Rocks
Chapter 4
Classifying Rocks
Mineral Composition - Look with a
magnifying glass to determine the
mineral that makes up the rock
Color - Light or dark color, glassy?
Texture – look and feel of the rocks
surface
Grain size – Fine, coarse or no visible
grain
Grain Shape – Rounded or jagged
Grain Pattern – non-banded, banded
What is a rock? Rocks
mixture of minerals, mineraloids,
glass or organic matter
3 Types of Rocks:
1. Igneus Rock – molten material
inside earth or from a volcano cools
slowly due to the insulation of rock
allowing the atoms to arrange
themselves into large crystals called
mineral grains
Remember:
magma forms from
great temperatures
(1400oC) and
pressure in the
earth this melts the
minerals and forms
magma
When magma flows
to the surface it is
called lava
Intrusive Rocks
Intrusive Rocks– formed when
magma insulated and trapped
below the earth surface, holds
heat and cools slowly – large
mineral grains
Granitic – light colored rocks,
lower density contains a lot of
silicon and oxygen, mountains
Extrusive Rocks
Extrusive Rocks – Formed when
magma cools on the earths surface
and cools rapidly –
fine grained texture Basaltic –
dense, heavy, dark colored (magma)
examples iron, magnesium, covers
ocean floor
Combination:
Andresitic – in between basaltic and
granitic in mineral composition
Igneous Rocks - Texture
Intrusive—Large crystals
Extrusive—Small Crystals (maybe
not visible: obsidian & Pumice)
Mafic—Dark
Felsic—Light
Obsidian
Pumice
Andesite
Rhyolite
Diorite-Dark color w/ large black
crystals
Basalt
Gabbro
Granite
Sedimentary Rock
Sedimentary rock -comprises 75 % rocks on
earth -formed when sediments pressed or
cemented together – may form layers
Sediments – loose materials such as rock
fragments, grains and bits of plant and
animals
Compactation – pressure causes sediments
to form rock
Cementation – water dissolves minerals
(quartz, calcite, hematite, limonite) natural
cements
Classification of sedimentary
rock:
Clastic – broken, named for size &
shape of sediments example –
conglomerate, breccia, sandstone,
shale
Chemical – minerals precipitated
from a solution or evaporates
example – limestone, rock salt
Organic – from once living things
examples – coal, chalk
Sedimentary Rocks
Compacted or Cemented (layering is key)
Sandstone
Shale
Coal (rock or not? Some debate)
Clastic
Breccia—sharp angles
Conglomerate—rounded edges
(concrete)
Coquina—Shells
Coal
Sandstone
Mudstone-gray & silky soft
Shale
Conglomerate
Breccia
Coquina
Coral Reefs
Formed from skeletons of tiny coral animals
that grow together to form the coral reef
structure
Almost all coral reef growth occurs within
the top 40 meters of the surface where
sunlight penetrates
Coral organisms need warm water, found
only in tropical oceans
Limestone deposits formed from coral reefs
Metamorphic Rock
Metamorphic rock – rocks changed due to
temperature and pressure increases or
change in composition
Classification of Metamorphic Rock
Foliated – mineral grains flatten and line up in
parallel bands Example slate and gneiss
Slate forms from shale which is arranged in
layers when exposed to heat and pressure
Nonfoliated – no banding occurs, the mineral
grains change, grow and rearrange but no bands
Example is marble which forms from
sedimentary rock calcite – shiny. Hornblende
and serpentine – green. Hermatite - red
Metamorphic Rocks
Gneiss—Zebra strips
Schist—Metallic shine
Marble—Contains Calcite (bubbles
w/HCl)
Or Morphed sedimentary rock
Slate (shale that has hardened)
Gneiss
Slate
Schist
Marble
Phyllite
Rock Cycle