Structure and Composition of the Earth (3)

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LU 3 STRUCTURE AND COMPOSITION OF THE EARTH

THE EARTH

Spheres

A physical subdivision of the natural world, that acts semi-independentlyand in which one or more classes of processes occur, is into what is calledspheres.

The Atmosphere

The atmospheric is a layer of gases enveloping the earth.

It is about 250 miles thick whose bottom 5 - 11 miles (7 - 16 km) contains

most (75%) of the air.

Stratification:The Earths atmosphere is stratified as it is composed of a number oflayers. These layers are shown below.

Composition:The atmosphere is a mixture of gases.

The gaseous composition of clean and dry air according to percent byvolume is shown below.

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The Hydrosphere

The hydrosphere includes allwateron Earth.71% of the Earth is covered by

water and only 29% is terra firma.The abundance of water on Earthis a unique feature that clearlydistinguishes our"Blue Planet"from others in the solar system.

Not a drop of liquid water has been found anywhere else in the solarsystem.It is because the Earth has just the right mass, right chemicalcomposition, right atmosphere and is the right distance from the Sun

(the "Goldilocks" principle) thatpermits water to exist mainly as a liquid.

However, the range of surface temperatures and pressures of our planetpermit water to exist in all three states: solid (ice), liquid (water), and gas(water vapor).Most of the water is contained in the oceans and the high heat capacityof this large volume of water (1.35 million km3) buffers the Earth surfacefrom large temperature changes such as those observed on the moon.

Water is the universal solvent and the basis of all life on our Planet.

Scientists estimate that the hydrosphere contains about 1.36 billion km3of watermostly in the form of a liquid that occupies topographicdepressions on the Earth.

The Biosphere

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The biosphere is the life zone of the Earth and includes all livingorganisms, including man, and all organic matter that has not yetdecomposed.

Life evolved on earth during its early history between 4.5 and 3.8 billionyears ago and the biosphere readily distinguishes our planet from allothers in the solar system.

The chemical reactions of life (e.g., photosynthesis-respiration,carbonate precipitation, etc.) have also imparted a strong signal on thechemical composition of the atmosphere, transforming the atmospherefrom reducing conditions to and oxidizing environment with free oxygen.

The Geosphere

The geosphere is the solid Earth that includes the crust as well as thevarious layers of the Earth's interior.

The interior of the earth is layered both chemically and mechanically

THE EARTH SYSTEM

A system is defined as a collection of interacting objects.

A system consists oftwo basic elements:(1) a functioning set ofcomponents,(2) a flow of energy which powers them

The Earths components define the Earth system.

The Earth system is a complex functioning system that includes all thecomponents of the various "spheres" like the solid Earth surface orgeosphere, the gaseous envelope surrounding the Earth that is the

atmosphere, biospherecomprised of all living organisms and thehydrosphere or "water sphere".

The Earth system represents flows of energy and mass that connect andintertwine the four spheres.

Solar energy drives many of the environmental processes operating inthe four spheres.

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Additional sources of energy to drive earth systems come from the Earth'sinternal heat engine and the gravitational attraction of the moon.

Therefore, forces that shape the Earth derive their energy from a number of

different sources.

The Earth System Interactions

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The combined forces of nature cause materials to move about Earth fromsphere to sphere.

This movement of matteroften includes chemical transformationsconducted by geologic, hydrologic, atmospheric or biologic agents.

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Cycles of Matter

Thepathways and rates of material or energy transferbetween spheres arereferred to as cycles.

Cycles on Earth are often separated into two sub-cycles, the endogenicand exogeniccycles (for interior and exterior or surficial process).

Processes

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Exogenic processes are those driven by exogenicforces that primarilyderive their energy from solar radiation.

Example: Soil erosion caused by the force of wind acting on bare ground.We can trace the energy that causes wind erosion to the receipt

of solar radiation.

How?

Wind is a product of horizontal differences in pressure over distancecaused by the unequal heating of the Earth's surface.

Low pressure is created when heated air rises from the surface and thenflows outward at a higher elevation.

As air is moves upward, the surface pressure decreases relative to theair around it.

The variation in surface pressure causes air to move into the region oflow pressure to replace that which is rising, thus creating a wind.

Soil is detached when wind blows over an erodible surface. Water andglacial erosion are other examples of exogenic processes

Endogenic processes are those that get their energy from endogenicforces originating deep within the Earth.

Example: Many of the great mountain systems are a product of themovement of geosphere thought to result from convectioncurrents in the mantle.

How?Deep within the core of the Earth, heat is generated by the radioactive

decay of elements like uranium, thorium, and potassium.

The heat is transferred upward to warm the mantle causing it to slowlycirculate and tug on the crust above.

The movement of geosphere fractures and folds rock, and their collisioncreates vast mountain chains and volcanic cones.

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THE GEOSPHERE

Four Main Layers Making Up The Geosphere:

1. CoreInner Core

It is a mass with a temperature of about 7000 F.In spite of such temperatures immense pressure on it keeps it in asolid form.

