The Rock CycleAn Exploration of the World Under YourFeet
What is a “Rock”?• Rock: is a fairly general term that refers to anysolidified mixture of inorganic or fossilized organicmaterials.– There are a multitude of extremely diverse rocktypes that can be differentiated and classifiedbased upon their elemental composition, physicalproperties, and formation mechanism.• Mineral: inorganic solid with a specific internalstructure and a definite chemical composition.– Minerals are the basic building blocks of rock.– Almost all minerals contain some combination ofthe following elements:2• Oxygen - O• Silicon - Si• Magnesium - Mg• Iron - Fe• Aluminum - Al• Calcium - Ca
33 Major Rock Types• Igneous– Formed from the solidification of moltenrock (magmaor lava).• Sedimentary– Formed at the Earth’s surface from theaccumulation and cementation offragmented pieces of older rockproduced by weathering.• Metamorphic– Rocks that have undergone physicalchanges as a result of exposure toextreme pressure, temperature andfluids.
The Rock Cycle• Law of Conservation of Matter: Matter cannot be createdor destroyed it can only change forms.• This fundamental law of the universe applies to thechemical elements that make up the earths rock.• Particular rock formations are created and destroyed manytimes over geologic timescales but the chemical materialitself is recycled over and over again through the rockcycle.4
Types of Igneous Rock• Igneous (ignis= fire)– Formed from the solidification of molten rock(magma or lava).• Magma: is molten rock below the surface of the earth.• Lava: is molten rock that has been extruded on to thesurface of the earth.– Igneous Rocks are generally classified based onwhere in the earth crystallization takes place.• Plutonic (Intrusive): Magma slowly cools and crystallizesbelow the earths surface.– Since the cooling process is very slow intrusive igneous rocks havevery large crystals (coarse grained).• Volcanic (Extrusion): Lava cools quickly at the surface ofthe earth.– Since the cooling process is very fast extrusive igneous rocks havevery small crystals (fine grained). 7
Igneous Classification:TextureVolcanic Glass:Extrusive igneous rockformed when lava comesin contact with water andcools instantly. Appearsglass-like.Example: Obsidian8
Igneous Classification:TextureAphanitic: Typically form from lavawhich crystallizes rapidly on or nearthe Earth’s surface.• Because extrusive rocks makecontact with the atmosphere theycool quickly, so the minerals donot have time to form largecrystals.• The individual crystals in anaphanitic igneous rock are notdistinguishable to the nakedeye.• Example: Basalt 9
Igneous Classification:Texture• Phaneritic: (phaner = visible)textures are typical of intrusiveigneous rocks, these rockscrystallized slowly below theEarths surface.• As a magma cools slowlythe minerals have time to growand form large crystals.• The minerals in a phaneriticigneous rock are sufficiently largeto see each individual crystal withthe naked eye• Example: Fine Grain Granite 10
Igneous Classification:Texture• Pegmatitic: Texture that occursduring very slow magma coolingwhen some minerals may growso large that they becomemassive.• Crystal size ranges from a fewcentimeters to several meters.• Example: Large Grain Granite11
Igneous Classification:Composition• Felsic Rocks: Are those that are light incolor and are mostly made up offeldspars and silicates. Low Density.• Intermediate Rocks: fall somewhere inbetween being Felsic and Mafic.Intermediate Density.• Mafic Rocks: Are darker colored andare mostly made up of magnesium andiron.– Ultramafic Rocks: Very dark colored. 13
Weathering, Erosion, and DepositionWeathering – the breakdown of rocks into smallerpieces, called sediments.Transport (Erosion)– the process where the sedimentsare carried by wind, gravity, glaciers, man, and runningwater.Deposition – the process whereby these sediments arereleased by their transporting agents (dropped).Weathering breaks down the rocks, erosion moves the particles,and deposition drops the sediments in another location.
Types of Weathering• There are two main types of weathering:– Physical Weathering: the breakdown of rocksand minerals into smaller pieces without a changein chemical composition.– Chemical Weathering: the breakdown of rocksand minerals into smaller pieces by chemicalaction. The rocks breaks down at the same timeas it changes chemical composition. The endresult is different from the original rock.
Types of Weathering22
Frost Wedging: rock breakdowncaused by expansion of ice incracks and joints.- Water expands when it freezes
Exfoliation: As previously buried rockformations are exposed they experience adecrease in confining pressure causing themto expand and fall off in parallel sheets.
Plant and Animal Activity• Lichens and mosses grow onrocks.– They wedge their tiny rootsinto spores and crevices.– When the roots grow, therock splits.• Larger trees and shrubs maygrow in the cracks of boulders.• Ants, earthworms, rabbits,woodchucks, and other animalsdig holes in the soil.– These holes allow air andwater to reach the bedrockand weather it.26
There Are 3 Types of Chemical Weathering:Oxidation– oxygen combines with the elementsin the rock and it reacts. This the scientific namefor rust.Hydration– water can dissolve away many earthmaterials, including certain rocks.Carbonation– carbon dioxidedissolves in water to form carbonicacid. This makes acid rain whichchemically weathers (dissolves)rocks. Other acids also combine withwater to make acid rain.
