Themes for Today

• A huge variety of minerals, but relatively few that make up most of the crust

• Rocks are composed of minerals

• The rock cycle describes the origin, transformation, and erosion of the three types of rocks:

–igneous

–sedimentary

–metamorphic

Earth Materials – Minerals and Rocks

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Earth Materials – Minerals• Minerals have many essential uses

– common minerals like quartz: glass, sandpaper, optics

– not-so-common like corundum (ruby): nail files, gemstones

– everything in between

• Basic units that make up most of Earth’s materials

Earth Materials – Rocks• Rocks are solid aggregates of

minerals – (few exceptions)

• Rocks also have many uses– crushed for aggregate in cement and

for roadbeds– sawed and polished rocks for

buildings, mantle pieces, and counter tops

– soils are formed by alteration of rocks

Matter and Its Composition

• Matter– is anything that has mass and

occupies space– exists as solids, liquids, and gases– consist of elements and atoms

• Element– is a chemical substance that cannot

be chemically decomposed into simpler substances and is composed of tiny particles called atoms

Building a rock...

Atoms

• Atoms are the smallest units of matter that retain the characteristics of the element

• Atoms have– a compact nucleus containing

• protons – particles with a positive electrical charge• neutrons – electrically neutral particles

– particles orbiting the nucleus • electrons – negatively charged particles

Structure of an Atom

• Dense nucleus of an atom consisting of protons and neutrons

• Surrounded by a cloud of orbiting electrons

Atoms

• Atomic number = the number of protons – is constant

• Atomic mass number = number of protons + number of neutrons– may vary due to varying numbers of

neutrons

Isotopes

• An element that has varying numbers of neutrons (and different atomic weights) has multiple isotopes

• Different isotopes of the same element – have different atomic mass numbers– behave the same chemically

• Isotopes are important in radiometric dating

Carbon Isotopes

• Carbon atoms (with 6 protons)

– have 7 neutrons = Carbon 13 (13C)– have 8 neutrons = Carbon 14 (14C)

– thereby making up three isotopes of carbon.

– have 6 neutrons = Carbon 12 (12C)

Minerals—The Building Blocks of Rocks

• A mineral’s composition is shown by a chemical formula– a shorthand way of indicating how many

atoms of different kinds it contains

Quartz: SiO2

Ratio: 1: 2

– Quartz consists of 1 silicon atom for every 2 oxygen atoms

– Orthoclase consists of 1 potassium, 1 aluminum, and 3 silicon for every 8 oxygen atoms

KAlSi3O8

1: 1: 3: 8

Native Elements

• A few minerals consist of only one element

• They are not compounds

• They are known as native elements

• Examples: – gold – formula: Au– diamond – formula: C

Minerals

California Academy of Sciences in San Francisco

Minerals

• Geological definition of a mineral:– naturally occurring– crystalline solid

• crystalline means that minerals have an ordered internal arrangement of their atoms– a narrowly defined

chemical composition– characteristic physical

properties such as• density• hardness• color...

Crystalline Solids• By definition, minerals are crystalline

solids– with atoms arranged in a specific 3D

framework• If given enough room to grow freely,

– minerals form perfect crystals with – planar surfaces, called crystal faces– sharp corners– straight edges

Narrowly Defined Chemical Composition

• Some minerals have very specific compositions– examples are halite (NaCl) or quartz (SiO2)

• Others have a range of compositions – because one element can substitute for

another if the atoms of the two elements have• the same electrical charge• and are about the same size

– example: olivine • (Mg,Fe)2SiO4

• iron and magnesium substitution in any proportion

Mineral Properties

• Mineral properties are controlled by– Chemical composition

– Crystalline structure

• Mineral properties are used for mineral identification and include:

•cleavage • fracture•hardness•specific

gravity

•color•streak• luster•crystal form

How Many Minerals Are There?

• More than 3500 minerals are known• Only about 2 dozen are particularly

common• Many others are important resources• Mineral groups:

– minerals with the same negatively charged ion or ion group belong to the same mineral group

• Most minerals in the crust – belong to the group called silicates

Silicates

• Silicates are minerals containing silica – Si and O

• They make up perhaps 95% of Earth’s crust– and account for about 1/3 of all known

minerals

Types of Silicates

• Ferromagnesian silicates – contain iron (Fe), magnesium (Mg), or

both

• Nonferromagnesian silicates – do not contain iron or magnesium

Ferromagnesian Silicates

• Common ferromagnesian silicates include:

