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Geology of the Hawaiian IslandsGeology of the Hawaiian IslandsClass 12Class 12

19 February 200419 February 2004

Exam ScoresA B C D F

Mean = 71Median = 82/77

828257578484606085856969meanmean858591916060737386869292

2929646473738787949451516666777788889797

Any Questions?Any Questions? Sedimentary RocksSedimentary RocksRocks made of fragments of other rocksRocks made of fragments of other rocksRocks precipitated from a solutionRocks precipitated from a solution

Example: LimestoneExample: Limestone

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Sedimentary Stages in the Rock CycleSedimentary Stages in the Rock Cycle

SedimentSedimentMaterial that has been moved by geologic Material that has been moved by geologic processes such as wind, gravity, streams, processes such as wind, gravity, streams, ice, ocean currents, and wavesice, ocean currents, and wavesMay be carried as solid fragments or in May be carried as solid fragments or in solution to be precipitated latersolution to be precipitated later

Sedimentary rocks Sedimentary rocks Compacted sedimentCompacted sedimentNot very voluminous in HawaiiNot very voluminous in HawaiiOverall very important components Overall very important components of Earth's crustof Earth's crust

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Sedimentary Rock ClassificationSedimentary Rock ClassificationLevel 1: Level 1: DetritalDetrital vsvs ChemicalChemicalDetritalDetrital sediments formed of material sediments formed of material eroded and transported before being eroded and transported before being depositeddepositedChemical sediments precipitated Chemical sediments precipitated directly from solutiondirectly from solution

Sedimentary Rock ClassificationSedimentary Rock ClassificationLevel 2: SizeLevel 2: SizeSize of fragments > 2mm = Size of fragments > 2mm = conglomerateconglomerate2 to 1/16 mm = sandstone2 to 1/16 mm = sandstone1/16mm to 1/256 mm = siltstone1/16mm to 1/256 mm = siltstone< 1/256 mm= < 1/256 mm= claystoneclaystone or shaleor shale

Fragment size Fragment size Partly a function of original size of the Partly a function of original size of the fragments as they were eroded from the fragments as they were eroded from the source rockssource rocksMuch of the size difference in sedimentary Much of the size difference in sedimentary rocks occurs during the transportation of the rocks occurs during the transportation of the fragmentsfragmentsFragments may be abraded or broken, so Fragments may be abraded or broken, so that they generally become smaller when that they generally become smaller when they have been transported long distancesthey have been transported long distances

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QuartzQuartzThe dominant mineral found in The dominant mineral found in sedimentary rocks in most continental sedimentary rocks in most continental areasareas

QuartzQuartzThe dominant mineral found in The dominant mineral found in sedimentary rocks in most continental sedimentary rocks in most continental areas areas ---- WHY?WHY?

QuartzQuartzThe dominant mineral found in The dominant mineral found in sedimentary rocks in most continental sedimentary rocks in most continental areas areas ---- WHY?WHY?Most stable mineral at the Earth’s Most stable mineral at the Earth’s surfacesurface

QuartzQuartzA sedimentary rock cannot have quartz A sedimentary rock cannot have quartz unless the source terrane has quartzunless the source terrane has quartzAlmost no source rocks in Hawaii with Almost no source rocks in Hawaii with quartz, quartz, So quartz is very rare in Hawaiian So quartz is very rare in Hawaiian sedimentary rockssedimentary rocks

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ClasticClastic SedimentSedimentMost of the sediment formed on and Most of the sediment formed on and near the Hawaiian islands near the Hawaiian islands What are the What are the clastsclasts ? ? Volcanic, because the sediment comes Volcanic, because the sediment comes from the erosion of volcanic rocksfrom the erosion of volcanic rocks

ClasticClastic SedimentSedimentTemporarily accumulates in the valleys Temporarily accumulates in the valleys and bays, but eventually most of the and bays, but eventually most of the sediment is washed into the oceansediment is washed into the oceanBecause the islands have very steep Because the islands have very steep slopes, most of the sediment washed slopes, most of the sediment washed into the ocean is carried away from the into the ocean is carried away from the islands into the deep ocean basinislands into the deep ocean basin

Local Sedimentary RocksLocal Sedimentary RocksAccumulate around the coast lines on Accumulate around the coast lines on or near beaches and in the shallow or near beaches and in the shallow water offshorewater offshoreMost are made up of chemical Most are made up of chemical precipitatesprecipitatesStart out as coral or as shells secreted Start out as coral or as shells secreted by marine animalsby marine animals

Calcium Carbonate (CaCOCalcium Carbonate (CaCO33))Most important mineral type in local Most important mineral type in local sedimentary rocks sedimentary rocks Forms broken down pieces of Forms broken down pieces of shells and coralshells and coral

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Transport affects sediment in several waysTransport affects sediment in several ways

SortingSorting: : measure of the variation in the range measure of the variation in the range of grain sizes in a of grain sizes in a clasticclastic rock or sedimentrock or sediment

WellWell--sorted sediments have been subjected to sorted sediments have been subjected to prolonged water or wind action.prolonged water or wind action.