2.Particle Size – Larger particles weather slower and smallerparticles weather at a faster rate.There are 4 factors that effect the rate of weathering:1.Surface Area (exposure) - Exposing more surface area willincrease the rate of weathering.3.Chemical Composition (what a rock is made of) – Certain rocksand minerals are naturally weaker than others, while others are moreresistant (stronger).4. Climate – Warmer, moister climates have the most weathering.Heat & Water speed up all chemical reactions. This is the mostimportant factor in weathering.
Transport and Deposition• Sediments are transported away anddeposited in other locations.• These depositional environments are oftenlayered or stratified.– Older sediments at the bottom– Newer sediments on top• One can deduce the transport mechanismbased upon:– Grain Size– Sorting– Roundness 31
Grain Size• Based on thefundamental idea that asthe amount of weatheringincreases a rock willbreak into smaller andsmaller pieces.• The grain size asediment tells you howmuch weathering it hasundergone.– Also tells you the strengthof the transport process.• Wentworth Scale32
RoundnessAs weathering increases a sediments welldefined sharp edges wear away(angularity decreases) and the rockbecomes more rounded33
Sorting• Sorting describes the distribution of grainsize of sediments.• Very poorly sorted indicates that the sedimentsizes are mixed (large variance)• Well sorted indicates that the sediment sizesare similar (low variance).• The degree of sorting may also indicate theenergy, rate, and/or duration of deposition, aswell as the transport process responsible forlaying down the sediment.36
You can identify which agent of erosion transported eachsediment by looking at a few characteristics:Running Water – sediments that have been transported throughrunning water appear rounded and smooth and are deposited insorted piles.Glaciers – sediments that have been transported by glaciers appearscratched, grooved, and are deposited in completely unsortedpiles, because they were dropped during melting. Also, boulderscan only be transported by glaciers.Wind - sediments that have been transported by wind are appearpitted (random holes) and frosted (glazed look) and are depositedin sorted piles. Only very small particles can be transported bywind.Gravity – sediments that are transported by gravity are found inpiles at the bottom of cliffs or steep slopes. They appear angularand unsorted.
Classifying SedimentaryRocks• Sedimentary rocks are classified by theircomposition and by the manner in whichthey formed.• The three types of sedimentary rocks are– Detrital: Sedimentary rocks that are made fromthe broken fragments of other rocks.– Chemical (Evaporite): Mineral crystal formedwhen water evaporates and leaves behind anydissolved (chemically weathered) minerals.– Organic: Sediment formed from the remains ofdead plants and animals.
Compaction andCementation Forms DetritalSedimentary Rocks• Where sediments are deposited, layer upon layer builds up.• Pressure from the upper layers pushes down on the lowerlayers.• If the sediments are small, they can stick together and formsolid rock. This process is called compaction.• If sediments are large, like sand and pebbles, pressurealone can’t make then stick together– Large sediments have to be cemented together.• As water moves through soil and rock, it picks up materialsreleased from minerals during weathering.• The resulting solution of water and dissolved materials movesthrough open spaces between sediments cementing themtogether.
Detrital Sedimentary Rocks43
Chemical Sedimentary Rocks(Evaporites)44
GeodesFormed when air pockets in rock fills with water formingevaporite crystals.45
• Form from BiogenousOoze– >30% of debris from planktonicorganisms– Calcareous oozes (CaCO3)• Shells of foraminifera(zooplankton) andcoccolithophorids(phytoplankton).• Accumulate on seafloor.• Forms hard limestone underpressureBiologic Sedimentary Rocks
Metamorphic Rocks• Metamorphic rocks arise from thetransformation of existing rock types, in aprocess called metamorphismwhich means"change in form".• The original rock is subjected to heat(temperatures> 150 °C) and pressure (>1500bars)causing profound physical and/orchemical change.50
Types of Metamorphism• Contact Metamorphism: Rock thatcomes in close contact with magma andundergoes metamorphism as a result ofthe extreme temperature.• Regional Metamorphism: A largevolume of rock is buried in the crust andexposed to high temperature andpressure.51
Gem Stones• Many precious mineralsform at metamorphiczones.• Minerals with lowmelting points liquefy,while those with highermelting points remainsolid.– These liquefied mineralsaccumulate in voids,solidify, and becomeprecious minerals.52
54MAGMAVolcanicIGNEOUSPlutonicSEDIMENTSEDIMENTARYMETAMORPHICUpliftBurialIncreased P&TMeltingCrystallizationWeatheringErosionTransportDepositionCan you seeany shortcuts?
• The rock cycledemonstrates therelationships among thethree major rock groups• It is powered by theinterior heat of the Earth• It involves processes onthe Earth’s surface aswell as the Earth’s interior• It connects the“hydrologic cycle” with the“tectonic cycle”.In Conclusion…
60Major Rock Groups• Igneous– Formed from the solidification of molten rock(magmaor lava).– Plutonic (intrusive):slow cooling and crystallization– Volcanic (extrusion): quick cooling at the surface• Sedimentary– Formed at the Earth’s surface from the accumulationand cementation of fragmented pieces of older rockproduced by weathering.– Clastic (Mineral Fragments or grains, clays)– Chemical (crystalline chemical/biochemicalprecipitates)• Metamorphic– Rocks that have undergone physical changes as a