uniformitarianism:
1795 – James Hutton
“the present is the key to
the past”
we can learn about Earth’s
history by studying present
processes
What is a rock?
rock: an aggregate
(group) of minerals
3 Types of Rocks
igneous
sedimentary
metamorphic
Igneous Rock
a rock that forms when
melted rock inside the
Earth cools
- formed from volcanic
activity
Igneous rocks can form
on Earth’s surface or
deep inside Earth.
magma: molten rock inside
the Earth
intrusive: rocks that form beneath
Earth’s surface
- mineral grains are clearly seen
- form slowly (1000’s of years)
ex. granite
extrusive: rocks that form at
Earth’s surface
- no distinct mineral grains
- form relatively quickly
ex. obsidian
porphyry: rocks with
large crystals
surrounded by fine
crystals
Families of
Igneous Rock
Granite Family - light minerals
granite: coarse grained
rhyolite: fine grained
obsidian: glassy
pumice: spongy
Gabbro Family - dark minerals
gabbro: coarse grained
basalt: fine grained
basalt glass: glassy
scoria: spongy
Diorite Family: both light
and dark minerals
diorite: coarse grained
andesite: fine grained
Igneous Rocks and the
Environment
Intrusive igneous rocks are usually
very strong and resist weathering so
they make good foundations for large
scale construction projects such as
dams and buildings (think-granite!)
 Sometimes extrusive rocks can be
fractured and can cause problems
such as cave-ins

Sedimentary Rock
a rock that forms from
pieces of other rocks or
the remains of onceliving things
- often formed from
weathering and erosion
Types of Sedimentary Rocks
clastic
chemical
organic
Clastic Rocks:
rocks that form from the
weathering of rocks that
already exist
- most clastic rocks are
formed from weathering from
running water
Dissolved minerals (natural
cements) settle into the
spaces between sand grains
and pebbles.
This “cement” binds the
fragments together
Ex. shale, sandstone,
conglomerate
Sorting of Sediments:
Chemical Sedimentary
Rocks
rocks that are formed when
minerals fall out of solution
(when water evaporates or by
chemical actions)
ex. chemical limestone; rock
salt
Organic Rocks
rocks that are formed from the
remains of plants and animals
ex. organic limestone; coal
• much of the rock found in
this area (PA) is limestone
stratification:
arrangement of rocks
in visible layers
Sedimentary Rocks and the
Environment
 Shale
and silt
can be very
weak
 Limestone
can be
cavernous
and can cause
sinkholes
Metamorphic Rock
A rock that is formed
when rocks that already
exist are exposed to
heat and pressure
- often formed from
mountain building
Regional Metamorphism
definition: large areas
of rock are under
intense heat and
pressure
Examples of regional
metamorphism:
granite  gneiss
shale  slate  phyllite  schist
foliation: mineral grains
are flattened and lined
up in parallel bands
layers are easily
separated ex. slate
Contact Metamorphism
definition: hot magma
forces its way into
overlying rock and
changes it
-no foliation
ex.
shale  hornfels
sandstone  quartzite
limestone  marble
Metamorphic Rocks and the
Environment
 Slate
makes excellent
foundation material and is
also used in pool tables,
chalkboards and roofs
THE ROCK CYCLE
the process by which
Earth materials
change to form
different types of
rocks
Remember…



Rocks become igneous though melting
and cooling
Rocks become sedimentary through
weathering and erosion
Rocks become metamorphic through
heat and pressure