Earth & Environment Midterm Notes PDF
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These notes cover igneous rocks and volcanism, including the rock cycle, different types of melting, and how volcanic eruption products are determined by factors such as composition and volatile content. The notes also discuss the processes of fractional crystallization and how different minerals within a rock have various melting points.
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IGNEOUS ROCKS AND VOLCANISM 1. What is the Rock Cycle? 1. Answer: The rock cycle is a continuous process whererocks are transformed between three types: sedimentary, igneous, and metamorphic. It shows how rocks are recycled over tim...
IGNEOUS ROCKS AND VOLCANISM 1. What is the Rock Cycle? 1. Answer: The rock cycle is a continuous process whererocks are transformed between three types: sedimentary, igneous, and metamorphic. It shows how rocks are recycled over time through various geological processes such as melting, cooling, erosion, and heat/pressure changes. 2. melts become crystals as they cool down. crystals become rocks as they come together and become completely solid i. process called differentiation 2. Step-by-Step Process: 1. Weathering and Erosion: Rocks at the surface are brokendown into smaller particles (sediments) through physical, chemical, and biological processes. 2. Sediment Transportation: The sediments are transportedby wind, water, or ice to different locations. 3. Sediment Deposition: Sediments are deposited in layers,often in bodies of water. 4. Lithification: Over time, pressure compacts the sediments,and they are cemented together to form sedimentary rock. 5. Metamorphism: If sedimentary rock is buried deep withinEarth, heat and pressure cause it to recrystallize and transform into metamorphic rock. 6. Melting: When rocks are subjected to high temperatures,they melt and become magma. 7. Cooling and Solidification: As magma cools, it solidifiesinto igneous rock. 8. Uplift and Exposure: Rocks are pushed up to the surface,where they are again subjected to weathering and erosion, completing the cycle. What are the three main types of rocks? Answer: ○ Igneous rocks: Formed by the cooling and solidificationof molten rock (magma or lava). ○ Sedimentary rocks: Formed by the compaction and cementationof sediments. ○ Metamorphic rocks: Formed when existing rocks arealtered by heat, pressure, or chemical processes. Where igneous rocks form? Subduction zones (volcanic arcs) → plate boundary Melting beneath a mid-ocean rift→ plate boundary Melting beneath continental rift → plate boundary Hot spot volcano (e.g. Hawaii in the middle of ocean) Where does the heat come from? 1. Latent heat, which includes: ○ Heat from the formation of the solar system (kinetic energy from heavy bombardment stages) ○ Differentiation and frictional heating ○ Solidification of the inner core 2. Radioactivity of elements in the crust (mainly Uranium, Thorium, and Potassium) 3. Heat movement through: ○ Mantle convection ○ Lithosphere conduction he Earth is cooling over time. Primary sources and mechanisms of heat generation and T transfer within our planet. What are the types of melting? 1. Decompression: Decrease in P while T stays the same. a. The piece will contain liquid as well as solid rock, this happens after the solidus and before the liquidus. b. This causes mantle plumes, rifting, mid ocean ridges. Mantle flows up in each case. c. Mantle plumes: These are upwellings of hot rock from deep within the Earth's mantle d. Rifting: This refers to the process of continental breakup or the formation of rift valleys e. Mid-oceanic ridges: These are underwater mountain ranges where new oceanic crust is formed i. These locations are areas where decompression melting is likely to occur due to changes in pressure or upward movement of mantle material. 2. Addition of water:lowers melting temperature. a. This happens at subduction zones: water enters the mantle. b. V olatile (liquid or gaseous phase, mostly water) is squeezed out both physically and through metamorphism (as it gets deeper and hotter). c. This causes the volcanic arc, since the asthenosphere underneath melts (due to subduction). d. Nothing to do with increasing temperature 3. Heat transfer (conductive heating): a. Rising magma causes other material in the crust to melt as well. b. Bring up something hot from below and the surrounding material melts. c. Q: Why do volcanic eruptions produce different products and behave differently? Volcanic eruptions differ due to the composition of the magma and its volatile content, which influence its viscosity. High viscosity leads to explosive eruptions with products like gas, ash, and pyroclastic flows low viscosity results in flowing lava. hat factors determine the type of volcanic rock, the nature of volcanic eruptions, and the W environment in which the rock forms? 1. Texture a. size, shape, and arrangement of the crystals in the rock. b. influenced by how quickly or slowly the rock cools c. faster cooling = smaller crystals d. slower cooling = larger crystals . Chemical and Mineralogical Composition 2 a. elements and minerals that make up the rock. b. Different compositions lead to different rock types, likemafic(rich in magnesium and iron) orfelsic(rich in silica). What are the main factors that influence volcanic eruption products? 1. Composition (including silica content) and temperature. 