The Seafloor

Oceanography Fundamentals

Course 1 · Chapter 2 · Basins and the Seafloor

In the 1950s the American geologist Marie Tharp, working with Bruce Heezen at Columbia University, plotted thousands of echo-sounding profiles into the first detailed maps of the Atlantic seafloor (check the dates). Women were not allowed on the research ships of the time, so she worked from the data the men brought back. Her maps showed a continuous mountain chain running down the middle of the ocean, with a deep cleft along its crest. Heezen at first dismissed the idea of a rift as "girl talk", and then came round to it (check the anecdote). That rift turned out to be one of the keys to the theory of plate tectonics. This chapter walks the seafloor from the coast to the deepest trench and explains how it is made.

A Profile from Coast to Deep

Imagine sailing west from the Scottish coast into the Atlantic and lowering a sounding line every few kilometres. The depths would trace a typical ocean-margin profile.

Feature	Typical depth	What it is
Continental shelf	0 to about 130 m	The gently sloping drowned edge of a continent; the average width is around 80 km, but it ranges from almost nothing to hundreds of kilometres (check)
Shelf break	About 130 m on average	Where the slope suddenly steepens
Continental slope	130 m to 2,000 m or more	The steep descent, often cut by submarine canyons
Continental rise	2,000 to 4,000 m	A gentle apron of sediment at the foot of the slope
Abyssal plain	3,000 to 6,000 m	The flat, muddy floor that covers a large part of the ocean bed
Trench	6,000 to 11,000 m	A narrow, very deep groove where one plate sinks beneath another

The North Sea sits entirely on the continental shelf. It averages under about a hundred metres in depth (check), which is why it was dry land in the last ice age and why the shallow, productive water supports so much fishing.

Shelf seas

Shallow, well mixed, rich in nutrients from rivers and the land. Most of the world's fishing happens here.

Abyssal plains

Among the flattest places on Earth, smoothed by a slow rain of fine sediment. Dotted with seamounts and abyssal hills.

Ridges and trenches

The mountain chains and canyons of the ocean. They mark the edges of tectonic plates and are where the crust is made and destroyed.

The Mid-Ocean Ridge

The longest mountain chain on Earth is almost entirely underwater. The mid-ocean ridge system winds through the Atlantic, Indian, Pacific and Southern Oceans for roughly 65,000 kilometres (check). The Mid-Atlantic Ridge, which runs from Iceland to the far south, is the part Tharp mapped. Iceland is a place where the ridge rises above the surface, which is why it has so many volcanoes and why you can walk between two tectonic plates at Thingvellir.

The ridge is a place where the crust is pulling apart. Molten rock rises from the mantle, cools, and becomes new seafloor on either side. This process is called seafloor spreading.

How the idea was proved In the early 1960s Harry Hess proposed that the seafloor spreads out from the ridges and sinks back into the mantle at trenches (check the date). Then magnetic surveys found stripes of rock with alternating magnetic direction running parallel to the ridge, matching the known reversals of Earth's magnetic field. In 1963 Frederick Vine and Drummond Matthews explained the stripes as a tape recording made as new rock cooled on both sides of the ridge (check). Later drilling showed that the rock and sediment get older with distance from the ridge, exactly as the theory required.

How Old Is the Ocean Floor?

Continental rocks include samples billions of years old. The ocean crust is very different: because it is constantly being made at ridges and recycled at trenches, none of it is older than about 180 million years (check; the oldest known patches are in the western Pacific and the Mediterranean region). Spreading speeds vary, from a couple of centimetres a year in the Atlantic (about the speed at which fingernails grow) to well over ten centimetres a year in parts of the Pacific (check).

Property	Continental crust	Oceanic crust
Typical thickness	About 30 to 50 km	About 7 km
Main rock	Granite-like	Basalt
Density	Lower	Higher, so it sits lower and holds the oceans
Oldest known age	Around 4 billion years	About 180 million years

Trenches, Seamounts and Hotspots

Trenches. Where an oceanic plate meets another and sinks, the seafloor bends into a trench. The Pacific has most of them, including the Mariana Trench, the deepest on Earth. They also produce the largest earthquakes and the tsunamis described in Chapter 4.
Seamounts. Underwater mountains, usually extinct volcanoes. There are tens of thousands, and many are hotspots for marine life (check the numbers).
Hotspot chains. A plume of hot rock under a moving plate builds a line of volcanoes, like the Hawaiian Islands. Mauna Kea, measured from its base on the seafloor, is taller than Everest, though only part of it shows above the sea (check).

Connecting the courses The plates, mantle plumes and magnetic stripes mentioned here are explained in detail in the Geology and Plate Tectonics Fundamentals course. This chapter takes the ocean's view of the same story.

How We See the Bottom

Light does not reach far, so the seafloor is mapped with sound and gravity.

Method	How it works	Strength and weakness
Lead line	A weighted rope lowered by hand	Used for centuries; one point at a time
Single-beam echo sounder	A pulse of sound, timed as it returns from the bottom	Common from the 1920s; a single line of depths under the ship
Multibeam sonar	A fan of beams across a wide swath	Detailed maps, but a ship must cover the whole area, slowly
Satellite altimetry	Radar measures sea-surface bumps caused by the gravity of seabed features	Covers everything, but at coarse resolution
Submersibles and robots	Vehicles carrying cameras and sonar close to the bottom	Very sharp detail, small areas

The sums behind an echo sounder are simple: depth equals half the speed of sound in water multiplied by the time the echo takes. Sound travels at roughly 1,500 metres per second in seawater, though the value varies a little with temperature and salinity (check; Chapter 3). So an echo that returns after four seconds means a depth of about 3,000 metres.

Maps are models A coloured seafloor map often blends precise ship surveys with coarse satellite estimates, and the join is not always visible. When you see a vivid image of a trench or ridge, check the source and resolution. Where nobody has sailed with a sonar, the map is an inference.

Exercises

Exercise 1

A research ship's echo sounder records an echo returning after 6.0 seconds. Taking the speed of sound in seawater as 1,500 m/s, calculate the depth, and name the seafloor feature it is most likely to be.

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Exercise 2

The Atlantic is spreading at about 2.5 cm a year. How far apart will two points on opposite sides of the ridge have moved after 10 million years? Compare the answer with the width of the Atlantic.

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Exercise 3

Sketch (or describe in words) the profile of the seafloor from a coast out to a trench, labelling the shelf, slope, rise, abyssal plain and trench, and say which one is the flattest and which the steepest.

→ Solution

Quick Reference

Margin profile: shelf, slope, rise, abyssal plain, with trenches at plate edges
Shelf break: about 130 m on average
Mid-ocean ridge: roughly 65,000 km; new crust is made here by seafloor spreading
Evidence: magnetic stripes, age of rocks growing with distance from the ridge
Age: ocean crust is under about 180 million years old
Mapping: echo sounder (depth = speed x time / 2), multibeam sonar, satellite altimetry
Check: ridge length, ages, spreading rates, shelf width

Coming next Chapter 3 turns from the container to what it holds: seawater, with its salt, temperature, density and layers, and why sound and light behave as they do in it.