Travel deep dive
The Azores: What Sitting on Three Tectonic Plates Actually Produces
The Azores' crater lakes tend to get explained one island at a time — Sete Cidades' twin-coloured lake, a hot spring here, a dormant crater there — but all of it traces back to one underlying fact that rarely gets stated plainly: these nine islands sit directly on the boundary where three tectonic plates meet, making the archipelago one of the most geologically active stretches of the Atlantic. That single position is the mechanism behind the crater lakes, the geothermal hot springs and the still-active volcanic system beneath all of it.
Updated 2026-09-19
A triple plate boundary: why the Azores exist at all
Most of the Atlantic Ocean floor is quiet, geologically speaking, but the Azores sit at a genuine rarity — a triple junction, the point where three of Earth's tectonic plates (the North American, Eurasian and African plates) meet and interact directly. That convergence is what pushed volcanic material up from the ocean floor to form the islands in the first place, and it's also why the region remains active today rather than being a dormant, long-settled landmass like most other mid-ocean islands.
That ongoing activity is the direct cause of everything distinctive about the islands' landscape: the volcanic peaks, the collapsed calderas now filled with water, the geothermal features bubbling at the surface, and the earthquakes the region experiences periodically all trace back to the same triple-plate mechanism rather than being separate, unrelated features of a volcanic island chain.
Sete Cidades: one caldera, two lakes, two colours
São Miguel's Sete Cidades lake sits inside a single collapsed volcanic caldera roughly five kilometres across, formed when a massive eruption emptied the magma chamber beneath a volcano and the unsupported ground above collapsed inward — the same basic mechanism behind calderas worldwide, here filled by centuries of rainwater into the lake seen today. The lake's split into a blue half and a green half, despite sitting in one connected caldera, comes down to how each half's specific algae population and mineral content interact with light rather than any physical separation between them, a detail easy to miss when the two colours look so distinctly separate from the crater rim above.
The caldera rim itself, walkable in sections and offering the classic overlook of both lake colours at once, is effectively standing on the collapsed edge of what was once a active volcanic peak — a genuinely different way to experience the same eruption-and-collapse process most visitors only encounter as a scenic photo opportunity.
Terceira: a crater you can actually walk into
Terceira's Algar do Carvão takes the same volcanic-collapse story in a different direction — rather than a wide caldera filled with a lake, it's a narrower volcanic chimney whose interior is accessible via a staircase leading roughly 100 metres down into the mountain itself, ending in a chamber with its own small pool and striking mineral formations lining the rock walls. Unlike Sete Cidades' broad, lake-filled caldera, Algar do Carvão gives a direct, physical sense of descending into the volcanic plumbing that shaped the island, rather than viewing the aftermath from a rim above.
Both formations come from the same underlying tectonic activity but read completely differently in person — Sete Cidades as a landscape-scale collapsed crater best appreciated from above, Algar do Carvão as an intimate, walkable descent into the mechanics of how a volcano's interior actually looks once emptied out.
Hot springs: the same heat, reaching the surface a different way
The geothermal hot springs scattered across several islands, particularly São Miguel's Furnas area, come from the same underlying volcanic heat source as the crater lakes, just expressed differently — rather than a collapsed chamber filling with rainwater, hot springs form where groundwater seeps down toward that same heat, gets warmed, and rises back to the surface through cracks in the rock, sometimes bringing dissolved minerals that give the water its distinctive colour and smell. Furnas specifically also uses this same underground heat for cozido das Furnas, a traditional stew cooked for hours by burying the pot directly in the geothermally heated volcanic soil rather than over an open flame.
That shared heat source is why crater lakes, hot springs and still-active fumaroles (steam vents) can all sit within a short distance of each other on a single island — they're not separate volcanic phenomena that happen to coexist, but different surface expressions of the identical underlying heat driven by the archipelago's position on the tectonic boundary.