The Earth beneath the sea

Why do land and ocean exist?

• Bimodal distribution in frequency plot of topography and bathymetry
• Mean evelvation of land=0.84km  vs. Mean depth of ocean=3.8km
• 29% rock is above sea level- continental crust=granitic rocks 2.7g/cm3
• 71% rock is beneath sea-Oceanic rock=basaltic- 3.3g/cm3
• Both float on top of mantle 

Isostasy refers to the balance of an object floating upon a fluid medium. Height of the mass above and below the surface=controlled by the thickness of the mass and its density.

• Greater the density of the mass- more it will sink into the medium
• Continent=less dense so floats higher- higher elevation
• Sea floor=thin (4-10km thick)- does not rise high above the mantle- greater density- deeper depths

Features on the ocean floor: major topographic features. 

• Bounce sound waves down to the seafloor- record the echos
• Biggest mountain ranges and plains are in the ocean
• Manned submersibles; remotely operated vehicles. swath-mapping sonar on ships, seafloor observatory. 
• We've only mapped a tiny amount of the ocean floor in high detail. 
• We have directly sampled (with grabs from ROVs, submarines etc) an area of deep sea the size of greater London. Tiny tiny tiny area compared to global seafloor area. "We know the dark side of the moon better."

Major physiological features: Continental margin…