PoorlyPoorly--sorted sediments are either not farsorted sediments are either not far--removed from their source or deposited by removed from their source or deposited by glaciers.glaciers.

SortingSortingWellWell--sortedsorted PoorlyPoorly--sortedsorted

Transport affects sediment in several waysTransport affects sediment in several ways

Roundness Roundness : measure of how rounded the : measure of how rounded the corners arecorners are

SphericitySphericity: measure of how much it is like a : measure of how much it is like a spheresphere

Sorting, roundness, and Sorting, roundness, and sphericitysphericity all increase all increase with amount of transportwith amount of transport

Roundness and Roundness and sphericitysphericity

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WellWell--sortedsortedAngularAngularSandSand

PoorlyPoorly--sortedsorted

RoundedRoundedSandSand

Types of chemical sedimentary rocksTypes of chemical sedimentary rocks

LimestoneLimestone CaCOCaCO33

ChertChert SiOSiO22

SaltSalt NaClNaCl, , KClKCl, K, K22SOSO44

GypsumGypsum CaSOCaSO44 • 2H• 2H22OO

CoalCoal altered organic debrisaltered organic debris

Common Sedimentary EnvironmentsCommon Sedimentary Environments

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Age of the Islands Age of the Islands Relative Relative vsvs Absolute AgeAbsolute AgeRelative: Rock “A” is older (or Relative: Rock “A” is older (or younger) than rock “B” younger) than rock “B” Absolute = knowing the age of a rock Absolute = knowing the age of a rock in yearsin yearsUsually geologists first establish Usually geologists first establish relative ages then try to get absolute relative ages then try to get absolute age datesage dates

Layer 1Layer 2Layer 3Layer 4

Principle of SuperpositionPrinciple of SuperpositionSedimentary rocks are deposited in a Sedimentary rocks are deposited in a

layerlayer--cake fashion:cake fashion:

Each layer is older than the one Each layer is older than the one above and younger than the one above and younger than the one

belowbelow

Oldest rocksOldest rocks

Youngest rocksYoungest rocks

Principle of SuperpositionPrinciple of Superposition

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Principle of SuperpositionPrinciple of SuperpositionAlthough this is really obvious, it was Although this is really obvious, it was not stated until 1669not stated until 1669This principle generally applies to This principle generally applies to volcanic rocks as well as sedimentary volcanic rocks as well as sedimentary rocks rocks

Principle of Original Principle of Original HorizontalityHorizontality

Layers of sediment are deposited in a Layers of sediment are deposited in a nearly horizontal positionnearly horizontal position

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Principle of Original HorizontalityPrinciple of Original HorizontalityNote that original horizontality is not Note that original horizontality is not strictly applicable to volcanic rocks strictly applicable to volcanic rocks ----why? why? Because they are often deposited on Because they are often deposited on slopesslopes

Principle of CrossPrinciple of Cross--cutting cutting RelationshipsRelationships

Something (such as a dike or fault) Something (such as a dike or fault) that cuts across a layer must be that cuts across a layer must be younger than the layeryounger than the layer

Layers of rock are said to be Layers of rock are said to be conformableconformable when they are when they are

found to have been deposited found to have been deposited essentially without essentially without

interruptioninterruption

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Unconformity Unconformity Results from interruption of deposition Results from interruption of deposition Represents a long period of time during Represents a long period of time during which there either was no deposition, or which there either was no deposition, or earlier deposited material was eroded earlier deposited material was eroded awayaway

Unconformity Unconformity Often not easy to recognize if the Often not easy to recognize if the layers are all parallellayers are all parallelMuch easier to recognize when there is Much easier to recognize when there is a period of folding of the rocks before a a period of folding of the rocks before a period of erosion and renewed period of erosion and renewed depositiondeposition

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The Great Unconformity of the Grand CanyonThe Great Unconformity of the Grand Canyon

Angular unconformity, Grand CanyonSummary of Geologic Events in a Summary of Geologic Events in a small areasmall area

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These methods work well in small areas These methods work well in small areas where we can see the relationships where we can see the relationships between rock layers. between rock layers. What happens when we want to tell the What happens when we want to tell the relative ages of the strata on Oahu with relative ages of the strata on Oahu with respect to strata on Maui? respect to strata on Maui? We have to figure out some way to We have to figure out some way to correlatecorrelate the layers of interest.the layers of interest.

