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

igneoussedimentarymetamorphic

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 once-living things

- often formed from weathering and erosion

Types of Sedimentary Rocks

clasticchemicalorganic

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 RockA 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