How to Make a Coral Reef: Sunlight, Water & A Dash of Lava

Written by Sofia Perez

Edited by Sara Gagliardi Managlia

The earth as we know it today is like a soft piece of clay constantly changing shape as we speak. Yes, it is slow, but over millions of years even a gradual change is enough to shape the most intricate structures of nature into something irreplaceable. These slow and indomitable forces have also taken a large part in building up the world’s coral reefs through volcanoes, earthquakes, and the restlessly shifting tectonic plates that make up the earth’s crust. 

For now let’s narrow it down to volcanoes, which have taken on a key role in developing the diversity of several reefs across the globe. To clarify, there are three types of coral reefs: fringing, barrier, and atoll. These are all formed in different ways while sharing certain properties. Among these are the fact that reefs are made up of stony corals,  which are themselves made up of tiny organisms called polyps, which form colonies and bear a resemblance visually to sea anemones. As you might imagine, they are indeed related, as they are both invertebrates in the Cnidaria phylum. Furthermore, these polyps sit in a calyx, or calice, which is a cup that it creates made of a type of limestone called aragonite. The colony of polyps that make up a reef form a blanket of living tissue over this limestone skeleton.  

What about the differences between fringing, barrier, and atoll reefs? Simply put, fringing reefs grow on the fringe of the coast in shallow waters, as in these shallow waters they can receive the most sunlight for photosynthesis. Barrier reefs, one example being the famous Great Barrier Reef, are separated from the land by a lagoon, which acts as a barrier. Finally, atoll reefs, the reefs I’ll be focusing on, are located near the sea surface- to maximize photosynthesis, as you might’ve guessed- and are shaped like a ring. This peculiar shape comes from the fact that these reefs form around underwater islands or inactive volcanoes. 

However the role volcanoes play in the lifetime of an atoll reef don’t stop there. In fact, volcanoes also play a key part in creating the rich biodiversity that is characteristic of reefs today. The 2013 paper Faunal breaks and species composition of Indo-Pacific corals: the role of plate tectonics, environment, and habitat distribution by Sally Keith, Andrew Baird, Terry Hughes, Josh Madin and Sean Connolly discusses this in more depth, revealing for the first time that sudden changes in the composition of coral species is correlated with the movements of Earth’s tectonic plates, particularly earthquakes and volcanoes. This gives us clues as to the processes behind generating the patterns of reef biodiversity seen now and the reason for certain coral species being more widespread than others. 

Keane, B. (n.d.). [Online Image] Pexels. Available at: [Accessed 26 Jul. 2022].

With this in mind, let’s take a moment to reflect on the immense importance of conserving such carefully curated ecosystems. After all, these daunting tectonic forces have been carefully developing coral reefs for millions of years and replacing them is no small feat. That said, how does one actually go about conserving these reefs, whose rich biodiversity has been partly developed on behalf of a volcano?  

To answer this question, let’s take a voyage to French Polynesia’s Tetiaroa Atoll to glimpse upon a mischievous little land mammal with a staggering criminal record. In case you haven’t already guessed it, we’re talking about invasive rats. In fact, globally, invasive rodents are among the most damaging invasive species, particularly on islands, where they are linked to 86% of recorded extinctions. On Tetiaroa, they threaten native seabird populations, Green Sea Turtle hatchlings, crabs, and the seeds and saplings of native plants. Currently, the Tetiaroa Society, Island Conservation, and others are working to remove these pests from the atoll. This will hopefully lead to balance for the terrestrial and marine ecosystems, enhancing the reefs nearby as well.

Another threat to atoll reefs is deforestation, particularly because trees help land retain water and sustain coastal species by supplying the soil with rich nutrients and animals with a habitat. Without this, the soil is easily eroded by the wind and rain. When this washes into the reef, it pollutes the water and smothers reefs while decreasing their exposure to sunlight in a process called sedimentation. In the Palmyra Atoll this has been an issue, alongside invasive rats and copra farming, which is why a major native rainforest restoration project is currently underway. The hope is that once completed, it will increase terrestrial and marine ecosystem resilience to climate change impacts. 

In this sense, the paradigm of volcanoes as a source of destruction is flipped. Perhaps instead, they are a source of abundance, diversity, and mystery. The destruction, on the other hand, is found in the quick unraveling of these million-year-old marvels under the forces of manmade change.  

For more articles by Sofia Perez, go to or follow @greenalsogreen on Instagram.


Coral Reef Alliance. (n.d.). How Reefs Are Made. [online] Available at:

Encyclopedia Britannica. (2020). Coral reef – Origin and development of reefs. [online] Available at: (n.d.). Coral Basics | Flower Garden Banks National Marine Sanctuary. [online] Available at: [Accessed 26 Jul. 2022].

Heber, E. (2021). Restoring Tetiaroa Atoll. [online] Island Conservation. Available at: [Accessed 26 Jul. 2022]. (2019). How Do Coral Reefs Form? [online] ThoughtCo. Available at:

The Nature Conservancy. (n.d.). Palmyra Atoll: Frontline of Climate Change. [online] Available at:

US (2019). How Do Coral Reefs Form – Corals: NOAA’s National Ocean Service Education. [online] Available at: (2013). Coral reefs ‘ruled by earthquakes and volcanoes’ – ARC Centre of Excellence for Coral Reef Studies. [online] Available at: [Accessed 26 Jul. 2022].

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