Rocks Notes
Rocks – groups of minerals bound together in some way. There
are three categories of rocks: igneous, sedimentary, and
metamorphic.
Igneous Rocks – formed by the cooling and hardening of
magma or lava (molten rock)
Magma – melted minerals found underground
Lava- melted minerals found on the surface, comes
out of volcanoes
Igneous rocks can be recognized by two physical properties: color and texture.
Color: depends on the composition of the magma/lava the rock forms from.
Felsic is light-colored magma or lava.
Mafic is dark-colored magma or lava.
Felsic characteristics
Mafic characteristics
High silica content
Low silica content
Thick
Thin
Slow flowing
Fast flowing
Contains the minerals quartz, feldspar
Contains the minerals amphibole, biotite
Resulting rocks are light in color
Resulting rocks are dark in color
Texture: depends on where the rock cooled and hardened.
3 Types of Texture
Intrusive
Cooled slowly
Cooled inside earth
Large grains – we call this
a coarse-grained rock
Extrusive
Cooled quickly
Cooled near the surface
Small grains – we call this
a fine-grained rock
Glassy
Cooled super fast
Cooled on the surface
No grains – we call this a
glassy rock
When you put the color and texture together, you get the following igneous
rocks:
Texture
Color
Intrusive
Extrusive
Glassy
Felsic
Mafic
Granite
Gabbro
Rhyolite
Basalt
Obsidian & Pumice
Scoria
Sedimentary Rocks- formed by the
hardening and cementing of layers of
sediment (rock fragments, plant/animal
remains, or chemicals)
Three Types of Sedimentary Rocks
1. Clastic Sedimentary Rocks
 Formed from fragments of other rocks
 Cemented with silica, calcite, or iron oxide
 Rivers separate sediment by size
Examples of Clastic Sedimentary Rocks
Conglomerate
Sandstone
Shale
Rounded pebbles and sand
Sand-sized quartz grains
Tiny flakes of clay
2. Chemical Sedimentary Rocks
 Formed when minerals come out of a solution
 By evaporation or precipitation
Examples of Chemical Sedimentary Rocks
Limestone
Rock Salt
Rock Gypsum
Composed of calcite (will fizz)
Composed of halite
Composed of gypsum
3. Organic Sedimentary Rock
 From the remains of plants and animals
Examples of Organic Sedimentary Rock
Fossiliferous limestone
Coal
Coquina
Limestone with a lot of fossils
Remains of plants and animals
Shells and sand cemented
together
Other Important Features that help to identify Sedimentary Rocks
Stratification – arrangement of sediment in visible layers
Fossils – evidence of past life preserved in rock
Ripple marks – action of wind, waves, and currents on sediment
Mud cracks – when wet clay dries and cracks
Nodules/concretions – lumps of silica or calcite, usually layered like an
onion.
Geodes – hollow spheres of silica crystal
Metamorphic Rocks – formed when
existing rocks are changed by heat and
pressure and made into new rocks
Two Types of Metamorphism
1. Regional Metamorphism – large areas of rock are affected, usually
when mountains are being formed.
2. Contact Metamorphism – hot magma touches surrounding rock and
“bakes” it.
Foliation – minerals become arranged in parallel layers
Parent Rock
Limestone (sedimentary)
Sandstone (sedimentary)
Granite (igneous)
Basalt (igneous)
Shale (sedimentary)
New Metamorphic Rock
Marble
Quartzite
Gneiss
Schist
Slate
Rock Cycle – rocks change from one kind to another
in a continuous cycle