RocksBuilding Blocks of the

Earth’s Crust

Earth’s Layers

Chemical Composition of the Earth

Element Symbol Percent Iron Fe 34.6% Oxygen O 29.5% Silicon Si 15.2% Magnesium Mg 12.7% Nickel Ni 2.4% Sulfur S 1.9% Calcium Ca 1.1% Aluminum AL 1.1%

Total 98.5%

Chemical Composition of the Earth’s Crust

Element Symbol Percent Oxygen O 46.6% Silicon Si 27.7% Aluminum Al 8.1% Iron Fe 6.0% Calcium Ca 3.6% Sodium Na 2.8% Potassium K 2.6% Magnesium Mg 2.1% Total 98.5%

Where the Rocks Are … The Crust

What Is a Rock?A rock is a mixture of minerals!

What is a mineral?• naturally occurring (not man-made) • inorganic (not part of or a product of a (once)

living organism ) • solid (not made of liquid or gas)• chemical compound• with a definite chemical structure which

gives it unique physical properties.

Minerals are the building blocks of rocks.

Minerals

• minerals are compounds and compounds are composed of elements

• What are the most common elements in the crust? Oxygen (O) and Silicon (Si)

• the most common minerals are silicates (contain silicon and oxygen)– the most common silicate – quartz

Physical Properties of Minerals

• Hardness – what minerals can scratch it or be scratched by it

• Cleavage – how does it break … along planes?

• Streak – what does the powdered form look like

• Color – what colors are common

• Density – the mass of a given volume

Cleavage

Calcite Halite

Rocks are divided into three major groups based on the methods of formation

1. Igneous rocks … from cooling lava or magma

2. Sedimentary rocks … from compacted sediments

3. Metamorphic rocks … meta (change) morphic (form) … rocks changed by extreme forces

Rock Classification

Rocks Classification

• Rocks are further subdivided by

–mineral composition• what minerals are present in large

quantities

– texture• what is the size of the mineral

crystals (grains )

Igneous Rocks

Igneous RocksFormed when molten rock cools and solidifies

Magma

Lava

geollab.jmu.edu/Fichter/ IgnRx/Intrusiv.html

Volcano

Explosive vs Non-explosive

Magma vs. Lava

Magma• intrusive – below the

surface

• gathers in large bodies called plutons

• may become lava

• cools slowly – forms large crystals

Lava• extrusive - above

the surface• expelled from a

fissure or a volcano• begins as magma• cools rapidly – forms

small or no crystals

Mineral Composition of Igneous Rocks

Rich in silicates• lots of feldspar,

quartz, and other silicates

• lighter color• more viscous –

extrusives form steep volcanoes

• continental crust

Poor in silicates• lots of iron and

magnesium rich minerals

• darker color• less viscous –

extrusives form large flows

• oceanic crust

Types of Common Igneous Rocks

Extrusive

(Volcanic)

Intrusive

(Plutonic)

Rich in Silicates

Rhyolite Granite

Poor in Silicates

Basalt Gabbro

Igneous Rocks

Rhyolite

Gabbro

Granite

Basalt

Other Important Volcanics

• Pyroclastics – also called ejecta - thrown from the volcano – cool very quickly

Obsidian – volcanic glass, a unique rock containing NO MINERALS

Pumice – frothy volcanic glass

Sedimentary Rocks

Sedimentary Rocks

• formed from sediments (particles) that settle in great amounts and are cemented together under great pressure

• sedimentary rocks cover some 80% of the Earth's crust

• generally, the only type of rock to contain fossils

Sedimentary Rock Formation

Clastic – particles deposited by water, wind, or ice

Chemical – particles precipitate from solution

Organic – growth in place (eg, coral reefs)

Clastic–basic sedimentary rock.

–accumulations of pieces of broken up (eroded) rock (clasts) which have piled up and been "lithified“ (turned to rock) by compaction and cementation

Queen’s Throne, Utah

Clastic Sediments

• formed from the erosion of other rocks

• most sediments are carried by rivers and are deposited in great amounts in the ocean

• under great pressure, these particles are cemented together

• classified by particle size

Clastic Particles and the Corresponding Rocks

Particle Rock

Gravel Conglomerate

Sand Sandstone

Silt Siltstone

Clay Claystone

Shale

Chemical

–some form when water conditions change causing solids to precipitate out of solution

–some form when standing water evaporates, leaving dissolved minerals behind

Limestone

Chemical Sediments

Deposit Rock

Calcium Carbonate Limestone

Calcium Magnesium DolomiteCarbonate

Sodium Chloride (Salt) Halite

Organic– accumulation of sedimentary debris

caused by organic processes– shells, bones, and teeth (calcium)– these bits of calcium can pile up on the

seafloor and accumulate into a thick enough layer to form an "organic" sedimentary rock

– Limestone can form this way; also, lignite, bituminous coal

Metamorphic Rocks

Metamorphic Rocks

• metamorphics get their name from "meta" (change) and "morph" (form)

• the rock is moved into an environment in which the minerals which make up the rock become unstable and out of equilibrium with the new environmental conditions

• in most cases, this involves burial which leads to a rise in temperature and pressure

Metamorphic Rocks• the metamorphic changes in the minerals

always move in a direction designed to restore equilibrium

• any rock can become a metamorphic rock, including another metamorphic rock (can become a different metamorphic rock)

• a metamorphic rock exposed to too much heat will melt and become ???

• common metamorphic rocks include slate, schist, gneiss, and marble

Metamorphic Rock - Gneiss

www.nearctica.com/geology/ mineral/metamor.htm

Types of Metamorphism

Regional• areas of great

amounts of pressure (during mountain-building)

• usually associated also with high temperatures

Contact• near magma

chambers (plutons)

Types of Metamorphism

Regional• areas of great amounts of pressure (during

mountain-building), usually associated also with high temperatures

• foliation of rocks present … pressure causes tabular minerals in the rock to grow parallel to each other and perpendicular to the direction of pressure (stress), generating a mineral alignment termed a foliation

• rocks often look folded or wavy

Foliation

Slate

Metamorphic Rock

www.aqd.nps.gov/grd/usgsnps/ rxmin/rock3.html

Types of Metamorphism

Contact• near magma chambers (plutons)• rocks in “contact” or near-contact with the

magma are baked• new minerals are formed• rocks do not conduct heat well (they are good

insulators) so the zone of contact metamorphism is usually relatively narrow

• marble results from the contact metamorphism of limestone

Metamorphic Rock

Marble

The Rock Cycle

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• Many are extrusive volcanic islands– Eruption eventually above the surface of the water:

Montserrat, Mt. Pelee, many others– Eruption below the surface and uplifted: St. John,

St. Thomas

• Some are flat limestone islands– Aruba, Anegada, Anguilla, Bonaire, Curacao

• St. Croix is sedimentary, but has more complex origin

• Barbados is an accretionary wedge.

Likely geological origins of Caribbean islands