Drain the sea
Globereset
Bench:
wet glass
two fingers pan
·
Window follows the probe:
last 250 million years
around the age
Zoom gain:
wheel0.10
pinch0.65
Below is the whole Earth. Most of it is sea, and the sea hides a landscape nobody had seen until about seventy years ago, when ships began to measure its depth with sound. Drag the water line down the gauge at the map’s edge and the sea goes down. The shallow shelves round the continents dry first. Then a mountain range comes up out of the Atlantic, longer than any on land, running down the middle of the ocean like a seam.
The floor is young. Nothing on it is much older than 180 million years, while the oldest rocks on land are twenty times that. New floor is made along the seam and moves away on both sides, a few centimetres a year, about as fast as your fingernails grow. Turn on Age and the floor is coloured by when it was made: pale at the seam, darker the farther it has travelled.
The Earth’s magnetic field flips every so often: north becomes south, and a compass would point the other way. Floor that cools while the field is one way keeps that direction in its rock. Turn on Stripes and you can see it, in bands either side of the seam, mirrored. Tap the sea anywhere and a probe tells you how deep the floor is, how old it is, and which way a compass pointed when it was made.
A bench, not a story. The water line is a what-if past 130 metres: no sea was ever that low. The floor keeps its shape as the water leaves; the land does not rise.
Sources. The ground and the floor: ETOPO 2022 (NOAA NCEI), public domain. The floor’s age: Seton, Müller, Zahirovic, Williams, Wright, Cannon, Whittaker, Matthews and McGirr, 2020, A global dataset of present-day oceanic crustal age and seafloor spreading parameters, Geochemistry, Geophysics, Geosystems 21 (EarthByte; the paper is open access, the grid folder states no licence, cited as its authors ask). The stripes: Meyer, Saltus and Chulliat, 2017, EMAG2v3 (NOAA NCEI, doi:10.7289/V5H70CVX), a US government work. Which way the field pointed: the Geomagnetic Polarity Time Scale of GTS2020 (Ogg, 2020), as tabulated in PmagPy, to 121 million years; the older chrons are not in the table yet. The water taken out: worked out from ETOPO 2022 (tools/ocean-volume.py); the whole ocean comes to 1.34 billion cubic kilometres, within a tenth of a percent of the published figure. The sea through the ice ages: Spratt and Lisiecki, 2016; Bintanja and van de Wal, 2008; Lambeck and others, 2014. Continents: Merdith and others, 2021 (EarthByte). The floor’s ages are on the 2012 timescale and the reversals on the 2020 one, so near a boundary the needle can be a chron out.