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Rock Composition
Rocks aren’t minerals, even if minerals are rocks!
What is a rock?
• Rock: naturally formed, non-living Earth material.
• Rocks are made of collections of mineral grains that are
held together in a firm, solid mass.
• The different colors and textures of different rocks are caused by
the presence of different minerals
How is a rock different from a mineral?
• Rocks are made of minerals. The mineral grains in a rock
may be so tiny that you can only see them with a
microscope, or they may be as big as your fingernail, or
even your finger.
• Rocks are identified primarily by the minerals they contain
and by their texture. A rock may be made of grains of all
one mineral type, such as quartzite. Much more
commonly, rocks are made of a mixture of different
minerals.
• Texture is a description of the size, shape, and arrangement of
mineral grains.
The Three Rock Types
• The three types are:
• Igneous, Metamorphic, and Sedimentary
• These rocks all have different properties, and form in
different ways.
Igneous Rocks
• Igneous Rocks form from the cooling and hardening of
molten magma in many different environments.
• The chemical composition of the magma in question as
well as the rate at which it cools will determine what kind
of rock forms.
• More than 700 types of Igneous rocks are known
• They have these characteristics:
• Large, visible crystals
• Glasslike
• Holes from air bubbles while cooling
Sedimentary Rocks
• Sedimentary rocks form by the compaction and
cementing together of sediments.
• Sediments are small fragments of rocks and minerals such
as pebbles, sand, silt, and clay.
• Fossils are always found in Sedimentary rocks
• They have these characteristics:
• Appearance of sand, pebbles, rocks cemented together
• Layers
• Appearance of fossils
Metamorphic Rocks
• Metamorphic rocks form when the minerals in an existing
rock are changed by heat or pressure below the surface
• Metamorphic rocks start off as some kind of other rock.
The starting rock can be igneous, sedimentary, or even
another metamorphic rock. Heat and/or pressure then
change the rock into a metamorphic rock.
• The changes can be physical, chemical, or both
• They have these characteristics:
• Stripes in thin, parallel, wavy lines
• Tiny crystals that line up in the same direction
• Very hard, usually can scratch metal