2. Volatile content (e.g., water and gases). These factors influence the viscosity (or fluidity) of magma, which in turn ○ controls whether eruptions are explosive or flowing. Classification: texture E xtrusive:Formed on the surface from lava coolingquickly, producing small crystals (e.g., basalt). ○ ‘Effusive’ volcanism ○ Volcanic eruptions in Hawaii ○ fine grained - cooled faster (less crystals) Intrusive:Formed underground from magma chamberssolidifying and cooling slowly, resulting in large crystals (e.g., granite). ○ dikes: vertical openings, because volatile/melted rock is pushing through trying to get to the surface. Cuts through rock layers ○ Sills are horizontal offshoots of the dyke, flowing between layers. run horizontally and parallel to rock layers ○ coarse grained - cooled slower (bigger crystals) ○ Pluton: A mass of intrusive igneous rock that formed from magma cooling and solidifying below the surface. Over time, erosion can expose these plutons at the Earth’s surface. What are plutons, dikes, and sills, and how do they form + overtime? 1. M agma chambers form underground, with magma intruding into surrounding rocks. 2. As the magma cools slowly, it forms large crystals (e.g., granite). Dikes and sills may also form as the magma pushes through cracks in the surrounding rock. 3. Over geological time, erosion strips away the overlying material, exposing the pluton and associated features (like dikes and sills) on the surface. Classification of igneous rocks: Chemical and mineralogical composition Felsic vs. Mafic Composition: Felsic: ○ Composition: Rich in silicon, oxygen, aluminum, sodium,and potassium. ○ Appearance: Often lighter in color, can appear pinkish. ○ Behavior: High viscosity magma/lava, lower temperatures,tends to be explosive due to high silica content. ○ Rock Types: Granite (intrusive), Rhyolite (extrusive). ○ Felsic magmasare moreexplosivedue to high silicacontent and gas pressure. ○ High Viscosity: Formstall, steep stratovolcanoesdue to the thick, sticky felsic lava that doesn’t travel far from the vent. Mafic: ○ Composition: Rich in magnesium and iron. ○ Appearance: Darker in color (black or greenish). ○ Behavior: Low viscosity magma/lava, higher temperatures,more fluid and tends to result in gentle eruptions. ○ Rock Types: Basalt (extrusive), Gabbro (intrusive). ○ Mafic magmasareflowingdue to lower silica contentand gas pressure ○ Low Viscosity (Mafic): Results inbroad, shield volcanoeswith gentle, flowing eruptions. ○ Createswide, gentle-sloped shield volcanoes(e.g.,those in Hawaii), as mafic lava spreads easily. What processes affect composition? Melting and crystallisation Melting D ifferent minerals within a rock have different melting points. Rocks rarely melt completely; instead, they undergo partial melting where only some minerals melt while others remain solid. Partial melting happens when the temperature is high enough to melt some of the minerals in the rock, but not all. Minerals with lower melting points (often rich in silica) melt first. These form felsic magma, which has higher silica content. Less silica means the magma is more mafic, which is less viscous and more fluid, resulting in gentle eruptions. Less melt: more silica = felsic. More melt: less silica = mafic What is fractional crystallisation? f ractional crystallisation:separates minerals fromthe melt as they solidify, progressively enriching the remaining magma in silica. This explains why you can get both mafic (low silica) and felsic (high silica) rocks from the same body of magma as cooling progresses A fter the high-temperature minerals crystallize, the remaining melt becomes more felsic (rich in silica). If thisresidual meltescapesand freezes, it formsfelsic rockslike granite. Whilemaficandfelsicrocks don’t exist togetherin the same rock, they can come from thesame magmadue to fractional crystallization. ○ Mafic rocks(e.g., basalt, gabbro) form from the firststages of cooling. ○ Felsic rocks(e.g., granite, rhyolite) form from theresidual meltthat becomes enriched in silica as mafic minerals crystallize out. Imagine a large magma chamber: At thebottom, mafic minerals crystallize first, creatingmafic rocks. T heremaining meltrises and becomes felsic, leading tofelsic rockformation as it cools. What is Bowen’s reaction series? B owen's Reaction Series explains how fractional crystallization changes magma composition over time, starting from mafic minerals (high temperatures) and moving toward felsic minerals (low temperatures). The process leads to the formation of both mafic and felsic rocks from a single magma source. What are Bowen’s series minerals? Discontinuous series:minerals stopformingas thetemperature drops and a new mineral begins to crystallize. The earlier minerals can stillremainin the rock, but no more of them will form once the magma cools past the temperature at which that mineral is stable. ○ Olivine → pyroxene → amphibole → biotite →… continuous series:onlyplagioclase feldspar crystallizes,but its composition gradually changes from calcium-rich at high temperatures to sodium-rich at lower temperatures. Unlike the discontinuous series, where different minerals form, the change is gradual and not a complete switch to a new mineral. ○ r esulting plagioclase can contain both calcium and sodium, but in varying proportions depending on the temperature at which it crystallized. ○ Early-formed plagioclase will be more calcium-rich (Ca-plagioclase) mafic ○ later-formed plagioclase will be more sodium-rich (Na-plagioclase). felsic hy do different types of volcanoes form, focusing on how viscosity influences the shape W and behavior of a volcano? V iscosity is the property that determines how easily lava flows. The factors that influence viscosity are: 1. Temperature: Hotter lava is less viscous (flows more easily), while cooler lava is more viscous (thicker and flows less easily). 2. Volatile Elements: Higher gas and volatile content can increase explosiveness but decrease lava viscosity. 3. Amount of Silica (SiO₂): Silica content directly affects viscosity—more silica means higher viscosity. Low viscosity 1. Hotter lava 2. High gas and volatile content 3. Warmerand more fluid, meaning it flows easily. 