Use and Care Manual

3
How Rocks Are Classified
Igneous
Igneous rocks form when
molten rock (magma),
originating from deep within
the Earth, solidifies. The
chemical composition of the
magma and its cooling rate
determine the final rock type.
Sedimentary
Sedimentary rocks are
formed from pre-existing
rocks or pieces of once-living
organisms. They form from
deposits that accumulate on
the Earth’s surface.
Metamorphic
Extreme heat and pressure
cause intense physical
and chemical changes to
a precursor or pre-existing
igneous, sedimentary or
older metamorphic rock.
These changes (morphing)
transform the rock into a
different one. A large part
of the Earth’s crust is made
up of metamorphic rock and
is classified by texture and
chemical/mineral content.
Intrusive igneous rocks are formed from magma that cools and
solidifies deep beneath the Earth’s surface. The insulating effect of the
surrounding rock allows the magma to cool very slowly. This allows the
individual mineral grains to grow over a long period of time, resulting in
a relatively large, coarser grain size.
Intrusive
(Plutonic)
Characteristics:
• Dense, hard material
• Easy to maintain
• Low porosity, but can stain
Types of Igneous Stones:
• Granite
• Obsidian
• Basalt
Extrusive igneous rocks are formed when magma cools and solidifies at
or near the Earth’s surface. Exposure to the relatively cool temperature of
the atmosphere or water makes the erupted magma solidify very quickly.
Rapid cooling means the mineral grains have only a short time to grow,
so their final size is very tiny, or fine-grained.
Extrusive
(Volcanic)
Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing
rocks. These pieces are loosened by weathering, then transported to a
basin or depression where they are trapped and compacted beneath
deeply buried sediment, forming sedimentary rock.
Clastic
Chemical sedimentary rocks are formed when water travels through
rock, dissolving some of the minerals and carrying them away from
their source. Eventually, these minerals are re-deposited when the
water evaporates away or when the water becomes over-saturated with
minerals.
Chemical
Characteristics:
• Susceptible to staining
• May vary in hardness
• Wide color range
• Acid-sensitive
Types of Sedimentary Stones:
• Limestone
• Travertine
• Sandstone
• Gypsum
Biological sedimentary rocks form from once-living organisms, such as
carbon-rich plant materials or deposits of animal shells.
Biological
Foliation occurs when pressure squeezes the flat or elongated minerals
within a rock so they become aligned. These rocks develop a sheet-like
(platy) structure that will indicate the direction the pressure was applied.
Foliated
Characteristics:
• Caused by metamorphosis
or change
• Medium hardness
Types of Metamorphic Stones:
• Marble
• Slate
• Quartzite
Non-foliated metamorphic rocks don’t have a plated or sheet-like
structure. Instead, they are formed when hot igneous rock comes into
contact with pre-existing rock. This “contact metamorphism” changes
the mineral structure of the rock, without the addition of pressure.
Non-
Foliated