Lecture 03_Uniformity PDF

Lecture 3 Response: If reading the rocks is like reading a book, what assumptions do we need to make to give meaning to signs in rocks we did not see form? Welcome back Have your geologic section at the ready…...

Lecture 3 Response: If reading the rocks is like reading a book, what assumptions do we need to make to give meaning to signs in rocks we did not see form? Welcome back Have your geologic section at the ready… Email your geologic section to your paired group and be ready to interpret the one you received 1 from your cooperating group.  We will do a lot of in-class A word on groups and discussions in groups It is not a strategy to have one group work… person show up and give notes to the rest. This defeats the whole purpose of the group. Now, instead of developing your knowledge from an expert, you are relying on a novice Working together is important You will learn more and better You will also develop a rhythm and pace (comfortableness) with each other that will aid in more high stakes activities such as learning assessments. You will be evaluated by your teammates for your contributions to the group goals 2  Step: How to interpret… 1. (OLDEST) Submarine (?) deposition of layers Include Steno’s (Superposition) principles 2. Tilting (original horizontality) 3. Uplift/emergence/ erosion  unconformity (cross-cutting relationship) 4. Submergence/ deposition (superposition) 5. Faulting 6. Igneous intrusion 7. (YOUNGEST) Erosion into hill landscape 3  The beginnings of Stratigraphy How do we know these rocks here Giving meaning to signs in the rocks using stratigraphy, are related to those way over biostratigraphy, fossils, and telling relative time there? 4 Correlation: Connecting rocks in space and Using Steno’s principle time… of lateral continuity Strati/graphy – layers/drawing Drawing the layers to connect one outcrop to another First used by connecting rock types 5  Primary (primitive) Types of Rocks: rocks: These were crystalline in nature Rock classification (back andusually found beneath all other rocks. No fossils then) were found in these rocks. Two schools of thought 1. To Neptunists, crystalline rocks formed as a precipitate as the original flood evaporated 2. To Plutonists, crystalline rocks formed from a molten rock during the cooling of the earth from a ball of molten material. 6  Abraham Gottlob Werner– Using the signs in the Neptunism–rocks rocks to understand the precipitated from global ocean, creating global history of the earth rock formations Hutton–Plutonism– Crystalline rocks the result of cooling molten rocks. Earth began as molten ball. Primary rocks first to cool and crystalize. How could they be Looking at the same signs but getting two different meanings? Initial assumptions influence thinking 1726-1797 1750-1817 7 Rock classification Secondary rocks: These rocks occurred mainly on (back then) top of the primary rocks and were composed mostly of bits and pieces of primary rocks in flat horizontal layers. They often contained fossils, possibly signs of past life. Layers that did not have fossils were called “transition” rocks within the secondary classification. They were below the fossiliferous rocks and therefore considered older. 8 Rock classification A. Tertiary rocks: This referred sediments (back then) that draped over both primary and secondary rocks, containing many fossils. C B B. Quaternary rocks: Clearly very young (surficial) unconsolidated, alluvial sediments (today we would consider them recent A glacial deposits). C. Volcanic rocks: Current volcanic activity continued to form these rocks. 9 Rock classification (back then) Volcanic Quaternary Tertiary Secondary Transition Primary 10  Scottish Extending Steno’s ideas: Enlightenment (starting in 1750s) Highly educated Solicitor (lawyer) Doctor “Gentleman farmer” Newton was all the rage Naturalistic world view Cycles James Hutton Also read Steno’s De Solido 1726-1797 11 The present is the key Hutton’s observations: After a rain, the rivers on his to the past (hint: This is an property rose and erosion (wasting) quickened important idea) Sediments traveled to the ocean where deposition took place These processes created signs in the sediments Hutton’s assumptions: Modern day phenomena have been at work throughout the history of the earth Similar looking signs observed in rocks must have been created by similar phenomena in the past Deposits of sediment turned to stone and deformation resulted as they were pushed back into mountains for the cycle to start again. 12  When we see these structures (ripples) The present is the key created by wave action in a beach to the past environment (A), then look at a rock with similar signs (B), we can assume the same processes were involved in making them A B 13 Using Steno to understand James Hutton: the nature of some Asserted innumerous cycles of wasting, “primary” rocks deposition, and deformation into “new worlds.” Proposed cycles driven by “Earth’s internal heat” (Plutonism) Found evidence of crystalline rocks crosscutting sedimentary asserting they must be younger than the sedimentary rocks, supporting Plutonism Can you interpret the history of these rocks? 