4. Containsless SiO₂, making it moremafic. 5. Producesshield volcanoeswith broad, gentle slopesbecause the lava can spread out over large areas. High viscosity 1. Cooler lava 2. Low gas and volatile content 3. High silica 4. Coolerand stickier, meaning it resists flowing andbuilds up pressure. 5. Containsmore SiO₂, making it morefelsic. Producescomposite/stratovolcanoes, which have steeperprofiles and can lead to more explosive eruptions due to the buildup of pressure Hotspot Volcanoes, which are formed bymantle plumesfar from tectonic plate boundaries otpost volcanoes: mantle plumes + plate movements. Interior of tectonic plates, far away H from plate boundaries. H otspotsare areas where volcanic activity occursin the interior of tectonic plates, far from plate boundaries. Age and distanceof volcanoes in hotspot chains provideevidence of how they form, and the movement of tectonic plates can be tracked using these chains. Mantle plumesare columns of hot material that risefrom deep within the mantle. As a tectonic plate moves over a mantle plume, new volcanoes form. The plate motion causes older volcanoes to move away from the hotspot, where they becomeextinctas they areno longer over the plume.New volcanoes form where the plume meets the surface. Hotspot chainsshow the direction and speed of tectonicplate movement. Plate motion relative to the mantlecan be deducedfrom these chains. hotspotsandhotspot chainshelp scientists understandthemovement of tectonic platesand theformation of volcanoesfar from plateboundaries Summary Magmais molten rock that is beneath the Earth’s surface;lavais molten rock at or above the Earth’s surface. Melts can be produced throughdecompression,additionof volatiles, orheat transfer. The silica content of a melt determines a melt’sviscosityand, ultimately, the name given to a particular melt. Rock composition depends on melting temperature, crystallization sequence/amount, and degree of magma mixing and assimilation The composition and viscosity of magma sourcing a volcano controls is shape, size, and degree of explosivity Geological time and stratigraphy r elative ages:the age of one feature with respectto another numerical age:age in years What are Steno’s principles for relative dating? ○ Superposition: layers at the bottom are older thanlayers at the top (relative dating) ○ Lateral continuity: sediments accumulate in continuoussheets over large regions. If you find sedimentary layer cut: assume that layer once spanned the area that was later eroded by river that cut the canyon. ○ Original horizontality:sediments are deposited inhorizontally flat layers (gravity) ○ Cross-cutting relationships:If a geologic featurecuts across another one, the feature that has been cut must be older If igneous dike cut across sequence of sedimentary beds, the beds existed before the dike. If fault cuts across and displaces layers of sedimentary rock, then the fault must be younger than the layer. If a layer of sediment buries a fault, the layer is younger than the fault. ○ Inclusion: an inclusion (fragment of one rock incorporated in another) must be older than the rock that contains it. A layer of younger sediment deposited on older rock may contain inclusions (clasts) of the older rock ○ Baked contact: during the formation of an igneous intrusion, magma injects into cooler rock Heat from the intrusion bakes (metamorphoses) surrounding rocks Rock that has been baked by an intrusion must be older than the intrusion Margin of intrusion cools more rapidly than the rest. Finer-grained of younger. ❖ Relation between geological observations and time 1. Uniformitarianism: Velocity and intensity of processes remain constrained through time (present is key to past) 2. Catastrophism: Catastrophes are the main drivers of change through geologic time. (before we had the knowledge we have today) Gradualism: : Geological changes occur slowly and gradually over long periods of time. 3. Actualism: Process observed today were similar in the past Works for some process, but we also know that for example the temperature and continents have changed over time ossil Correlation: Geologists can date rock layersby comparing the fossil assemblages F they contain. Fossils from the same time period are found in rocks that formed at the same time. Assemblages: A group of fossils found together y comparing fossil ages, geologists can determine whether two units, even if composed of B different rock types, are the same age. eologists recognise gaps in the rock record through unconformities, which are surfaces G where no rock layers have been deposited or older layers have been eroded away. These unconformities are identified when layers are missing from the sequence. S tratigraphic columns can also be correlated by matching rock types (lithologic correlation). Determining the age of strata at one location with respect to the strata at another. When correlating formations among nearby regions, we can look for similarities in rock type. Geologists can correlate rock layers between different regions to see where layers are missing. ○ Lateral continuity If no fossils: Geologists must rely on radiometric dating (measuring the decay of radioactive elements in minerals) to establish the age of these older rocks. Once a fossil species disappears at a horizon in a sequence of strata, it doesn’t reappear higher in the sequence. Gap: time interval during which one strata eroded away and new strata had not yet accumulated ○ Unconformity:boundary surface between two rock units,which represents period of nondeposition and erosion. ○ Represents gap in geologic record How do geologists determine gaps in time? G aps in timeare