Outer CoreIt is a molten surrounding the solid inner core.

2. Mantle

It is a rock layerParts of this layer become hot enough to liquefy and become slowmoving molten rock or magma.

3. CrustA layer consisting of solid rock

The whole geosphere is made ofrocks & minerals.

Inside the earth there is a liquid core of molten rock and on the outsidethere is a hard crust. If you compare the earth to an egg, the shell on anegg is like the crust on the earth.

The core, mantle and crust of the earth can be envisioned as a giant rock

recycling machine. However, the elements that make up rocks are never created or

destroyed although they can be redistributed, transforming one rocktype to another.

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THE CRUST

The crust is made up ofrocks and minerals.Much of the crust is covered by water, sedimentary particles (e.g. sand),soil and ice.

If you dig deep enough, you will always hit rocks.Below the loose layers of soil, sand & crumbled rocks that are found on theEarths surface is bedrock, which is solid rock.

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ROCKS

The earths crust is made up of rocks. A rock is defined as a compact andconsolidated mass ofminerals.

Minerals as Constituents of Rocks

A rock is a solid aggregate ofone or more minerals that have beencohesively brought together by a rock-forming process.

The materials constituting the earths crust are a result of the processes atwork in and on the crust. The processes and products are best describedthrough the rock cycle.

The Rock Cycle:

The rock cycle is a general model that describes how various geologicalprocesses create, modify, and influence rocks.

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The rock cycle never stops.The rock cycle is shown on a global scale below.All this Earth action is linked by the rock cycle, as you can see in thedrawing below.

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Main Rock Types

The classification of the rocks into three main categories is thus a geneticclassification.

A simplified schematic version below reviews the origin of the rocks by theearth processes at work.

Each of these three classes has by virtue of their formation individualphysical characteristics as shown in the diagram below.

Igneous rock

Igneous rock forms when magma cools and makes crystals.Magma is a hot liquid made of melted minerals.The minerals can form crystals when they cool.When it pours out on Earth's surface, magma is called lava.

Yes, the same liquid rock matter that you see coming out of volcanoes.Igneous rock can form underground, where the magma cools slowly.Or, igneous rock can form above ground, where the magma coolsquickly.

Sedimentary rock

On Earth's surface, wind and water can break rock into pieces.They can also carry rock pieces to another place.Usually, the rock pieces, called sediments, drop from the wind or waterto make a layer.The layer can be buried under other layers of sediments.The deposited sediment undergoes lithification (the processes that turnit into a rock). These include cementation and compaction.

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It takes a long time for the sediments to be cemented togetherto makesedimentary rock. In this way, igneous rock can become sedimentaryrock.

Metamorphic rocks

All rock can be heated.But where does the heat come from?Inside Earth there is heat from pressure (push your hands togethervery hard and feel the heat).There is heat from friction (rub your hands together and feel the heat).There is also heat from radioactive decay (the process that gives usnuclear power plants that make electricity).

So, what does the heat do to the rock?It bakes the rock.Baked rock does not melt, but it does change.It forms crystals.

If it has crystals already, it forms larger crystals.Because this rock changes, it is called metamorphic.Remember that a caterpillar changes to become a butterfly. Thatchange is called metamorphosis.

As the sedimentary rock is buried under more and more sediment, theheat and pressure of burial cause metamorphism to occur.This transforms the sedimentary rock into a metamorphic rock.Metamorphosis can occur in rock when they are heated to 300 to 700 C.Earth's tectonic movements produce heat and build mountains andmetamorphose (met-ah-MORE-foes) the rock.

The rock cycle continues.Mountains made of metamorphic rocks can be broken up and washed awayby streams.

New sediments from these mountains can make new sedimentary rock.As the metamorphic rock is buried more deeply (or as it is squeezed byplate tectonic pressures), temperatures and pressures continue to rise.

If the temperature becomes hot enough, the metamorphic rockundergoes melting.The molten rock is called magma.

The Rock Cycle is a group of changes.

Igneous rock can change into sedimentary rock or into metamorphic rock.

Sedimentary rock can change into metamorphic rock or into igneous rock.

Metamorphic rock can change into igneous or sedimentary rock.

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Please also note that:

1. Any rock type can undergo weathering (breakdown) to formsediment, followed by transportation and deposition of the sediment.Both metamorphic and sedimentary rocks can undergo weathering.

2. Igneous rocks can undergo metamorphism (as a result of heat andpressure) to form metamorphic rocks.

Some Features of different types of Rocks

1. Igneous 2. Igneous 3.Metamorphic 4. Sedimentary(Coarse crystals) (Fine crystals) (Banding) (Grains)

Find out for yourself how different parts of the rock cycle work. The cycleis shown schematically in the diagram below. Each number in the diagramcorresponds to a process in the cycle.

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Common ions found in minerals.

Charges and relative sizes are shown below.

Definite composition indicates that a chemical analysis of a givenmineral will always produce the same ratio of elements.

