Geology of the Hawaiian Islands Class 12 19 February 2004 A B C D F 97 94 92 91 88 87 86 85 85 84 82 77 73 73 mean 66 64 60 69 60 57 51 29 Exam Scores Mean = 71 Median = 82/77 Any Questions? Sedimentary Rocks Rocks made of fragments of other rocks Rocks precipitated from a solution Example: Limestone Sedimentary Stages in the Rock Cycle Sediment Material that has been moved by geologic processes such as wind, gravity, streams, ice, ocean currents, and waves May be carried as solid fragments or in solution to be precipitated later Sedimentary rocks Compacted sediment Not very voluminous in Hawaii Overall very important components of Earth's crust Sedimentary Rock Classification Level 1: Detrital vs Chemical Detrital sediments formed of material eroded and transported before being deposited Chemical sediments precipitated directly from solution 1
Sedimentary Rock Classification Level 2: Size Size of fragments > 2mm = conglomerate 2 to 1/16 mm = sandstone 1/16mm to 1/256 mm = siltstone < 1/256 mm= claystone or shale Fragment size Partly a function of original size of the fragments as they were eroded from the source rocks Much of the size difference in sedimentary rocks occurs during the transportation of the fragments Fragments may be abraded or broken, so that they generally become smaller when they have been transported long distances Quartz The dominant mineral found in sedimentary rocks in most continental areas Quartz The dominant mineral found in sedimentary rocks in most continental areas -- WHY? Quartz The dominant mineral found in sedimentary rocks in most continental areas -- WHY? Most stable mineral at the Earth s surface Quartz A sedimentary rock cannot have quartz unless the source terrane has quartz Almost no source rocks in Hawaii with quartz, So quartz is very rare in Hawaiian sedimentary rocks Clastic Sediment Most of the sediment formed on and near the Hawaiian islands What are the clasts? Volcanic, because the sediment comes from the erosion of volcanic rocks Clastic Sediment Temporarily accumulates in the valleys and bays, but eventually most of the sediment is washed into the ocean Because the islands have very steep slopes, most of the sediment washed into the ocean is carried away from the islands into the deep ocean basin 2
Local Sedimentary Rocks Accumulate around the coast lines on or near beaches and in the shallow water offshore Most are made up of chemical precipitates Start out as coral or as shells secreted by marine animals Calcium Carbonate (CaCO 3 ) Most important mineral type in local sedimentary rocks Forms broken down pieces of shells and coral Transport affects sediment in several ways Sorting: measure of the variation in the range of grain sizes in a clastic rock or sediment Well-sorted sediments have been subjected to prolonged water or wind action. Poorly-sorted sediments are either not far- removed from their source or deposited by glaciers. Well-sorted Sorting Poorly-sorted Transport affects sediment in several ways Roundness : measure of how rounded the corners are Sphericity: : measure of how much it is like a sphere Sorting, roundness, and sphericity all increase with amount of transport Roundness and sphericity Well- sorted Angular Sand Poorly- sorted Rounded Sand Types of chemical sedimentary rocks Limestone Chert Salt Gypsum Coal CaCO 3 SiO 2 NaCl, KCl,, K 2 SO 4 CaSO 4 2H 2 O altered organic debris 3
Common Sedimentary Environments Age of the Islands Relative vs Absolute Age Relative: Rock A is older (or younger) than rock B Absolute = knowing the age of a rock in years Usually geologists first establish relative ages then try to get absolute age dates Principle of Superposition Sedimentary rocks are deposited in a layer-cake fashion: Layer 4 Layer 3 Layer 2 Layer 1 Each layer is older than the one above and younger than the one below Youngest rocks Principle of Superposition Oldest rocks Principle of Superposition Although this is really obvious, it was not stated until 1669 This principle generally applies to volcanic rocks as well as sedimentary rocks Principle of Original Horizontality Layers of sediment are deposited in a nearly horizontal position 4
Principle of Original Horizontality Note that original horizontality is not strictly applicable to volcanic rocks -- why? Because they are often deposited on slopes Principle of Cross-cutting Relationships Something (such as a dike or fault) that cuts across a layer must be younger than the layer Layers of rock are said to be conformable when they are found to have been deposited essentially without interruption Unconformity Results from interruption of deposition Represents a long period of time during which there either was no deposition, or earlier deposited material was eroded away Unconformity Often not easy to recognize if the layers are all parallel Much easier to recognize when there is a period of folding of the rocks before a period of erosion and renewed deposition 5
The Great Unconformity of the Grand Canyon Angular unconformity, Grand Canyon Summary of Geologic Events in a small area 6
These methods work well in small areas where we can see the relationships between rock layers. What happens when we want to tell the relative ages of the strata on Oahu with respect to strata on Maui? We have to figure out some way to correlate the layers of interest. Correlation Within sedimentary layers there are often the remains of small animals (fossils) Fossils are quite useful for correlating between two sections that are not laterally continuous Correlation Fossils have evolved through time, so when we find a fossil of the same type in two different areas, we are pretty sure that the rocks are about the same age 7
This his technique is not very useful in Hawaii Why? Fossils helpful in sedimentary rocks, but usually no fossils in volcanic rocks Sometimes small amounts of sediment between layers of volcanic rock (such layers might have fossils), but most rocks in Hawaii do not have fossils Magnetostratigraphy Technique that works best in volcanic rocks Time scale based on polarity reversal of Earth's magnetic field Major problem is that Earth's magnetic field has been constant for the past 700,000 yrs (no reversals), so this doesn't work for very young rocks Magnetic time scale 0-700,000 -- Normal 700,000-2.5 my -- Reversed > 2.5 my -- Normal Ko`olau lavas mostly reversed in polarity, so they must be older than 700,000 yrs, but younger than 2.5 my Lavas on Kaua`i and in Wai`anae Range are positive, so they must be older than 2.5 my Radiometric Dating Use of radioactive decay to determine the age of a rock Key principle: Half Life = time required for 1/2 of the nuclei in a sample to decay Radioactive Decay of Rubidium to Strontium Radioactive Decay of Uranium 238 to Lead 206 Radiometric Dating In addition to Uranium - Lead series, geologists also use Rubidum-Strontium, C 14 and Potassium - Argon for dating Carbon-14 used for very young rocks. Radiometric Dating Shows that the earth is much older than people had previously suspected Earth formed about 4.6 billion years ago 8
The Geologic time scale Divisions in the worldwide stratigraphic column based on variations in preserved fossils Built using a combination of stratigraphic relationships, cross- cutting relationships, and absolute (isotopic) ages Questions? For Tuesday Hydrologic Cycle and Stream Erosion Please read Chapters 8+9 in Text 9