MR. SURRETTE VAN NUYS HIGH SCHOOL

CHAPTER 1: ROCKS AND MINERALS

CLASS NOTES

INTRODUCTION TO ROCKS

There are three types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks are formed by the cooling and crystallization of hot, molten rock material called magma. Sedimentary rocks are formed from weathered material (sediments) that are carried by water, wind, or ice.

INTRODUCTION TO ROCKS

Metamorphicrocks are formed from preexisting rocks that are transformed by high temperature or pressure without melting.

MINERALS

Minerals are the building blocks of rocks. A mineralis a naturally formed, generally inorganic, crystalline solid. Minerals are composed of an ordered array of atoms and have a specific chemical composition.

CRYSTAL FORM

The orderly arrangement of atoms in a crystal is expressed in its shape, or crystal form. Every mineral has its own crystal form.

POLYMORPHS

Two or more minerals that contain the same elements but have different structures are called polymorphs. For example, the polymorphs graphite and diamond are both composed of pure

carbon.

CARBON POLYMORPHS

MINERAL HARDNESS

The resistance of a mineral to being scratched or its ability to scratch other minerals is a measure of hardness.

MOH’S SCALE OF HARDNESS

CLEAVAGE AND LUSTER

Cleavageis the tendency of a mineral to break along planes of weakness. Planes of weakness are determined by crystal structure and chemical bond strength. The lusterof a mineral is its ability to reflect light.

COLOR AND STREAK

Although color is an obvious feature of a mineral, it is not very reliable. For example, the common mineral quartz (SiO2), can be found in a variety of colors, depending on slight impurities. Streak, the name given to the color of a mineral in its powdered form, is a better indicator.

DENSITY

Density is how heavy an object feels for its size. It can be determined using the equation:

 = m / V

(Density = mass / volume)

Example 1. What is the density of silver if 26.25 grams occupies 2.5 cm3 volume?

1A.

(1)  = m / V

(2)  = 26.25 g / 2.5 cm3

(3)  = 10.5 g/cm3

SPECIFIC GRAVITY

The standard measure of density is specific gravity. Specific gravity is the ratio of a substance to an equal volume of water. The density of water is 1 g/cm3:

Specific Gravity =  / (1 g/cm3)

Example 2. 6 cm3 of a mineral weighs 3 grams. What is its specific gravity?

2A.

(1)  = 6 cm3 / 3 grams

(2)  = 2 cm3 / gram

(3) Specific gravity =  / (1 g/cm3)

(4) Specific gravity = 2g/cm3 / 1g/cm3

(5) Specific gravity = 2

ELEMENTS AND MINERALS

Of all the elements in the periodic table, 88 occur naturally in the Earth’s crust. These 88 elements combine to make over 3,400 minerals. Almost half the mass found in minerals consists of oxygen. The second most abundant element found in minerals is silicon.

THE SILICATES

Oxygen and silicon combine to form the most common mineral group, the silicates. The silicate quartz, the second most common mineral, is composed entirely of oxygen and silicon.

THE OXIDES

Within the oxide minerals, oxygen combines with one or more metals. Oxides commonly contain the metals iron, chromium, manganese, tin, and uranium.

THE CARBONATES

All carbonatescontain the carbonate ion (CO32-). Two common carbonate minerals are calcite and dolomite. Calcite and dolomite are the chief minerals that make up the limestone rocks.

THE SULFIDES AND SULFATES

The sulfide and sulfate minerals contain sulfur. Sulfidesinclude the majority of ore minerals. The most common sulfide is pyrite(fool’s gold), FeS2. In sulfates, sulfur is present as the sulfate ion (SO42-).

IGNEOUS ROCKS

95% of the earth’s crust is composed of igneous rock. On the continents, the most common igneous rocks are graniteand andesite. On the ocean floor, basalt is predominant. All igneous rock originates as magma.

MAGMA

Magma is a mixture of liquid, melted rock, and dissolved gases. The most abundant chemical elements in magma are silicon and oxygen. The oxide SiO2, silica, is the most abundant oxide in magma.

VISCOSITY

The term viscositydescribes a fluid’s resistance to flow, or its internal friction. Lava becomes more viscous as it cools. The amount of silica in lava also affects viscosity. Lava that is low in silica is less viscous.

FRACTIONAL CRYSTALLIZATION

Igneous rocks form from the cooling and crystallization of magma. Crystallizationis very similar to melting, but in the reverse order: solid crystals form out of a liquid mixture. Minerals with high melting points crystallize first, followed by minerals with low melting points. This process is called fractional crystallization.

SHIELD VOLCANOES

When low-silica magma erupts, lava tends to flow freely and far. If it erupts from a single opening (vent) or closely spaced vents, it forms a broad shield volcano.

SHIELD VOLCANO

CINDER CONES

Cinder cones, such as Sunset Crater, generally have smooth steep slopes of 25o to 40o and bowl-shaped summit craters.

CINDER CONE

COMPOSITE CONES

A composite coneforms by many eruptions of material with medium or high silica content. They erupt violently when pressure builds up in the magma. Composite volcanoes are tall and have steep slopes because lava does not flow easily.

COMPOSITE CONE

INTRUSIVE AND EXTRUSIVE IGNEOUS ROCKS

Igneous rocks crystallize from cooling magma and lavas. Intrusive igneous rocks have cooled slowly beneath the Earth’s surface. They are made of large crystals. Extrusive igneous rocksform from magma brought to the Earth’s surface. These rocks cool rapidly and contain small crystals.

SEDIMENTARY ROCKS

Sedimentary rocks provide information about geological events that occurred at the Earth’s surface. The most abundant sedimentary rocks are shale, composed of very fine particles, and sandstone, composed of medium sized particles.

SEDIMENTATION

Weatheringbreaks down and decomposes surface rock. There are two kinds of weathering: mechanical and chemical. Both types of weathering produce sediment. Mechanical weatheringphysically breaks rocks into smaller and smaller pieces. Chemical weathering, which involves reactions with water, also decomposes rock into smaller pieces.

EROSION

As rock is weathered, it erodes. Erosionis the process by which weathered rock particles are removed and transported away by water, wind, or ice.

FOSSILS

Because sedimentary rocks are formed at the Earth’s surface, they often contain fossils, the remains of preexisting life forms.

METAMORPHIC ROCKS

Igneous or sedimentary rocks may undergo change (metamorphism) if they are heated and/or compressed for long periods of time. For example, shale subject to enough heat and pressure becomes slate. It is important to note that no minerals are melted during metamorphism. Change occurs by the recrystallizationof preexisting minerals and the mechanical deformationof rock.

METAMORPHISM

Recrystallizationoften occurs when rocks subjected to high temperatures and pressures go through a change in mineral assemblage. Rocks often lose water (H2O) or carbon dioxide (CO2) during this process. Mechanical deformationoccurs when a rock is subjected to physical stress.

TYPES OF METAMORPHISM

Metamorphism can be dynamic, contact, or regional.

DYNAMIC METAMORPHISM

Dynamic metamorphisminvolves mechanical deformation; for example, the shearing and grinding that takes place in a fault zone.

CONTACT METAMORPHISM

Contact metamorphismoccurs when a body of rock is intruded by magma.

CONTACT METAMORPHISM

REGIONAL METAMORPHISM

Regional metamorphismis the widescale alteration of rock. This is found in all the major mountain belts of the world.

THE ROCK CYCLE

Igneous, sedimentary, and metamorphic rocks have different origins. Although formed by different processes, the three rock types are related, as seen in the rock cycle.

THE ROCK CYCLE

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