---
source: https://iinventedlight.com/library/notes/coral-bleaching.md
title: "Coral bleaching and how coral reefs form"
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 coral bleaching?, US government work, public domain; NOAA, Corals Tutorial: How Do Coral Reefs Form?, 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: marine life and currents"
tags: ['noaa', 'corals', 'reefs', 'ocean']
sources:
  - id: S1
    title: "What is coral bleaching?"
    publisher: NOAA
    url: https://oceanservice.noaa.gov/facts/coral_bleach.html
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "Corals Tutorial: How Do Coral Reefs Form?"
    publisher: NOAA
    url: https://oceanservice.noaa.gov/education/tutorial_corals/coral04_reefs.html
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# Coral bleaching and how coral reefs form

**Answer:** Coral bleaching happens when corals are stressed by changes in conditions such as temperature, light, or nutrients. They expel the symbiotic algae living in their tissues, which turns them completely white.

## Can coral recover?

If the stress-caused bleaching is not severe, coral have been known to recover. If the algae loss is prolonged and the stress continues, coral eventually dies.

## What builds a reef

Coral reefs are created by hard, or stony, coral. Soft corals don't form reefs, but are often found on reefs, adding essential structural complexity to the reef ecosystem.

## Why reefs take so long

- Massive corals grow 0.3 to 2 centimeters per year, and branching corals up to 10 centimeters per year.
- At those rates it can take up to 10,000 years for a coral reef to form.
- Depending on their size, barrier reefs and atolls can take from 100,000 to 30 million years to fully form.

Barrier reefs and atolls are some of the most beautiful and biologically diverse habitats in the ocean, and also some of the oldest.

## Claims
- id: C1
  claim: Stressed corals expel their symbiotic algae and turn white.
  source: S1
  evidence: "they expel the symbiotic algae living in their tissues, causing them to turn completely white."
- id: C2
  claim: Coral can recover if bleaching is not severe.
  source: S1
  evidence: "If the stress-caused bleaching is not severe, coral have been known to recover."
- id: C3
  claim: Coral reefs are created by hard, or stony, coral.
  source: S2
  evidence: "Coral reefs are created by hard, or stony, coral."

## Sources
- [S1] What is coral bleaching?. NOAA. https://oceanservice.noaa.gov/facts/coral_bleach.html . License: US government work, public domain. Accessed 2026-11-04.
- [S2] Corals Tutorial: How Do Coral Reefs Form?. NOAA. https://oceanservice.noaa.gov/education/tutorial_corals/coral04_reefs.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).