– augite, a pyroxene-group mineral

– hornblende, an amphibole-group mineral

– biotite mica

– olivine

Nonferromagnesian Silicates

Quartz Potassium feldsparPlagioclase feldspar Muscovite

Other Mineral Groups

• Carbonates – minerals with carbonate ion (CO3)-2

– as in calcite (CaCO3), • found in limestone

– and dolomite [CaMg(CO3)2], • found in dolostone

• Other mineral groups are important, but more as resources than as constituents of rocks

Rock-Forming Minerals

• Most rocks are solid aggregates of one or more minerals

• Thousands of minerals occur in rocks, – but only a few are common: rock-forming

minerals

• Most rock-forming minerals are silicates, – carbonates are also important

• Accessory minerals are present in small amounts – largely ignored in classifying rocks

Rock Cycle• The rock cycle represents events

leading to – the origin, destruction, change, and

reformation of rocks• Rocks belong to 3 major families

– igneous– sedimentary– metamorphic

• The rock cycle shows – how these rock families are

interrelated and can be derived from one another

Igneous Rocks

• All igneous rocks – cool and crystallize from magma, – solidify from lava, – or consolidate from pyroclastic materials

• Magma is molten material below the surface

• Lava is molten material on the surface

• Pyroclastic materials are particles such as volcanic ash

Pyroclastic material

Lava

Igneous Part of the Rock Cycle

Categories of Igneous Rocks

• Extrusive or volcanic rocks – formed at the surface – from lava or pyroclastic materials

• Intrusive or plutonic rocks – formed from magma injected into the

crust– or formed in place in the crust

Igneous Rocks

Igneous Rock Textures

• Texture – is the size, shape and arrangement– of crystals, grains and other

constituents of a rock• Igneous rocks have 4 textures

– that relate to cooling rate of magma or lava

4 Cooling-Rate Textures• Phaneritic,

– with visible grains • cooled slowly

• Aphanitic, – with grains too small to see without

magnification • cooled quickly

• Porphyritic, – with larger grains surrounded by a finer-

grained groundmass • cooled slowly first, then more quickly

• Glassy, – with no grains

• cooled too quickly for minerals to grow

Igneous Rock Textures

• Other textures reveal further details of the formation of the rock

• Vesicular texture, with holes (vesicles) – indicates the rock formed as water

vapor and other gases became trapped during cooling of lava

• Pyroclastic or fragmental texture – containing fragments, – formed by consolidation of volcanic

ash or other pyroclastic material

Igneous Rock Textures

Rapid cooling

Aphanitic texture

Slow cooling

Phaneritic texture

2-stage cooling

Porphyritic texture

Igneous Rock Textures

Glassy texture

cooling was too rapid for

mineral growth

Vesicular texture

gasses trapped in cooling lava

Pyroclastic texture

particles fragmented during eruption

Classifying Igneous Rocks

• Texture and composition are the criteria – used to classify most igneous rocks

• Composition categories are based on silica content – felsic (>65% silica)– intermediate (53-65% silica)– mafic (45-52% silica)

• More felsic magmas have higher Na, K, Al• More mafic magmas have higher Ca, Fe,

Mg

Classifying Igneous Rocks

Common Igneous Rocks

Basalt Gabbro

Andesite Diorite

Common Igneous Rocks

Rhyolite Granite

Classifying Igneous Rocks with Special TexturesTextu

re

Vesicular

Glassy

Pyroclastic or

Fragmental

CompositionPumice

Scoria

Obsidian

Volcanic breccia

Tuff/welded tuff

MaficFelsic

Igneous Rocks with Special Textures

Tuff has pyroclastic texture.

Pumice is glassy and extremely vesicular.

Sedimentary Rocks• Sedimentary rocks form

– by the lithification of sediment• In the rock cycle, sediment

originates when: – mechanical and chemical weathering

• breaks rocks down into smaller particles and into solution

– transport removes sediment from its source area and carries it elsewhere• running water, glaciers, wind, and waves

– deposition:• settling of particles, • chemical and biological extraction of

minerals from solution

Sedimentary Part of the

Rock Cycle

Lithification• Lithification means

– turning loose sediment into rock• Lithification occurs by

– burial • when additional sediment accumulates on top

– compaction • reduction of the amount of pore space between

particles • because of the weight of overlying sediment

– cementation • precipitation of minerals within pores • that effectively binds sediment together

– calcium carbonate (CaCO3) cement is common– silica (SiO2) cement is common– iron oxide (Fe2O3) cement is less common

