Geology - The Ocean Floor - Quick Guides

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Study GuideGeology–The Ocean Floor1. ConƟnental MarginsWhat Are ConƟnental Margins?Continental marginsare the areas where continents meet the oceans. Scientists classify them asactiveorpassivebased on whether plate tectonic activity is occurring.1.1 AcƟve ConƟnental MarginsAnactive continental marginis found where tectonic plates interact. Because of this movement,these margins are often linked toearthquakes and volcanoes.Active margins usually have:•Acontinental shelfclose to land•Avery steep continental slope•A deepocean trenchat the base of the slope•Anuneven ocean floor,often with volcanic hillsThese features form because plates are colliding, sliding, or sinking beneath one another.

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Study Guide1.2 Passive ConƟnental MarginsApassive continental marginforms where there isno active plate movement. These margins arequieter and more stable.A passive margin includes:•Awide, shallow continental shelf•Agentle continental slope•Acontinental rise, made of accumulated sediments•Aflat abyssal plainfarther offshoreBecause tectonic activity is absent, these margins tend to be smoother and more predictable inshape.

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Study Guide1.3 ConƟnental ShelvesAcontinental shelfis a shallow, nearly flat area that extends outward from the edge of a continentbeneath the ocean.Key features of continental shelves:•Near the coast, sediments are mostlysand•Farther offshore, sand gradually changes intofine-grained mud•Width varies froma few kilometers to more than 1,000 kilometers•Water depth increases froma few meters to about 200 metersSince shelves lie within the range of sea-level changes during ice ages, their sediments often recordmarine transgressions(sea level rising) andregressions(sea level falling).Continental shelves are made ofsialic crust, which is rich in silicon and aluminum and is part of thecontinental crust. Even though they are important, continental shelves make up onlyabout 8% of theocean floor.1.4 ConƟnental SlopesBeyond the edge of the shelf lies thecontinental slope. This area marks the transition from shallowcoastal waters to the deep ocean.Importantcharacteristics:•Extends down to depths of about15,000 feet•Has an average slope of4–5 degrees•Covers atransition zonebetween continental and oceanic crustThis transition zone can be complex and may includeblock faults and thrust faults, showingevidence of past tectonic movement.

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Study Guide1.5 Submarine CanyonsSubmarine canyonsare deep, steep-sided valleys that cut through continental shelves and slopes.How they form:•Some canyon heads were carved byriverswhen sea levels were lower during thePleistocene ice ages•Others are shaped by ongoingsand movementfrom coastal currentsKey features:•OftenV-shaped•Can be more than3 kilometers deep•Frequently end inabyssal fans, also calleddeep-sea fansAbyssal fans look likealluvial fansfound on land. Strong bottom currents help shape and smooththese fans over time. Very rapid sand movement down steep slopes is called asand fall.1.6 Turbidity CurrentsTurbidity currentsare powerful underwater flows of dense, sediment-filled water.They occur when:

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Study Guide•Sand and mud on the continental slope are loosened byearthquakes or landslides•Sediments mix with water, creating a flow that isdenser than seawaterImportant facts:•These currents can travel at speeds of up to60 km per hour•They move downhill like underwater avalanches•The sediments they leave behind are calledturbiditesAlthough scientists have never directly observed large turbidity currents, strong evidence supportstheir existence. They are believed to be responsible for:•Breakingundersea telephone cables•Creatinggraded sediment layers•Transportingshallow-water fossilsto deep ocean floorsThese clues show that turbidity currents play a major role in shaping the deep ocean environment.2. Ocean Floor SedimentsThe ocean floor is covered bysediments, which are small particles that settle out of water over time.Scientists group sea floor sediments intothree main typesbased on where they come from:•Terrigenous•Pelagic•HydrogenousEach type forms in a different way and is found in different parts of the ocean.2.1 Terrigenous Sediments: From the LandTerrigenous sedimentscome from the continents. They form when rocks on land break downthrough weathering and erosion.Key points:•Carried to the ocean byrivers, glaciers, wind, and gravity

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Study Guide•Mostly deposited on thecontinental shelf,continental rise, andabyssal plain•Made up of sand, silt, and clayOnce these sediments reach the ocean,strong currents, especially along the continental rise, helpreshape and spread them out. This is why terrigenous sediments are most common near continentsand less common far offshore.2.2 Pelagic Sediments: From the Ocean ItselfPelagic sedimentsform far from land and settle slowly through the water column to the deep oceanfloor.They are made of:•Tiny clay particles•Microscopic shells(skeletons) of marine organismsSome pelagic sediments are calledoozesbecause they are thick and soft:•Calcareous oozecomes from organisms with calcium carbonate shells•Siliceous oozecomes from organisms with silica shellsThe clay particles (and sometimes volcanic ash) are often blown from land bywindand fall gentlyonto the ocean’s surface before sinking to the bottom.Pelagic sediments areleast common on mid-ocean ridge crestsbecause active volcanism thereprevents thick sediment layers from building up.2.3 Hydrogenous Sediments: From SeawaterHydrogenous sedimentsform directly fromseawater, not from land or organisms.Important features:•Rich inminerals•Form when dissolved mineralsprecipitateout of seawater•Common examples includemanganese nodules, which grow very slowly on the ocean floor

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Study GuideThese sediments usually form in deep ocean areas where sediment from land and organisms isscarce.3. AcƟve ConƟnental MarginsActive continental marginsare places where tectonic plates are moving and interacting. A well-known example is along much of thePacific coast. Because plates collide, slide pasteach other, orsink beneath one another in these regions, active margins are areas of intense geologic activity.Key Features of AcƟve MarginsActive continental margins are easy to recognize because they are associated with:•Frequent earthquakes•Volcanoes•Mountain beltsUnlike passive margins,active margins do not havea continental rise or a broad abyssal plain.Instead, their structure is much steeper and more dramatic.

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