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CorrelationCorrelationWithin sedimentary layers there are Within sedimentary layers there are often the remains of small animals often the remains of small animals ((fossilsfossils))Fossils are quite useful for correlating Fossils are quite useful for correlating between two sections that are not between two sections that are not laterally continuouslaterally continuous

CorrelationCorrelationFossils have evolved through time, so Fossils have evolved through time, so when we find a fossil of the same type in when we find a fossil of the same type in two different areas, we are pretty sure two different areas, we are pretty sure that the rocks are about the same agethat the rocks are about the same age

TThis technique is not very useful in his technique is not very useful in HawaiiHawaiiWhy?Why?Fossils helpful in sedimentary rocks, Fossils helpful in sedimentary rocks, but usually no fossils in volcanic rocksbut usually no fossils in volcanic rocksSometimes small amounts of sediment Sometimes small amounts of sediment between layers of volcanic rock (such between layers of volcanic rock (such layers might have fossils), but most layers might have fossils), but most rocks in Hawaii do not have fossilsrocks in Hawaii do not have fossils

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MagnetostratigraphyMagnetostratigraphyTechnique that works best in volcanic Technique that works best in volcanic rocksrocksTime scale based on polarity reversal of Time scale based on polarity reversal of Earth's magnetic fieldEarth's magnetic fieldMajor problem is that Earth's magnetic Major problem is that Earth's magnetic field has been constant for the past field has been constant for the past 700,000 yrs (no reversals), so this 700,000 yrs (no reversals), so this doesn't work for very young rocksdoesn't work for very young rocks

Magnetic time scale Magnetic time scale 00--700,000 700,000 ---- NormalNormal700,000 700,000 -- 2.5 my 2.5 my ---- ReversedReversed> 2.5 my > 2.5 my ---- NormalNormalKo`olauKo`olau lavas mostly reversed in lavas mostly reversed in polarity, so they must be older than polarity, so they must be older than 700,000 yrs, but younger than 2.5 my700,000 yrs, but younger than 2.5 myLavas on Lavas on Kaua`iKaua`i and in and in Wai`anaeWai`anaeRange are positive, so they must be Range are positive, so they must be older than 2.5 myolder than 2.5 my

Radiometric DatingRadiometric DatingUse of radioactive decay to determine Use of radioactive decay to determine the age of a rockthe age of a rockKey principle: Half Life = time required Key principle: Half Life = time required for 1/2 of the nuclei in a sample to for 1/2 of the nuclei in a sample to decaydecay

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Radioactive Radioactive Decay of Decay of

Rubidium to Rubidium to StrontiumStrontium

Radioactive Radioactive Decay of Decay of

Uranium 238Uranium 238toto

Lead 206Lead 206

Radiometric DatingRadiometric DatingIn addition to Uranium In addition to Uranium -- Lead series, Lead series, geologists also use geologists also use RubidumRubidum--Strontium, Strontium, CC1414 and Potassium and Potassium -- Argon for datingArgon for datingCarbonCarbon--14 used for very young rocks. 14 used for very young rocks.

Radiometric DatingRadiometric DatingShows that the earth is much older than Shows that the earth is much older than people had previously suspectedpeople had previously suspectedEarth formed about 4.6 billion years Earth formed about 4.6 billion years agoago

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The Geologic time scaleThe Geologic time scaleDivisions in the worldwide Divisions in the worldwide stratigraphic column based on stratigraphic column based on variations in preserved fossilsvariations in preserved fossilsBuilt using a combination of Built using a combination of stratigraphic relationships, crossstratigraphic relationships, cross--cutting relationships, and absolute cutting relationships, and absolute (isotopic) ages(isotopic) ages

Questions? Questions?

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For TuesdayFor Tuesday

Hydrologic Cycle and Hydrologic Cycle and Stream ErosionStream ErosionPlease read Chapters 8+9 in TextPlease read Chapters 8+9 in Text