A guyot is an isolated underwater volcanic mountain with a flat top that lies more than 200 metres below the sea surface. It is also called a tablemount, and it forms when a volcanic island is flattened at the waterline and then sinks.
Key takeaways
- A guyot is a flat-topped seamount, with its top more than 200 m below the sea surface.
- The flat top was cut near the surface by waves or reef growth, then the cooling seabed sank.
- Our list names guyots and tablemounts such as Limalok Guyot (1,172 m) and Allison Guyot (3,870 m).
- Harry Hess spotted guyots in 1945 using echo-sounding data from his ship.
How a guyot forms
A guyot begins as a volcano rising from the seafloor. When it grows high enough to near or break the surface, wave action or coral reef growth tends to flatten its top [1]. Later, the ocean crust under the volcano cools, becomes denser and sinks, so the former island slowly subsides [1]. Corals may keep pace with the sinking for a while and form a ring-shaped atoll. Once the corals sink too deep to grow, the feature becomes a guyot [1].
The key evidence is the flat top. Erosion mostly acts in the zone near the surface, so a flat summit that now sits far below the waves shows that the mountain was once at or close to sea level [1]. Wikipedia's definition sets the flat top at more than 200 m below the sea surface, and the flat summit can be more than 10 km across [1].
Harry Hammond Hess first recognised guyots in 1945, from echo-sounding data collected by a ship he commanded during the Second World War [1]. He named them after the geosciences building at Princeton University [1].
All guyots are seamounts, because they are volcanic mountains standing on the seabed. NOAA lists flat summits, which it calls guyots, among the shapes a seamount can take [2].
An example from our data
The depth in our data for a seamount is the depth to its summit, the shallowest cell. Limalok Guyot has its summit 1,172 m down, 61 km from the Marshall Islands. Sylvania Tablemount is 1,296 m, 51 km from the Marshall Islands. Allison Guyot reads 3,870 m and Horizon Guyot 2,037 m. Limalok Guyot lies near the Marshall Islands, a group famous for atolls, which form on the same kind of sinking volcano [3].
Two guyots lie near Antarctica. Belgica Guyot has a summit 374 m down, and Lecointe Guyot is 253 m, so their summits are much shallower than the other guyots here. Our names are not a full list of flat-topped seamounts, because the map's feature names come from Wikidata, so some flat-topped seamounts appear under another name.
The thing people get wrong
A flat-topped mountain far below the surface can look as if the sea has risen over a landscape. In fact the usual cause is the opposite: the mountain was at the surface, was planed flat, and then the crust beneath it sank [1]. The drain slider lowers the sea by 120 m at its Ice Age step, in a simple bathtub model, which is far less than the depth of most guyot summits listed above.
Another mistake is to read the depth on the map as the height of the mountain. The feature page for a seamount or guyot gives the depth to its summit. The height of the mountain above the seabed around it is a different number.
Guyot vs atoll
| Guyot | Atoll | |
|---|---|---|
| Top | Flat, drowned below the waves | Ring of coral at the surface |
| Coral | Corals sank too deep to grow | Corals kept pace with the sinking |
| What it is | A sunk volcano | A sunk volcano wearing a coral rim |
Frequently asked questions
Why are guyots flat on top?
Waves, or growth of a coral reef, flattened the volcano's top while it was at or near the sea surface. The seabed beneath it later sank, carrying the flat top down.
What is the difference between a guyot and a seamount?
A guyot is a type of seamount with a flat summit. Most seamounts are cone-shaped.
Sources
- Guyot, Wikipedia.
- What is a seamount?, NOAA Ocean Service.
- Atoll, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).