Rocks

All You Need To Know

Rock Types

• Three Main Types of Rocks:1. Igneous2. Sedimentary3. Metamorphic

Igneous Rocks• Formed from magma (molten rock below earth’s

surface) or lava (molten rock above earth’s surface).• When molten rock crystallizes (cools/hardens), Two

types of Igneous Rocks can Form:1. Intrusive: cool slowly inside earth’s crust allowing for

large crystals to grow2. Extrusive: cool quickly above earth’s surface not

allowing for any crystals to grow or allowing for very small crystals to grow

• Examples include:– Granite and gabbro (intrusive) and basalt and obsidian

(extrusive)

Sedimentary Rocks• Formed from pieces of other rocks (sand,

shells, pebbles etc…) i.e. sediment that gets glued and compacted together.

• Different Types Include:1. Clastic – made from other pieces• Examples sandstone, conglomerate, shale

2. Chemical – precipitates or evaporates• Example rock salt (halite)

3. Organic – formed from once living things• Examples coal, limestone

• Fossils are found in sedimentary rocks.

Metamorphic Rocks

• Formed from intense heat and pressure.• Two Types:

1. Foliated: ribbonlike/banded layers• Examples slate, phyllite, schist, gneiss

2. Non-foliated: crystals (no banding)• Examples quartzite, marble

• May have shiny crystals as well as the minerals in the rock change due to the intense heat and pressure.

Key Identifying Characteristics• Igneous Rocks:– Gas bubbles holes created from trapped gas– Glassy surface shiny smooth surface from quick

cooling rocks– Crystals small, flat surfaces that are shiny or sparkly

• Sedimentary Rocks:– Sand or pebbles individual stones, pebbles, or sand

grains visible in the rock– FossilsI imprints of shells, insects, leaves etc…

• Metamorphic Rocks:– Ribbonlike layers wavy or straight stripes/bands of

different colors in the rock

Different RocksRock Name Rock Type How Formed Characteristics

Conglomerate Sedimentary Made from pebbles, stones pressed together by action of waves or water

Rounded pieces

Limestone Sedimentary Formed from seashells and skeletons of marine animals that were compressed into the seafloor

Contains Fossils

Basalt Igneous(Extrusive)

Formed from cooling lava that had gas trapped in it then blew out creating the holes

Gas bubbles

Obsidian Igneous(Extrusive)

Formed from lava that cools so fast no crystals form

Glassy surface

Gneiss Metamorphic(Foliated)

Formed from intense heat and pressure deep inside earth’s crust

Has ribbonlike layers (banding)

Marble Metamorphic(Non-Foliated)

Formed when limestone is subject to intense heat and pressure

Has crystals

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How Rocks Change

• Rocks change due to various processes.• Processes Include:– Heat and Pressure– Melting and Cooling– Weathering and Erosion– Compacting and Cementing

Heat and Pressure• Heat from the earth’s interior can melt rock and

pressure from overlying rock can also morph (change) rocks from one type to another.

• Heat increases dramatically the deeper a rock gets pulled under the earth’s surface – prior to melting a rock can change chemically to become another rock.

• Pressure from overlying rock also can change a rock chemically into another type of rock.

• Heat and pressure work together and this is known as metamorphism thus creating metamorphic rocks.

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Melting and Cooling• Melting:– Temperatures between 600 and 1300 degrees Celsius

(1100 – 2400 degrees Fahrenheit) melt rock and turn it into magma

• Cooling:– any rock that forms from cooling magma is an

igneous rock– Two Types of Igneous Rock:

1. Intrusive form from slowly cooling magma inside earth2. Extrusive form from quickly cooling lava above/on

earth’s crust

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Weathering and Erosion

• Weathering:– the breaking apart or wearing down of rock due to

water, wind, gravity, temperatures, and other elements at the earth’s surface.

– Creates sediments.• Erosion:– Transportation of weathered material via water,

wind, or gravity.

Compacting and Cementing• Compacting:– Sediment formed from weathering and erosion

accumulates (is deposited/left behind) in oceans, lakes, and valleys this allows for layers of sediment to build up

– Each layer presses down on the layer below it, thus compacting (pushing down on) bottom layers.

• Cementing:– When water passes through spaces in between

particles of sediment this helps cement the particles together forming sedimentary rock

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The Rock Cycle

• James Hutton (1726-1797) is attributed with the concept of the rock cycle.

• Main Idea of Rock Cycle:– Rocks are continually changing from one type to

another and back again– Elements that make up rock aren’t destroyed or

created, they are just being recycled

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