lecture 1: ocean basins

how scientists map and characterise seafloor features

  • how do we know what the ocean floor looks like?

  • what types of geographic features would you expect to find there?

the ocean floor is mapped by bathymetry

  • bathymetry → the discovery and study of the ocean floor topography

  • ancient methods: early depth measurements used weighted ropes or poles

  • 18th to 19th century: systematic lead line soundings by naval expeditions (e.g. HMS challenger, 1872-76)

  • early 20th century: introduction of echo sounding (sonar) for faster, more accurate depth measurement

  • mid 20th century: development of multibeam sonar systems for wide-area seabed mapping

  • 1970s - 1980s: use of satellite altimetry to infer seafloor topography over large scales

  • ongoing efforts: projects like seabed 2030 aim to map the entire ocean floor by 2030

echo sounding

  • simple depth sounder

    • 2 seconds for a sound pulse to strike the bottom and return to the ship when water depth is 1500 meters

    • basic depth sounders lacked teh resolution and were unable to provide detailed iamges of the ocean floor

multibeam sonar

  • multibeam echo sounders work by bouncing sound off the seafloor, like other echo sounders

  • use up to 121 beams radiating from the ship’s hull

  • fan out at right angles to the direction of travel

  • a ping is sent every 10 seconds toward the seabed, the reflected sound is recorded from narrow corridors

  • provide more accurate data than single-beam systems and are less prone to errors due to more focused coverage

satellites

  • satellites measure small variations in the elevation of surface water

  • because of their precise position, the average height of the ocean surface can be known with great accuracy

  • geosat, TOPEX/Poseidon, Jason-1, and Jason-2 allowed rapid, alrge.scale seaflor mapping

  • they revealed several underwater features and provided crucial data for understanding global seafloor topography

features of earth’s solid surface

  • the ocean makes up 70.8% of the earth’s surface, and the rest 29.2% is land

  • within the 70.8%,

    • 22.1% is oceanic ridges

    • 29.8% is the ocean basin floors

    • 3.7% are volcanic island arcs, trenches, submarine volcanoes, and hills

    • 3.8% is continental rise

  • of the 29.2% of land,

    • 10.3% are mountains

    • 18.9% are continental lowlands

    • 11.4% are continental shelves and slopes

ocean floor topography varies with location

  • earth’s surface is a dynamic mosaic of moving lithospheric plates

  • seabed topography results from plate movement and isostatic balance

  • continental crust → thicker and less dense (granite)

  • oceanic crust → thinner and denser (basalt)

  • isostatic equilibrium explains differences in eelvation between alnd and seabed

    • high density of oceanic lithosphere keeps most of the seafloor >3000 meters deep

  • transition to basalt marks the edge of a continent

  • divides the ocean floor into two main provinces

    • continental margin → the submerged outer edge of a continent

    • ocean basin → the deep sea floor beyond the margin

  • the shallow submerged extension of a continent is called the continental shelf

characteristic features of continental margins and how those features may vary in different locations

Continental margins can be active or passive

  • active continental margin → a geologically active coastline located at a tectonic plate boundary characterised by frequent earthquakes, volcanic activity, mountain building, and a narrow continental shelf that slopes abruptly into a deep ocean trench due to subduction

  • passive continental margin → a geologically stable region marking the transition from thick continental crust to thinner oceanic crust, formed by continental rifting and subsequent seafloor spreading, and not by an active plate boundary

characteristic features of continental margins

  • passive margins →

    • forms where plates are moving apart (diverging boundaries)

    • characterised by broad continental shelves, thick sediments, and gentle slopes

    • little tectonic activity (earthquakes and volcanic)

    • common around the Atlantic ocean - called atlantic-type margins

  • active margins →

    • forms where plates collide (converging) or slide past one another (transform boundaries)

    • narrow shelves, steep slopes, deep ocena trenches often present high tectonic activity (earthquakes and volcanic)

    • common around the pacific ocean - called pacific type margins

features of a passive continental margin (Atlantic type)

