---
source: https://iinventedlight.com/library/notes/seamounts.md
title: "Seamounts: underwater mountains made by volcanoes"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: NOAA, What is a seamount? - NOAA Ocean Exploration, US government work, public domain; NOAA, Do volcanic eruptions happen underwater? - NOAA Ocean Exploration, US government work, public domain."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "ocean science: seafloor"
tags: ['noaa', 'seamounts', 'ocean', 'volcanoes']
sources:
  - id: S1
    title: "What is a seamount? - NOAA Ocean Exploration"
    publisher: NOAA
    url: https://oceanexplorer.noaa.gov/facts/seamounts.html
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "Do volcanic eruptions happen underwater? - NOAA Ocean Exploration"
    publisher: NOAA
    url: https://oceanexplorer.noaa.gov/facts/volcanoes.html
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# Seamounts: underwater mountains made by volcanoes

**Answer:** A seamount is an underwater mountain with steep sides rising from the seafloor. Most seamounts are remnants of extinct volcanoes, and to count as a seamount a feature must rise at least 1,000 meters (3,300 feet) above the surrounding seafloor.

## Shapes

Seamounts are typically cone shaped, but often have other prominent features such as craters and linear ridges. Some, called guyots, have large, flat summits.

## How many

Seamounts are found in every world ocean basin. Based on satellite altimetry and bathymetric mapping from survey ships, the number of seamounts at least 1,000 meters high is thought to be greater than 100,000. Less than one-tenth of a percent of the seamounts in the world have been explored.

## The volcanoes behind them

Underwater volcanic activity is a constant process that shapes the features of the ocean.

- Submarine eruptions occur along all seafloor spreading centers. Spreading center eruptions typically produce basalt, the principal rock that makes up oceanic crust.
- Submarine basalt flows often have a distinctive "pillow" shape.
- Hotspot volcanoes form where a magma plume rises through the crust over an area of melting in the mantle. They often form chains of volcanic islands and seamounts that are older with increased distance from the rising magma plume.

## Example

A ~4,200-meter (~13,800-foot) high seamount dubbed "Kahalewai" was mapped during a NOAA expedition in the Central Pacific Basin and found to be almost 1,000 meters taller than previously thought.

## Claims
- id: C1
  claim: A seamount is an underwater mountain with steep sides.
  source: S1
  evidence: "A seamount is an underwater mountain with steep sides rising from the seafloor."
- id: C2
  claim: Most seamounts are remnants of extinct volcanoes.
  source: S1
  evidence: "Most seamounts are remnants of extinct volcanoes."
- id: C3
  claim: Underwater volcanic activity constantly shapes the ocean's features.
  source: S2
  evidence: "Underwater volcanic activity is a constant process that shapes the features of the ocean."

## Sources
- [S1] What is a seamount? - NOAA Ocean Exploration. NOAA. https://oceanexplorer.noaa.gov/facts/seamounts.html . License: US government work, public domain. Accessed 2026-11-04.
- [S2] Do volcanic eruptions happen underwater? - NOAA Ocean Exploration. NOAA. https://oceanexplorer.noaa.gov/facts/volcanoes.html . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).