detected throughunconformities,which are surfaces that represent missing time in the geological record due to erosion or non-deposition of sediments. ○ E.g. sudden change between strata and conglomerate above, e.g. change in direction For Cambrian and younger rocks, gaps are often determined through fossil correlation, where the absence of certain fossil species in the expected sequence suggests a gap in time. ○ lithologic correlation, where rock layers are matched between different locations, and sedimentary structures help to identify these gaps. Lateral continuity. Precambrian rocks do not contain fossils, so geologists rely on radiometric dating to determine gaps. By measuring the decay of radioactive elements in minerals, they can establish the age of rocks and detect missing time periods. What are the 3 types of unconformities? A ngular uncoformity: layers undergo folding (e.g. mountains form and layer fold) → erosion produces a flat surface → sea level rises and new layers of sediment accumulate Recognise by old,folded layers → suddenly new horizontal layers ○ No nconformities (igneous): pluton intrudes → erosioncuts down into the crystalline rock → new sedimentary layers accumulate above erosion surface ○ Sedimentary rocks overlie older intrusive igneous rocks and/or metamorphic rocks ○ Anonconformityis a specific type ofunconformitywheresedimentary rocks lie on top of older igneous or metamorphic rocks.This represents a significant gap in the geological record because the older igneous or metamorphic rocks formed deep within the Earth, and after a long period of erosion and exposure, sedimentary layers were deposited on top. ○ Recognise by igneous/metamorphic then suddenly sedimetary Disc onformities (sedimentary): layers of sedimentaccumulate → sea level drops and an erosion surface forms (no longer flat-lying strata) → sea level rises and new sedimentary layers accumulate ○ Parallel layers but boundary represents a hiatus ○ Recognise by slightly not straight sedimentary layers but still parallel ○ Gap in the fossil record or evidence of erosion reveals the uncomformity What is lithostratigraphy? Allows to correlate rock layers9 Lithostratigraphy: Correlates rock layers based onphysical characteristics (rock type, sequence) across different locations (A, B, C). Different Thicknesses: Same rock layers (e.g.,MiloLimestone,Rufus Limestone) can have different thicknesses or even be absent at different locations due to environmental changes. What is meant by correlating fossils to find ages? Fossils succession: predictable order in which fossils appear and disappear Every geologic time interval has a specific fossil content Index fossils: Organism existed for a short period; widely distributed Why is there a fixed order? Evolution is irreversible ○ Once a fossil species disappears at a horizon in a sequence of strata, it doesn’t reappear higher in the sequence. What is the global geographic timescale? Correlation of stratigraphic columns from around the world provides a composite record of Earth’s history What is radiometric dating and how does it work? Absolute dating technique P arent isotope:undergoes decay Daughter isotope:decay product All radioactive decay reactions change the atomic number of the nucleus thus the identity of the element Since an isotope’s half-life stays constant, we can calculate the age of a mineral by measuring the ratio of parent to daughter atoms in the mineral 1. Unstable Isotopes: Some atoms (isotopes) are unstable and contain an excess of energy in their nucleus. 2. Decay to Stability: These unstable isotopes transform over time into more stable forms by emitting particles and energy in a process called radioactive decay. 3. Parent to Daughter Isotopes: During decay, the original unstable isotope (the parent) changes into a different isotope or element (the daughter) that is more stable. 4. Use in Radiometric Dating: By measuring the ratio of parent to daughter isotopes and knowing the half-life, geologists can calculate the age of a mineral or rock Why can’t we date sedimentary rocks directly? W e cannot directly date sedimentary rocks because they are composed of particles that have been eroded, transported, and then cemented together. T hese particles, or clasts, come from pre-existing rocks, meaning the age of the sedimentary rock reflects the age of the particles, not the time of its formation. Radiometric dating, which is effective for igneous and metamorphic rocks, isn't suitable for sedimentary rocks because they typically don't contain the radioactive isotopes needed for this method. Sediments and sedimentary rocks Types of rocks igneous rocks ○ created when molten magma or lava cools and solidifies, forming a crystalline texture if they cool slowly underground (intrusive) or a smooth, glassy texture if cooled quickly on the surface (extrusive), often found near volcanic activity or deep within the Earth's crust. ○ clear intergrown crystals ○ usually hard ○ no preferred orientation in crystals ○ vesicles (gas bubbles) ○ e.g. granite, basalt, gabbro sedimentary rocks ○ form from the compaction and cementation of sediments like sand, silt, and clay, often displaying distinct layers and sometimes containing fossils of plants or animals, typically found in environments like rivers, lakes, or oceans where sediments accumulate ○ reworked particles/clasts ○ chemical precipitates ○ layers, strata, or grainy textures (can be fine grained) ○ conglomerate rock (clasts in matrix). reworked pebbles. ○ limestone. made from calcium carbonate, dead organisms’ shell. ○ mud stone - a little bit metamorphic (it has been metamorphose). breaks in a straight line when broken. also called slate. made from dead organic material. ○ sandstone - made from grains of sand ○ rock