For example, the mineral quartz will always have one silicon forevery two oxygen atoms.Therefore, minerals can be expressed by chemical formulas, suchas SiO2 for quartz.

Regular internal crystalline structure indicates that the atoms arearranged in a regular repeating pattern.

This diagram shows the structure of the mineral halite.

Atoms ofchlorine and sodium are arranged in a three-dimensional repeating pattern.

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Another basic arrangement, the silica tetrahedron, consists ofonesilicon atom surrounded by four oxygen atoms at the corners of thetetrahedron.The silica tetrahedron is a basic building unit for a major group of

minerals called the silicates.

The diagram below shows four representations of the silica tetrahedron.A. Oxygen is represented by the white spheres and siliconby the smaller red sphere.B. An expanded view with rods representing bondsbetween the atoms.C. Diagrammatic representation of the tetrahedron, withfour points representing the locations of oxygen atoms.D. Diagrammatic representation of the tetrahedron lookingdown from above

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By sharing adjacent oxygen atoms, the tetrahedron can form chains,sheets, and three-dimensional frameworks.

A. The structure of the mineral olivine is based on isolated silicatetrahedra.

B. The pyroxene minerals are made of a single chain of tetrahedra.

C. The amphibole minerals are made of a double chain of tetrahedra.

D. Micas, like biotite, are sheets of tetrahedra.

E. Framework silicates, like plagioclase and quartz, are three-

dimensional networks of silica tetrahedra.

Mineral Classification.

Mineral classification is based primarily on the chemical composition,atomicstructure, degree of ionic substitution, and colorand crystallinestate of minerals.

A. Mineral Classes

- Minerals are classified primarily on the main anion (O-2, S-2, etc.), anioniccomplex (oxyacid anion) (OH-1, SO4

-2, CO3-2, PO4

-3, BxOy-Z, SixOy

-Z, etc), orlack of an anion (native elements)

- Some of the classes are as below

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Native elements (comprised of atoms of only one element and no anion)

Sulfides, including Sulfarsenides, Arsenides, Sulfosalts (main anion is S -2)

Oxides (main anion is O-2

)Hydroxides (main anion complex is OH-1)

Halides (main anion is a halogen as Cl-1, Fl-1, Br-1, I-1)

Carbonates (the oxyacid anion, CO3-2)

Nitrates (the oxyacid anion, NO3

-1)

Borates (the oxyacid anion, BxOy-Z)

Phosphates (the oxyacid anion, PO4-3)

Sulfates (the oxyacid anion, SO4

-2)

Tungstates (the oxyacid anion, WO4-2)

Silicates (the oxyacid anion, SixOy-Z)

B. Mineral Subclasses

Some classes can be subdivided on chemical or structural groundse.g.1. Native Element Class which is divided into minerals with metallicbonding (metals), those with mostly covalent bonding (nonmetals), andthose with a mixture (semimetals);

2. the S ilicate Class, into 6 subclasses (neso-, soro-, cyclo-, phylo-,tecto-silicates) based on the linkage of the silica tetrahedra

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Silicates

Importance of Silicates

Silicates account for91.6% of the Earth's crust.

Silicates - Structure

Tetrahedron - Shared Oxygens

Basic Building Block: Silica Tetrahedron

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Composed ofone silica atom bonded to four oxygen atoms

50/50 mix ionic and covalent bonds - mixed bond

Oxygens form isodesmicbonds, meaning that they equally distributetheir charge between the central Si and some other ion. (i.e., -1 to Siand -1 to other = total -2)

Silicates - Classification

Classes of Silicate Structures

Silicates are subdivided into 6 groups based on the arrangement of Si-

tetrahedra within their structure:

Subgroup StructureSi:ORatio

Nesosilicates

Single tetrahedrons (no oxygens shared betweenneighboring tetrahedra) joined by bonds with othercations

Red circles denote other cations between the tetrahedra.

1:4

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Sorosilicates

Two neighboring tetrahedra share one point

2:7

Cyclosilicates

All tetrahedra share 2 oxygens, each one with a differentneighbor, building up 3, 4, 6 or 12 tetrahedra into a ringstructure

1:3

InosilicatesSingle Chain

All tetrahedra share 2 oxygens, each one with a differentneighbor, building a chain structure

1:3

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InosilicatesDouble Chain

Alternate tetrahedra share 2 then 3 oxygens, each onewith a different neighbor, building a side-by-side doublechain structure

4:11

Phyllosilicates

Each tetrahedron shares 3 basal oxygens, each one witha different neighbor, building a sheet structure

2:5

TectosilicatesEach tetrahedron shares all 4 oxygens, each one with adifferent neighbor, building a 3-D framework structure

1:2

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1. Single tetrahedralolivine

2. Single chainspyroxene

3. Double chains

amphibole

4. Sheetsmuscovitebiotite

5. Frameworksquartz

feldsparpotassium feldspars (orthoclase and microcline)plagioclase feldspars

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