14 The longest word Hutton also: noticed that changes to the you’ll learn in this land on his farm appear class slow and constant, with the surface always in dynamic equilibrium. This would eventually become known as uniformitarianism. surmised each cycle (wasting, deposition, and deformation) takes longer than currently imagined, and proposed that an uncountable number of cycles have and will The unconformity at continue to occur. “…no Jedburgh vestige of a beginning, no Unconformity = missing prospect of an end…” An eternal earth. time 15 What other signs could be information about a rock’s history? We have talked about PHYSICAL processes and the signs they leave What other kinds of processes might also leave signs that are useful? 16 What is a fossil? What? In the “old days” a How do we make one? fossil was anything that you dug up out of the ground. Today, we define fossil as signs of past life. How? Many living organisms have both hard and soft parts to their bodies… The soft parts don’t usually last too long. 17 Compression “messes” things up a How can we make it so bit there is some preservation? Quick burial Anoxic conditions (prevents decomposition) Mineralization 18 Different types of fossilization Original body material Insects get stuck in tree sap which later hardens as amber Baby mastodont, found freeze-dried in glacial ice Ammolite: aragonitic shell of ammonite, prized for its iridescence 19  Top: Permineralization 1. Trees get covered in Mineralization volcanic ash. Water replaces wood with dissolved silica 2. Erosion uncovers the now fossilized tree 3. Note the intricate preservation of the rings and cells of the original tree Bottom: Replacement 1. Ammonite swims and dies, sinks to the bottom 1. 2. 3. of the ocean, and gets buried 2. Sediments harden, shell dissolves in water leaving mold 3. New minerals precipitate in the mold creating the cast 20  Burgess shale Rare fossil finds Important because it appears that a catastrophic event buried the organisms quickly and completely Very fine preservation, even of soft bodied organisms Gives us insight into the earliest complex life on the planet 21  Calcium carbonate–CaCO3–is a Mineralization of popular skeletal mineral for fossils marine organisms, from shell- fish to corals, to certain kinds (common) of algae, and single-celled organisms (diatoms, foraminifera, and coccoliths). Mainly precipitated through biological processes as the mineral aragonite Aragonite is not stable at higher temperatures and pressures and so will often convert to the mineral calcite (same chemical formula, different crystal structure). Easily identified by its low hardness (3/10) and its vigorous reaction to HCl acid. Dolomite–(Ca, Mg)Co3–A product of exposure of calcite to magnesium-rich waters after burial. Identified by its less vigorous reaction with HCl acid 22  Silica–SiO2–is a mineral that is less popular for skeleton building (in terms of variety of organisms Mineralization of using it). Mainly single-celled marine fossils (not so organisms (Radiolaria) It is a harder mineral (6-7/10) common) Does not react with HCl acid Iron oxide–Fe2O3–precipitated after burial Pyrite–FeS2–Precipitated after burial, replacing original organism. Important to realize that when mineralization happens, it is a new sign in the rock, recording certain conditions and processes at the time of mineralization, but also obscures the original signs. Therefore, older rocks are harder to interpret than younger ones. It’s like someone walking into a crime scene and picking up some signs of the crime but also leaving signs of their presence as 23 well. They become part of the  Compression Preservation of carbon Marine arthropods (Pterygotus) buried, and exoskeleton becomes a carbon film in the rocks Plants get buried and the carbon is left as a sign of its existence Large deposits of these carbonized plant deposits (usually signifying swamps) will play a HUGE role in our developing understanding of Coal Earth’s history. (stay tuned  ) 24 Why fossils are Gives us insight to the types of organisms important living at different times They are signs. We can use similar modern organisms to infer activity and environments of the past (sound familiar?) We see this as a coral. We know corals (most often) live in shallow, tropical seas, in a subtidal environment. We infer the same environment for this rock. 25 Question for next Besides the signs on which the four class… principles of Steno are based, what other signs might we look for in the rocks that could help inform on the history of that rock? List three different signs Tell how each might be a clue to interpreting the history of the rock. Fire drill on Thursday 26