What are cays and atolls actually made of?
The key difference is the material at the surface. A volcanic island is a landmass of erupted rock. A coral cay, by contrast, is a small sandy island formed from reef debris—the kind of low coral-sand island studied on the Great Barrier Reef [5]. An atoll is a ring of coral reef around a central lagoon, not a volcanic peak [3].
The Cook Islands geology gives a concrete picture of the range: the Northern Group includes six atolls among seven islands, alongside a volcanic island, a hilly near-atoll, and a sand cay [3]. That mix shows atolls and cays are distinct landforms that can occur in the same region as volcanic islands without being volcanic themselves [3].
Why do volcanoes keep coming up in atoll stories?
Because the foundation often is volcanic. Coral reefs tend to occur in regions of high volcanic activity, and submarine volcanoes are described as the foundation for coral atolls [2]. In that sense a volcano can be the base on which an atoll's coral grows, while the visible atoll is coral-built [2].
American Samoa illustrates the same distinction in practice: surveys there covered five volcanic islands and two remote atolls as separate categories of reef system [1]. The atolls are treated as their own kind of place, not as volcanic islands with coral on top [1].
So is an atoll just a drowned volcano?
Not in the sense the question implies. The evidence supports volcanoes as a common foundation for atolls [2], and it supports atolls and cays as landforms distinct from volcanic islands [3]. But it does not support saying a cay or atoll is merely a volcanic island; the coral and sand construction is what makes them what they are [3][5].
One limit on the evidence: the supplied material does not detail the full step-by-step process by which a volcanic base becomes an atoll, so the relationship is best described as a foundation link rather than a complete identity [2][3]. The hydrodynamic modelling of Maldivian atolls, for instance, treats atolls as reef-and-lagoon systems to be mapped and modelled, not as volcanic islands [4].
Sources used in this answer
Status of coral reefs on the main volcanic islands of American Samoa
Reef surveys in American Samoa covered five volcanic islands and two remote atolls as distinct categories, supporting a comparison between the two island types.
Coral reef annihilation, persistence and recovery at Earth's youngest volcanic island
Coral reefs occur in regions of high volcanic activity, and submarine volcanoes are described as the foundation for coral atolls.
Geology of the Cook Islands
The Cook Islands include a mix of island types—volcanic island, hilly near-atoll, atoll, and sand cay—showing atolls and cays as landforms distinct from volcanic islands.
A digital framework for the hydrodynamic modelling of coral atolls
A high-resolution bathymetry and hydrodynamic modelling framework was developed for the multi-atoll Maldives archipelago, treating atolls as reef-and-lagoon systems.
Community ecology of a coral cay: a study of One-Tree Island, Great Barrier Reef, Australia
A community ecology study examined a coral cay on the continental shelf of the Great Barrier Reef, noting that applying its findings to cays on atolls would require additional work.