Time To Read
Date Last Modified
Place 04 · The Ridge and the Trench · Chapter 1 of 7
Mountains Nobody Saw
The longest mountain range on Earth is about 65,000 km long, and until the 1950s nobody knew it was one range at all, because it is almost entirely underwater.
About 15 minutes · 1 video · 1 live seafloor map · 2 quick activities
Start here
A ping, a wait, a depth
For most of history, the only way to find out how deep the sea was involved lowering a weighted rope over the side and waiting, sometimes for hours. Then came sonar: send a pulse of sound at the seafloor, time the echo, and do a little arithmetic. Sound travels through seawater at about 1,500 m a second, and the echo has to go down and come back, so depth = speed × time ÷ 2.
Watch this
See it move
SciShow — The Scientist Who Mapped the Seafloor: Marie Tharp. Five minutes on the woman who drew the bottom of the ocean from a desk, because for years she was not allowed on the ships.
While you watch, look for
- What kind of measurements Marie Tharp was turning into maps
- What she spotted along the middle of the Mid-Atlantic Ridge, and why it mattered
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The sound
Shouting at the seafloor
An echo sounder sends a ping of sound straight down and listens for it to bounce back. If the echo takes 4 seconds, the sound traveled 6,000 m in total, so the bottom is 3,000 m down. Ships crossing the Atlantic in the 1950s ran these sounders for days on end, drawing long, wiggly profiles of the bottom.
The mapmaker
The crack in the middle
Marie Tharp, at the observatory that is now Columbia University’s Lamont-Doherty Earth Observatory, turned those profiles into maps. In 1952 she noticed a deep notch running along the crest of the Mid-Atlantic Ridge, a rift valley, and suggested the seafloor was splitting apart there. Her colleague Bruce Heezen first dismissed it as “girl talk,” until the locations of undersea earthquakes lined up neatly along her rift.
The range
A 65,000 km seam
Put all the profiles together and you get the mid-ocean ridge: a chain of undersea volcanic mountains about 65,000 km long, winding around the planet like the seam on a baseball, about 90 percent of it underwater. Along much of it runs a rift valley 25 to 50 km wide. Between the ridges and the continents lie the abyssal plains, 3,000 to 6,000 m down and so flat that they rise or fall only 10 to 100 cm per kilometer.
Photos: Vanessa V., cottonbro studio, gang chen / Pexels
Play with it
See how much of the seafloor is really mapped
This is GMRT, a seafloor map built from ships’ multibeam sonar at Lamont-Doherty, the same place Marie Tharp drew hers. Zoom into the middle of the Atlantic: where ships have surveyed, the ridge is sharp and detailed; where they have not, the map falls back on a much blurrier estimate made from satellites.
Live map: Global Multi-Resolution Topography (GMRT) synthesis, Lamont-Doherty Earth Observatory of Columbia University (CC BY 4.0).
Two questions to answer while you are in there
- Find the Mid-Atlantic Ridge. Where the map is sharp, can you see the rift valley running along its crest?
- Zoom in somewhere in the middle of the Pacific. Do you see sharp stripes of survey data or smooth blur? What does that tell you about how much of the seafloor has actually been mapped?
Try it yourself
Check yourself
Try it
The Mapping Story
Drag seven words into place to follow the seafloor from a rope to sonar to a map.
Try it · interactive
Sonar Mapper
Sail a survey ship across the North Atlantic and ping as you go: each echo time becomes a depth (1,500 m/s × time ÷ 2), and dot by dot the continental shelves, the abyssal plains and the Mid-Atlantic Ridge with its rift valley appear out of the dark.
Open the Sonar Mapper in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Bathymetry
Measuring and mapping the depth of the seafloor
Echo sounding (sonar)
Timing a sound's echo to measure depth
Mid-ocean ridge
Undersea volcanic mountain chain circling the globe
Abyssal plain
Flat, sediment-covered deep seafloor, 3,000–6,000 m down
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: Mountains Nobody Saw
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 1.4 — Mapping the Seafloor — LibreTexts · 8 minutes
- Our World Ocean 6.4 — Seafloor Provinces — LibreTexts · 15 minutes
- Lamont-Doherty Earth Observatory — 8 Surprising Facts About Marie Tharp — Columbia University · 5 minutes
Photo credits (Pexels): Kindel Media, Eren Hızlı, Vanessa V., cottonbro studio, gang chen.