salt/halite - forms underwater forms by the evaporation of water (usually seawater) and the precipitation of dissolved mineral metamorphic rocks ○ produced when existing rocks are subjected to intense heat and pressure, causing physical and chemical changes, and can either have a banded or layered appearance (foliated) or a dense, crystalline structure (non-foliated, so not banded or layered, no sheet-like structure), often found in mountain ranges or regions with significant geological activity ○ show evidence of recrystallisation/deformation ○ e.g. schist, slate igneous/metamorphic → sedimentary, by weathering, erosion, transportation, and deposition weathering is a chemical/physical process erosion is a physical process Where do sedimentary rocks form? S edimentary rocks are a thin cover Sedimentary rocks form only in the uppermost part of the crust and cover igneous and metamorphic ‘basement’ rocks What are the classes of sedimentary rocks? P hysical and chemical weathering provide the raw materials (particles and dissolved ions) for all sedimentary rocks. 1. Clastic:loose rock fragments(clasts)cementedtogether 2. Biochemical: cemented shells of organisms 3. Organic: carbon-richremainsofonce-living organisms 4. Chemical: minerals that crystallize directly from water CBOC What are the important processes of sedimentation? 1. M aking sediment: breakdown of rocks into smaller particles through weathering and erosion. Weathering (physical vs. chemical): forming sediments and soil. breakdown of pre-existing rock at or near the Earth’s surface. Physical (i.e., mechanical): breaks intact rock into unconnected chunks jointing, frost wedging, root wedging, thermal expansion Chemical: altering or destroying minerals and rocks by chemical reaction due to contact with water solutions or air dissolution, oxidation Not all minerals chemically weather at the same rate. Related to maturity → the more stable the rock, the more slowly it weathers Sediment: loose fragments of rock or minerals broken off of bedrock, mineral crystals that precipitate directly out of water, and shells or shell fragments Soil formation Clastic sedimentary rocks grain size and grain size is a measure of the size of fragments or grains composition size ranges from very coarse to very fine (gravel, sand, silt, and clay) astransport distance increases, grain size decreases different compositions (or a lack thereof) hint at source area and transport processes bedding layer:horizontal layer in a rock where everythingwas deposited at the same time Clasts - individual mineral grains or rock fragments containing several mineral types. angularity angularity:degree ofedge or corner smoothness sphericity:degreeto which the shape of a clastapproaches a sphere both indicate how far the sediment was transported: *angular and non-spherical is not transported veryfar. Distance = decreasing angularity and increasing sphericality sorting sorting:measure of the uniformity of grain sizesin asediment population degree of sorting increases with transport distance maturity maturity:thedegree to which sediment changes frominitial weathering events. mature sediments are well-sorted and well-rounded 2. (Re)moving sediment: Erosion (rock cracks and falls, mostly in cold weatherbc water freezes in between and expands) Transport 3. Turning sediments (loose material) into rocks Deposition: sediment settles out of a transporting medium Compaction:pressuresqueezes out air and water frompores Cementation:minerals(quartz or calcite)precipitatesfrom groundwater and glues sediments together →Lithification(making rocks) → Sediment =loose garments of rock or minerals brokenoff of bedrock, mineral crystals that precipitate directly out of water → Regolith: anyloosematerial (sediment or soil(organic))thatcovers solid bedrock at Earth’s surface ❖ Soil formation Soil forms through debris production, interaction with water, and interaction with organics All of these produce unique layers (horizons) seenin a soil profile Soil-forming factors Climate (temp, rain, etc) Substrate: mineral composition and resistance to weathering Slope steepness: regolith easily washes from steep slopes, flat soils hold more moisture and develop thicker soils Time: Soil needs time (young = thinner) ❖ Sedimentary structures Features that form when sediments were deposited Cross Beds: Created by ripple and dune migration Turbidites Turbidity currents: Form in deep basins that receive periodic pulses of turbid water. As pulse wanes, water loses velocity and grains settle biochemical sedimentary rocks: limestones ediments derived from the shells of once-living organisms. S Hard mineral skeletons accumulate after the death of the organisms Warm, tropical, shallow, clear, O2-rich, marine water. CaCO3 in limestone comes from shells from a diversity of organisms (plankton, corals, clams, snails, etc.). Textural varieties include: fossiliferous limestone, micrite, and chalk. edimentary structures:features that form when sedimentswere deposited. useful to s geologists because they provide strong evidence about conditions in the depositional environment crossbeds Created byripple and dune migration. Sandmoves up thegentle side and then slips downthe steep face. The slip facemoves downcurrent and is buried bythe next avalanche of sand. * Cross-bed orientation indicates wind direction at the time of deposition turbidites and turbidity currents: form indeep basinsthat receiveperiodic pulses of turbid water. as graded beds pulse wanes, water loses velocity and grains settle.thecoarsest material settles first, medium next, then fine. forms graded beds (coarse to fine upward) that alsogive apaleo-way up mud cracks Mud cracksindicate alternate wet and dry terrestrialconditions. and scour marks Scours form