Science 3360

Lecture 14: The Rock Cycle

A look at rocks, minerals, and the evidence of a dynamic Earth

Questions to Answer…From our previous discussion of the hydrological cycle and the ocean nutrient cycle, two important questions arise:

We know that rivers deliver materials from the eroding continents to the oceans (remember weathering?). On average, rivers deliver 2-4 x1012 kg of material to the oceans every year. Why haven’t the oceans filled in?

We know from the ocean nutrient cycle that some of the nutrients are buried in the sediment. If this is occurring over millions of years, why haven’t the ocean’s biota run out of nutrients?

Answers…

The answers to these questions are buried in the processes related to the solid earth.

To fully understand these concepts, we must first investigate the structure of the Earth, rocks and minerals, and the rock cycle.

The Structure of the Earth

The Earth consists of 3 major zones:

The CrustThe MantleThe Core

We’ll discuss these in more detail next lecture.

Aside: Volcanoes

1. Magma chamber2. Bedrock3. Conduit (pipe)4. Base5. Sill6. Branch pipe7. Layers of ash emitted by the volcano8. Flank9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater

From Wikipedia

Rocks and MineralsA mineral is a naturally occurring inorganic element or compound with a definite internal arrangement of ions and a chemical composition that is fixed or only varies slightly.

Rocks are aggregates of minerals.

A useful analogy* is Letter work sentenceElement mineralrock

* Conte, D.J., D.J. Thompson, and L.L. Moses, EarthScience: An Integrated Perspective, Wm.C. Brown Publishers, Chicago, IL, 432pp, 1997.

MineralsThere are 92 naturally occurring elements but most minerals contain 8 main elements. These are the elements that make up must of the Earth’s crustal material.

Element Average Weight Percent in CrustOxygen 46.6Silicon 27.7Aluminum 8.1Iron 5.0Calcium 3.6Sodium 2.8Potassium 2.6Magnesium 2.1All others 1.5

NB: While O is the most abundant element, it only appears in combinations with other elements as oxides (i.e. silicates)

MineralsItems of note:1. Native elements = pure

elements2. Other categories

comprise compounds where ions are combined with a common element or group (e.g., sulfides, sulfates, oxides, halides, etc).

3. Silicates are most abundant (> 90% of all minerals found are silicates). They are composed of varying numbers of silicate groups(SiO4

2- ).4. Some minerals formed by

ionic substitution (e.g. plagioclase, olivine).You don’t need to memorize all these names.

Minerals• Within each group or class there can be many

individual minerals• Any given mineral will have a limited chemical

composition. If the elements in that crystal are changed, it will no longer be the same mineral.• sodium chloride is the mineral halite;• potassium chloride is the mineral sylvite;• calcium carbonate is the mineral calcite;• calcium sulfate is the mineral gypsum.

• There is some minor flexibility in chemical make-up caused by ionic substitution. Some of the ions are similar in size and ionic charges (Ex.: Fe+2 and Mg+2). If the mineral is crystallizing out of a liquid that is rich in both elements, the crystal may incorporate some of the "substitute" ions instead of the more appropriate ion.• This substitution is common in the silicate families

Minerals: Ionic Substitution

For example: The plagioclase series comprises minerals that range in chemical composition.

Na or Ca can substitute for each other as can differing amounts of Al and Si.

So plagioclase can range from pure NaAlSi3O8 (Albite) to pure CaAl2Si2O8 (Anorthite) and includes combinations of Albite and Anorthite.

Mineral IdentificationHow do we identify unknown minerals?

By comparing physical properties of the substance to known values we can logically identify it.

Properties:1. Hardness2. Color3. Streak4. Luster5. Density6. Cleavage/Fracture7. Crystal Form8. Magnetic Properties

Hardness

Hardness scale developed by Friedrich Mohs.

Runs from 1 to 10 with one being the softest.

Take an item of known hardness and try to scratch the unknown item. If the known is harder, it will scratch the unknown.

Common Objects and Their Hardness Values

2.5 3.5 5.5 6.5 8.5

Fingernail Penny Glass Steel Knife

Emery Cloth

So if a fingernail can’t scratch it but a penny can, its hardness is between 2.5 and 3.5

ColorColor is as simple as it sounds. Some minerals are easily identified by color because they always have the same color characteristics. An example of this is malachite which is always green.

However, other minerals can change color drastically due to chemical impurities. An example of this is quartz. In its purest for, quartz is colorless. Quartz with traces of iron becomes violet (amethyst). With traces of manganese it turns pink (rose quartz).

