Transcript How Minerals Form

Classifying
Rocks
P74-77
Rocks
Rock – a mixture of one or
more minerals.
Example: Granite – a rock made
of the minerals quartz, feldspar,
mica, and hornblende.
Rocks are classified
according to 3 things:
1) Color
2) Texture
3) Mineral Composition
Color
Like with minerals, color alone
doesn’t give enough
information to identify a rock.
Texture
1) Very useful in identifying a
rock.
2) Texture is the
size, shape, and
pattern of a rock’s
grains.
3) Grains – the minerals (or other
rocks) that make up a rock and
give it its texture.
4) Texture describes how
the surface of something
feels (rough, smooth,
grainy, gritty, coarse, oily,
silky, bumpy, waxy, chalky).
Grains
1) Grain Size
• coarse grain rock = grains are
large/easy to see (Ex - diorite)
• fine grain rock = grains are so
small they can only be seen
under a microscope (Ex - slate)
Grains (Continued)
2) Grain Shape – can be smooth
and rounded (like conglomerate
rock) or jagged (like breccia)
Grains (Continued)
3) Grain Pattern
– the way the grains lie in the
rock.
- can be flat layers (like a stack of
pancakes – Ex – Gneiss) or a
wavy, swirling pattern (Ex –
quartz)
Grains (Continued)
4) No Visible Grains
- When some rocks form (flint
and obsidian) they cool off so
fast that they have a shiny
texture like glass.
Mineral Composition
Geologists determine the
minerals that make up the rock.
- They look at the crystal shape
- Acid test tells whether the
rock contains carbonates
- Magnet test tells whether the
rock contains iron or nickel
Origin
3 Major Groups of Rocks:
Igneous
Sedimentary
Metamorphic
These terms refer to how the
rocks in each group formed.
Igneous Rock
Formed by cooling of molten rock
– magma below the surface or
lava at the surface.
Sedimentary Rock
Formed when particles of other
rock or remains of plants and
animals are pressed and
cemented together.
Metamorphic Rock
Formed when an existing rock
is changed by heat, pressure,
or chemical reactions. Form
deep underground.