EVSC20004 Lecture Notes - Lecture 18: Oceanic Basin, Oceanic Crust, Continental Crust
21 Jun 2018
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Ocean Basins
Oceans are large- covering 70% of the Earth’s surface (80% in the southern hemisphere) and
have average depths of 4000m with over 84% of the ocean bottom deeper than 2000m.
Light does not penetrate below 1000m and anything below this point is where the ocean is
lightless and cold. In contrast, the upper sunlit part of the ocean is generally warm and rich
in life.
Evolution of Ocean Basins
Earth structure
oThere are 2 different types of crust on the earth:
Oceanic crust- thin and predominantly basalt (heavy)
Continental crust- thicker and predominantly granite (lighter). This
allows it to have higher buoyancy than oceanic crust (Like an iceberg
that are thicker but lighter)
Formation of liquid water
oAbout 50% of the earth’s water existed very early on in the formation of the
earth- hydrogen and oxygen atoms coming together
oEnvironment was initially very hot- water molecules were present as vapour
oAs earth began to cool, vapour condensed to precipitation and began forming
water- contributing to formation of life on earth
oOther 50% is due to collisions of ice comets crashing onto the earth
Origins of salt
o97% of the Earth’s water is salty (marine)
oBy weight, seawater is 96.5% water, 3.5% dissolved substances (salt)
oSchematic of water cycle:
As water evaporates and is condensed onto the land can dissolve
some of the land/soil it lands on (erodes it). Flowing water weathers
crustal rocks, transporting sediments and minerals (salts) into the
ocean: Terrestrial weathering via the water cycle (excess volatiles)
Water evaporates from ocean, leaving salts behind
However, is crustal rock is the source of salt in our oceans, then salts
in ocean should be similar to concentrated river water- but they
aren’t
Chemical composition of seawater (chloride and sodium ions) and
river-water (Bicarbonate and calcium ions)
Sat in inland lakes differ from salts in ocean
Therefore, weathering and erosion is not the only source of salt
oUpper mantle of the earth (layer beneath the crust) contains many
substances found in seawater
Tectonic movement allows volatiles to escape into water (outgassing)
via underwater volcanoes (Chemical substance in the mantle are then
released into the water column)
oSalt comes from weathering AND outgassing: Why don’t the oceans become
progressively saltier?
Glacial melt- melting of freshwater ice
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Oceans are large- covering 70% of the earth"s surface (80% in the southern hemisphere) and have average depths of 4000m with over 84% of the ocean bottom deeper than 2000m. Light does not penetrate below 1000m and anything below this point is where the ocean is lightless and cold. In contrast, the upper sunlit part of the ocean is generally warm and rich in life. Earth structure: there are 2 different types of crust on the earth: Oceanic crust- thin and predominantly basalt (heavy) This allows it to have higher buoyancy than oceanic crust (like an iceberg that are thicker but lighter) Origins of salt: 97% of the earth"s water is salty (marine, by weight, seawater is 96. 5% water, 3. 5% dissolved substances (salt, schematic of water cycle: As water evaporates and is condensed onto the land can dissolve some of the land/soil it lands on (erodes it).