StreakWhen a mineral is rubbed firmly across an unglazed tile of white porcelain (a streak plate), it leaves a line of powder called a streak.

The color of the streak is always the same, regardless of impurities that may be contained in the mineral.

For example, quartz always leaves a white streak regardless of whether it’s clear (pure quartz), violet (amethyst), or pink (rose quartz).

LusterLuster is the way the surface of a mineral reflects light. Luster should always be observed on a fresh cut of freshly broken surface.

There are two general types of luster: metallic and non-metallic.

The terms used to describe luster are:

Metallic -- example: goldVitreous (glassy) -- example: quartzAdamantine (brilliant) -- example: diamondResinous (like resin or sap from a tree) -- example: sphaleriteGreasy or waxy -- example: turquoisePearly -- example: talcSilky -- example: asbestosDull or earthy -- example: bauxite

CleavageWhen a mineral breaks it does so by fracturing or cleaving. Crystal cleavage is a smooth break producing what appears to be a flat crystal face. How a mineral cleaves is controlled by its crystalline structure.

Some minerals break only in one direction while others cleave in two or more directions.

Examples: Basal Cleavage is cleavage exhibited on a horizontal plane of the mineral. Minerals with basal cleavage can sometimes be “peeled”. E.g. Mica

Cubic Cleavage: Mineral cleaves perfectly in 3 planes. E.g. Calcite

MineralsSo how are minerals formed?

Minerals are formed by the process of crystallization.

Within the Earth’s interior high temperatures and pressures cause rocks to melt, forming magma.

As the magma cools, various elements and ions come together to form solids with a specific structure (a crystal)

Different minerals form at different temperatures. This relationship is described by Bowen’s Reaction Series.

RocksRocks are aggregates of minerals.

The variety of rocks that are found is determined by the processes that lead to the formation of rocks and their mineral constituents. As a general rule, the minerals that form a given a rock will tend to crystallize at similar temperatures and pressures.

Rocks

Rocks fall into 3 broad categories depending on how they were formed

• Igneous rock is formed from the crystallization of magma– Extrusive rocks means they formed on the surface;

resulted from faster cooling– Intrusive rocks formed below the surface; resulted

from slow cooling• Sedimentary rock is formed by the sedimentation and

compaction of material (called lithification)• Metamorphic rock is rock that has been chemically

altered while in the solid state from exposure to high temperature and pressure (called recrystallization)

Igneous RockIgneous Rocks can be classified by where they were formed

Extrusive Rocks. Cooled quickly on the surface. An example of an extrusive igneous rock is Basalt. Basalt is comprised of olivine, pyroxene, and plagiocloase

Intrusive Rocks. Cooled slower below the surface. An example is granite. Granite is often comprised of feldspar, quartz, and mica

Sedimentary RockFormed by depositing layers of material down over time. Can be classified by the type of material that was compacted.

Detrial: Shale (made from mud) and sandstone (made from sand)

Biological: Limestone (made from shells, coral, etc) and Chert (made from diatoms)

Evaporites: Gypsum (form when chemical solutions i.e. seawater evaporate)

Metamorphic RocksFormed when rocks of another type in solid form were chemically altered by high temperature and pressures.

Examples: Slate. Dominant minerals are clay, chlorite, mica and quartz. Slate is metamorphized shale. How many planes of cleavage does slate have?

Marble. Dominant mineral is calcite. Marble is metamorphized limestone.

Gneiss. Dominant minerals are mica quartz, and feldspar. Gneiss is metamorphized granite or slate.

Metamorphic RocksSedimentary Rocks start as a collection of lose mineral grains.

Pressure pushes the grains closer together, eliminating void spaces. There is no change to the grains themselves yet.

Additional pressure changes the shape of the grains to form an interlocking lattice.

Grain sizes change as many small grains reform into large ones. There may or may not be a change to the chemical composition. Note the decrease in volume as the small grains recrystallize into larger ones.

The Rock CycleAll three rock types are linked by the rock cycle.

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But Wait!

• The schematics on the previous page describe how the rock-types are linked by processes, but it does not indicate:– How does sedimentary material get

transported to the deep interior where it can be metamorphized?

– What causes uplift to move the igneous and metamorphic rocks to the surface to be weathered?

– Where does the energy come from to drive the cycle?

Somehow there are dynamical processes moving material through the Earth. We’ll explore this next time with Plate Tectonics