Categories of Sedimentary Rocks

• Detrital sedimentary rocks – consist of solid particles derived from

preexisting rocks (detritus)• Chemical sedimentary rocks

– consist of minerals derived from materials in solution and extracted by either • inorganic chemical processes • or by the activities of organisms

– biochemical sedimentary rocks

Detrital Sedimentary Rocks

• are composed of fragments or particles known as clasts = clastic texture

• These rocks are defined primarily by size of clasts

• conglomerate – composed of gravel (>2mm)– with rounded clasts

• sedimentary breccia– also composed of gravel (>2mm)– but clasts are angular

• sandstone– composed of sand

Detrital Sedimentary Rocks

• Mudrocks consist of particles < 1/16 mm– siltstone

• composed of silt-sized particles - 1/16-1/256 mm, • feel slightly gritty, • but not visible without magnification

– mudstone• composed of a mixture of silt- and clay-sized

particles– claystone

• composed of clay-sized particles – <1/256 mm, feel smooth even to the teeth

– shale• mudstone or claystone that • breaks along closely spaced parallel planes (fissile)

Chemical Sedimentary Rocks

• Inorganic chemical processes or organisms extract minerals from solution

• This can result in different textures– crystalline texture

• has an interlocking mosaic of mineral crystals• results from chemical precipitation

– clastic texture• has an accumulation of broken pieces of

shells

Chemical Sedimentary Rocks

• Limestone – carbonate rock made of calcite precipitated chemically or by organisms

• Evaporites formed by inorganic chemical precipitation during evaporation– rock salt – evaporite made of halite– rock gypsum – evaporite made of gypsum

• Chert – compact, hard, fine grained silica, formed by chemical or biological precipitation (some consisting of microscopic shells of silica-secreting organisms)

• Coal – made of partially altered, compressed remains of land plants accumulated in swamps

Common Sedimentary Rocks

Conglomerate Sedimentary breccia

Quartz sandstone Shale

Common Sedimentary Rocks

Fossiliferous limestone

Rock salt

Chert Coal

Rock gypsum

Metamorphic Rocks

• Metamorphic rocks – result from transformation of other

rocks in the solid state, without melting• Changes resulting from

metamorphism– compositional

• new minerals form

– textural• minerals become reoriented• minerals recrystallize

– or both

Metamorphic Part of the

Rock Cycle

Agents of Metamorphism• Heat provides new conditions

– where different minerals may be stable – and increases the rate of chemical reactions

• Pressure – Lithostatic pressure provides new conditions

• where different minerals may be stable • and forms smaller denser minerals

– Differential pressure • exerts force more intensely from one direction • causing deformation • and development of foliation.

• Fluid activity enhances metamorphism – by increasing the rate of chemical reactions – by transporting ions in solution

Types of Metamorphism• Contact metamorphism

– heat and chemical fluids from an igneous body• alter rocks adjacent to the magma

• Regional metamorphism– large, elongated area– tremendous pressure– elevated temperatures– fluid activity

Metamorphic Textures

• Foliated texture– platy and elongate minerals aligned

parallel to one another– caused by differential pressure

• Nonfoliated texture– mosaic of roughly equidimensional

minerals or platy and elongate minerals

– with random orientations

Formation of Foliation

• When rocks are subjected to differential pressure– the minerals typically rearrange or grow

parallel to each other

Formation of Foliation

• Foliated metamorphic rock showing mineral alignment visible under microscope

Foliated Metamorphic Rocks

• Slate – very fine-grained, breaks in flat pieces

• Phyllite – fine-grained (coarser than slate but grains

are still too small to see without magnification)

– breaks in flat pieces• Schist

– clearly visible platy and/or elongate minerals • Gneiss

– alternating dark and light bands of minerals

Nonfoliated Metamorphic Rocks

• Marble – made of calcite or dolomite from

limestone or dolostone• Quartzite

– made of quartz from quartz sandstone

• Hornfels– results from contact metamorphism

• Anthracite– made of black lustrous carbon from

coal

Common Metamorphic Rocks

Slate Schist

Gneiss Marble Quartzite

Earth Materials and Historical Geology

• Our record of Earth’s history is preserved in rocks– sedimentary rocks are especially useful in

preserving a historical record

• Igneous and metamorphic rocks provide information – about processes deep in the crust– history of plate activity

Themes for Today

• A huge variety of minerals, but relatively few that make up most of the crust

• Rocks are composed of minerals

• The rock cycle describes the origin, transformation, and erosion of the three types of rocks:

–igneous

–sedimentary

–metamorphic