
sharp edges.
Organic rocks
• Organic rock forms from the remains of
material that was once living (plants and
animals), which is deposited in layers.
• Coal forms from the remains of swamp
plants buried in water.
• Limestone forms in the ocean, where
many living things, such as coral, clams,
and oysters, have hard shells or
skeletons that are deposited on the
ocean floor.
Chemical rocks
• Chemical rocks form when
minerals dissolved in a water
solution crystallize, or from
mineral deposits that are left when
seas or lakes evaporate.
• Example: rock salt is made of the
mineral halite, which forms by
evaporation.
Metamorphic rocks
• Any rock that forms from another rock
as a result of changes in heat and/or
pressure is a metamorphic rock.
How metamorphic rocks form
• Metamorphic rock can form out of
igneous, sedimentary, or other
metamorphic rock.
• The heat that can change a rock into
metamorphic rock can come from
pockets of magma rising through the
crust, or collisions between Earth’s
plates pushing rock down towards the
heat of the mantle.
How metamorphic rocks form
• Very high pressure can also
change rock into
metamorphic rock, such as
from plate collisions during
mountain formation, or the
great pressure deep under
Earth’s surface.
How metamorphic rocks are classified
• Metamorphic rocks are
classified according to
the arrangement of the
grains making up the
rocks.
• Foliated rocks have
their grains arranged in
either parallel layers or
bands.
shale
Slate
schist
How metamorphic rocks are classified
• Nonfoliated rocks have their grains arranged
randomly.
limestone
marble
The Rock Cycle
• Not only can rock be changed into metamorphic rock,
such as from heat and pressure, but any rock can be
changed into any of the other three rock types! All of
this occurs during the rock cycle.
• The rock cycle is a series of processes that occur on
Earth’s surface and in the crust and mantle that slowly
change rocks from one kind to another.
• The changes of the rock cycle are closely related to
the movement of Earth’s plates.