whendebris is dragged along the bottomof a river. The scour is filled inwith sediment and can make casts epositional environments:different environments vary in energy, sediment transport d medium, depositional processes, and chemical, physical, and biological characteristics.This all conspires to create unique sedimentary rocks glacial Transport medium/energy:ICE! Can carry and dumpevery grain size. Glacial till:poorly sorted mixtureof allgrainsizes, gravel, sand, silt, and clay.Unsorted sediment deposited by a glacier mountain rivers “Braided Streams” Transport medium/energy:WATER! Can carrylarge clasts during floods. Whenlow-flow dominates, most cobbles and bouldersdon’t move. Coarseconglomerates, sandstonesare common here. meandering Transport medium/energy: WATER! rivers Can carry large clasts during floods. When low-flow dominates, most cobbles and boulders don’t move. Coarse conglomerates, sandstones are common here. Lower energy than mountain rivers (lower topography,lower sediment load) Finer sandstones, silt, and clay-rich rocks arecommon here deposited as channel fill and on nearby floodplains desert Abundance ofwind-blown, well-sorted sand. Dunes move according to the prevailing winds and result inuniform sandstones with gigantic cross beds. lakes Large ponded bodies of freshwater. Gravels and sands are trapped near shore. Well-sorted muds are deposited in deeper water. Varves:thinly alternating finer and coarser sedimentreflecting seasonal changes in sedimentation (each succession =1 year!) marine deltas Delta: awedge of sedimentthat accumulates wheremoving water enters standing water. Sediment accumulateswhere river velocity dropswhen entering the sea. Many subenvironments are present. shallow marine Tropical, warm, clear, shallow, normal salinity, marine water. carbonates Protected lagoons accumulate mud. Wave-tossed reefs are made of coral and reef debris. deep marine Fine sediment that settles out far from landfaraway from intense wave and tide action. Fine silts and clays lithify into shale. Skeletons of planktonic organisms make chalk orchert. How does sea level change affect sedimentary basins? ❖ Sedimentary basins and sea level change Sediment deposition is strongly linked to sea level Transgression(level rise) shifts depositional bedslandward Regression(level fall) shifts environment towardsthe basin hat is the relation with sedimentary basins and plate tectonics?: rift, intracontinental, W foreland basins and passive margins R ift basinsform atdivergent (pull-apart) plate boundaries. Passive margin basinsare theedges of continentsthat are not tectonic-plate boundaries. Intracontinental basinsform in the interior of thecraton, far from continental margins or tectonic plate boundaries. Interior of a continent Foreland basinsform on thecraton side of collisionalmountain belt. Weight of the mountain belt pushes down on the crust’s surface. 1. How do transport processes, transport medium, and transport distance affect the type of rock that forms when sediments are deposited and lithified? Transport Processes: ○ Erosion: Breaks down rocks and moves sediments toa new location. ○ Deposition: Sediment settles out when the transportingmedium (e.g., wind, water, ice) loses energy. Larger particles settle out first, while finer particles travel further before being deposited. ○ Compaction and Cementation: Sediments get compactedby overlying layers and cemented by minerals from groundwater, forming sedimentary rock. Transport Medium: ○ Water: Transports a range of grain sizes, dependingon its speed and energy. High-energy rivers deposit larger clasts (cobbles and gravel), while low-energy water forms finer-grained rocks (siltstone, shale). ○ Wind: Mostly transports fine sediments like sand,resulting in well-sorted sandstone with cross-beds. ○ Ice (Glaciers): Carries apoorly sorted mix of largeand small grains (glacial till) because it has enough energy to move any size particle. Transport Distance: ○ Thelonger the transport distance, the smaller andmore rounded the grains become. ○ Short distance: Sediments are angular, poorly sorted(e.g., breccia). ○ Long distance: Sediments are well-rounded, well-sorted(e.g., sandstone). . Propose a plausible paleo-depositional environment where a sedimentary 2 rock formed, considering the grain size, composition, and maturity of the rock. G rain Size: A well-sorted, fine-grained rock (likesiltstone) indicates low-energy deposition, likely in a lake or deep marine environment. high-energy depositionis typically characterizedby the presence oflarger grain sizes, poor sorting, and sometimes specific sedimentarystructures that indicate rapid sediment transport e.g. turbidites or ice Composition: If the rock is composed of quartz grains,it suggests highmaturityand long transport, possibly from a river or desert environment. Maturity: A mature sedimentary rock like sandstone,with well-rounded and well-sorted grains, could indicate deposition in a desert (wind-transported sand) or along a beach (wave action). xample: A well-rounded, well-sorted sandstone withcross-beds and quartz composition E likely formed in adesertenvironment where wind wasthe primary transport medium. The high maturity (well-rounded grains) and large cross-beds suggest long transport by wind and deposition in dunes. Metamorphism What are metamorphic rocks? Metamorphic rocks:rocks that have becomechangedby intense heat or pressure while forming. One way to tell if a rock sample ismetamorphic is to see if the crystals within it are arranged in bands. In solid state - no melting ○ If the rock were to melt, it would instead enter the igneous rock cycle. What do metamorphic rocks’ form depend on? Protolith (parent rock) ○ Sedimentary, Igneous, or other metamorphic rock Perturbation ○ Pressure ○ Temperature ○ Fluids Changes ○ In texture, mineralogy, grain size, and grain shape ○ Occurs by chemical reactions and recrystallization ○ Same elements → reorganized (limestone to marble) What are the different types of metamorphism? regional ccurringover a broad area because of tectonic movement o -mountain building (orogenesis): moderate P,T -subduction: high P, low T Burial during subduction, closure of an ocean, ending in continental collision and mountain formation Most metamorphic rocks form in regional tectonic settings thermal eat from amagmatic intrusion (high T, low P) h - canalso occur during mountain building High temperaturesimmediately next to pluton Heat moves through the rock and decreases with distance from intrusion Coarser grainedmetamorphic mineralscloserto contact(hornfels) dynamic in afault or shear zone,large range of PT conditions burial in asedimentary basin,due topressure of overlyingsediments hydrothermal rocksinteracting with hot fluids shock extremelyhigh pressure, extreme conditions duringimpacts What is orogenesis? The process of mountain building Continental plates collide Crustal thickening increases P quickly, but T rises slowly since rocks are slow heat conductors Regional Metamorphism: Affects large areas during mountain formation. ○ Increased pressure and temperature cause rocks to metamorphose What is subduction zone metamorphism? High P low T Cold oceanic crust is brought to great depths by subduction Metamorphism leads to a density increase, which helps sustain plate sinking “slab pull” force Makes blue minerals! (Nabearing amphibole called glaucophane) Blueschist oceanic crust is forced deep into the Earth's mantle at a fast rate. What is exhumation? Metamorphic rocks come back to surface via exhumation (= uplift + erosion) Rocks heat up and lose water (dry out) during burial and cool down during exhumation Metamorphic reactions are faster when temperature is high and when fluid is involved, so cooling makes it hard to adjust to new conditions.Catalyze retrograde metamorphism: add water, add deformation! How does tectonic setting control pressure-temperature evolution? Subduction ○ HP ○ Blueschist - rock name implies basalt protolith Mountain building ○ MP/M-HT ○ Schists and gneisses may eventually melt Intrusions igneous, magma ○ LP/HT ○ Hornfels What are metamorphic facies + key points? M etamorphic facies: Groups of minerals stable underthe same P-T conditions. Facies namescome from the metamorphosed basalt name,likegreenschistfor basalt metamorphosed in low to moderate temperatures and pressures. Different protoliths (e.g., mudstone vs. basalt) subjected to the same conditions will form different mineral assemblages, even though they belong to the same metamorphic facies. his explains why different rocks can form under the same metamorphic conditions but have T different minerals based on their original composition. hy is it important (from a metamorphic perspective) to understand the depositional W environment of a sedimentary rock, or the formation environment of an igneous rock, before it gets buried and heated? he protolith’s composition affects the type of metamorphic rock that forms. Understanding T its original environment (e.g., sedimentary or igneous) helps predict the minerals and texture that will develop under specific metamorphic conditions. nderstanding the depositional or formation environment is crucial because it determines U theprotolith's composition, which influences howthe rock will react to heat and pressure uring metamorphism. Different starting minerals in sedimentary or igneous rocks will lead to d distinct metamorphic outcomes (e.g., basalt becoming blueschist, limestone becoming marble). Additionally, the original environment affectsmineral stabilityand the types of metamorphic reactions that occur under varying pressure-temperature conditions. ompare the metamorphism that takes place next to a pluton with that in a C subduction zone. What are the differences? What is the result of this on the metamorphic rocks that form? Contact Metamorphism (near pluton): ○ High temperature, low pressure. ○ Produces non-foliated rocks like hornfels near the heat source from a magma intrusion. (fine-grained) Subduction Zone Metamorphism: ○ High pressure, moderate to low temperature. ○ Produces foliated rocks like blueschist, with distinctive blue minerals (glaucophane) due to the cold, high-pressure environment. Key Difference: Contact metamorphism forms non-foliated rocks, while subduction zone metamorphism forms highly foliated rocks due to pressure differences. ow is it possible that we find high-grade metamorphic rocks on the Earth's surface? H Why were they not transformed by retrograde metamorphism? E xhumation: High-grade rocks are brought to the surface by uplift and erosion during tectonic processes. No Retrograde Metamorphism: Rocks remain unchanged because lack of fluids and cooling prevent reverse reactions. Additionally, the minerals formed at high temperature remain stable as the rocks cool. Mountain building and deformation What is a mountain range? Series of mountains and hills arranged in a line and connected by high ground Built by ‘constructive’ processes, e.g. continent-continent collision Orogenesis ❖ Formed by tectonic activity: crustal shortening and thickening ❖ Convergent plate boundaries, constructive processes ❖ Torn down by destructive processes A mountain can only become so big, until it collapses under its own weight ❖ Dynamic processes combined Deformation Metamorphism ountain building involves faulting, folding, jointing, melting, metamorphism, glaciation, M erosion, sedimentation… 1. Collision between two continents a. Crumpling and thickening of the continental crust creates mountain ranges and “fold and thrust belts” b. Collision between a big continent and a crustal fragment i. Accretion of pieces:Crustal fragments taken withoceanic crust, but they have a lower density → does not want to subduct → smashes into edge of continent 1. Crustal fragment = pieces of continent, thickened oceanic crust: hotspot volcanoes/volcanic arcs 2. Subduction zone a. Compression (shortening) of the overlying plate leads to mountain building and volcanic activity. b. Regions behind the Vulcanic arc may experience stretching. Suture =structure marking where an old subductionzone used to be What is deformation and what does it cause? Orogenesis applies force to rocks, causing deformation ○ Bending, breaking, shortening, stretching, shearing Changes character of the rocks: NOT deformed = not strained ○ Horizontal beds, spherical sand grains, no folds, no faults IS deformed = is strained ○ Tilted beds, flattened or elongate grains, folds, faults Deformation takes place when rocks are under stress Stress is force applied per unit area hat are the 3 types of stress? Strain: deformation due to stress. W Compression (pushing) Tension (pulling) Shear What are the 3 different ways rocks can be deformed due to tectonic forces? What is brittle and ductile deformation? Brittle: Shallow crust, Pressure and temperature relatively low, Rocks break, rather than bend Ductile: Higher temperature and/or pressure in the deep crust (>10-15 km), Folding and continuous shape change without breaking Brittle and ductile Depends on: ○ Pressure and temperature ○ Strain rate ○ Rock type (soft clay vs. hard granite) ○ Presence of Fluid What are folds and their categories? Folds: ductile deformation structures ○ Two flanks or limb, Hinge (line marking where direction changes), Fold axis, Axial Plane ○ Anticline (A) and syncline A xial plane orientation: Folds can have vertical or tilted axial planes, influencing their symmetry and the orientation of their limbs. Plunging structures:Folds with tilted fold hinge What are faults? Brittle deformation structures Surfaces along which rock layers have moved What are the 3 different types of fault orientations? 1. Normal faults a. Extensional b. Tensional stress → Crustal stretching. c. Hanging wall moves down 2. Thrust faults a. Compressional stress → crustal shortening b. Creates overlap c. Hanging wall moves up 3. Strike-slip fault a. Both blocks move parallel to the trace of fault b. Usually oriented very steep or nearly vertical c. Sinistral: opposite block moves left Dextral: opposite block moves right 1. How does a mountain belt form? M ountain belts form throughorogenesis, which is drivenbytectonic activitysuch as thecollision of tectonic plates. This leads totheshortening and thickening of the Earth's crust. Convergent plate boundariesplay a key role, wheretwo continental plates or a continental plate and a crustal fragment collide, causingfolding, faulting, and crustal thickening. These processes createmountain rangesandfold andthrust beltsas the crust crumples and thickens. Additionally,subduction zoneslead tocompressionand volcanic activity, contributing to mountain building. 1: Subduction and consumption of an ocean basin 2: Collision of fragments 3: Ocean is completely consumed, leading to continent-continent collision . What determines whether a rock will deform in a brittle or ductile way 2 when subjected to stress? Brittle vs. Ductile deformationdepends on severalfactors: ○ Pressure and Temperature: Rocks at shallow depths(lower pressure and temperature) tend to behavebrittlely, breaking ratherthan bending. At deeper levels in the crust, where temperatures and pressures are higher, rocks deform in aductilemanner, bending and foldingrather than breaking. ○ Strain Rate: Faster rates of deformation favor brittlebehavior, while slower rates lead to ductile deformation. ○ Rock Type: Some rocks, likegranite, are more likelyto deform brittlely, while softer rocks, likeclay, tend to deform ductilely. ○ Presence of Fluids: The presence of fluids can promoteductile deformation by reducing the strength of the rocks. . Deformation: What types of faults and folds are there, and under what 3 conditions do they form? Faults: Normal Faults: ○ Form undertensional stress, where the crust is beingstretched. ○ Thehanging wallmoves down relative to the footwall. ○ Occur in regions experiencingcrustal extension. Thrust (Reverse) Faults: ○ Form undercompressional stress, where the crust isbeing shortened. ○ Thehanging wallmoves up, creating overlap of crustalblocks. ○ Common inmountain-buildingregions due to compressionalforces. Strike-Slip Faults: ○ Occur undershear stress, where two blocks move horizontallypast each other. ○ These faults are steep or nearly vertical. ○ Movement can besinistral(left-lateral) ordextral(right-lateral). Folds: A nticline: A fold in which rock layers bend upwardsinto an arch. These structures form fromcompressional stressand are typically foundin mountain ranges. Syncline: A fold in which rock layers bend downwardsinto a trough. These also form fromcompressional stress. Symmetrical vs. Asymmetrical Folds: ○ Folds can besymmetrical, with equal limbs, orasymmetrical,with one limb steeper than the other. ○ If the fold becomes extreme, one limb may beoverturned,meaning it is tilted beyond vertical. 4. What does the map image of a syncline or anticline look like? A nticlinesappear as a series of parallel lines ona map, with theoldest rock layers in the center and progressively younger layers outward. Synclinesalso show parallel lines, but with theyoungestrock layersin the center and older layers outward. When the foldsplunge, the pattern looks like a seriesof v-shapes, with anticlines plunging towards their noses (closed end) and synclines plunging away from their noses.