  • broad gently sloping shelf extends far form shore

  • extends from shore in a gentle slope

  • 350 km in width and end at a depth of about 140 m, where a steeper drop-off begins

continental slopes connect the continental shelves to the deep-ocean floor

  • the continental slope is the transition between teh continental shelf and the deep ocean floor

  • continental slopes are formed by sediments accumulating at the shelf edge

  • sediments are transported downslope to the ocean floor

continental rises form as sediments accumulate at hte base of the continental slope

  • sediments that form the continental rise are trasnported by turbidity currents

  • earthquakes can trigger underwater landslides or sediment flows. These fast-moving flows are called turbidity currents (27 km/h)

the topography of deep ocean basins differs from the continental margin

  • the seafloor is a blanket of sediment up to 5 km thick covering basaltic rocks

  • deep-ocean basins constitute more than half of earth’s surface

  • the sediments on the deep ocean floor reflect the history of the surrounding continents, the biological productivity, and the ages of the basins

origin, characteristics, and features of oceanic ridges

  • continuous ridge of underwater mountains in the deep ocean (>1500m)

  • the mid ocean ridge is technically the longest mountain range on earth

  • they occur along divergent plate boundaries, where new ocean floor is created as the earth’s tectonic plates spread apart

  • the thickness of the red lines indicates the rate of spreading and the numbers indicate the spreading rates in cm/year

spreading rates controls the topography

  • as the plates separate, molten rock rises to the seafloor, producing enomous volcanic eruptions of basalt

  • spreading speed shapes the ridge:

    • slow spreading → steep, rugged terrain (rift valley); e.g. mid-Atlantic ridge - rate of 2 to 5 cm per year

    • fast spreading → wide gentle slopes; e.g. East Pacific Rise - rate of 6 to 16 cm per year

cross section of mid ocean ridge

  • hydrothermal vents are hot springs on active ocean ridges

major features of ocean basins: abyssal plains and trench

abyssal plains (and abyssal hills) cover most of earth’s surface

  • abyssal plains → they are flat featureless expanses of sediment covering the ocean floor

  • they lie between the continental margins and the oceanic ridges

  • the Canary Abyssal Plain (west of the Canary Islands in the north atlantic) has an area of about 900,000 square kilometers

seamounts and oceanic trenches

  • Seamounts → volcaninc projections that do not rise above the surface of the sea

  • trench → is an arc shaped depression in the deep ocean floor. They occur where a converging oceanic plate is subducted

Mariana Trench

  • Mariana Trench Marine National Monument consists of 246,608 square kilomoeters of submerged lands and waters of the Mariana Archipelago east of the Phillipines

trenches and trash

  • the mariana trench is the deepest spot on our earth. Do you think humans have had an environmental impact on this location? what types of impacts do you think are present?

    • plastic trash has been found in the bottom, nad mercury pollution has been found within the area

  • how would scientists be able to explore and detect if humans have impacted this region?

    • scientists found it by looking through the deep sea debris database

the various zones that the marine environment is genenrally divided into

major features of ocean basins

the marine environment is calssified in distinct areas based on physical features

  • these zones or areas are based on light, temperature, salinty, depth, latitude, water density, or almost any of the other physical dimensions

  • pelagic (open water zones):

    • neritic zone → shallow water over the continental shelf

    • oceanic zone → deep water beyond the shelf (92% of total marine area and 65% of earth’s surface)

      • epipelagic: sunlit surface layer (photic zone)

      • mesopelagic, bathypelagic, abyssopelagic: deeper darker layers

      • abyssopelagic: water in deep ocean trenches

  • benthic (ocean bottom) zones:

    • littoral zone: shoreline area affected by tides

    • sublittoral zone: seafloor near shore and out to the shelf edge

    • bathyal zone: slope regions leading to deep sea

    • abyssal zone: deep ocean floor

    • hadal zone: deepest trenches and trench walls

neritic x oceanic zones: nutrients

  • as you get closer to Antarctica, the concentration of nutrients increases

  • as you get closer to the equator form the south pole, you’ll notice that the nutrients decrease in amount

  • there are high amounts of silicate present in the bottom close to the antarctic