A steep volcanic coast under building cloud, wide

The arc, and what it decides

Three different things wearing one name

The Greater Antilles are pieces of continent. The Lesser Antilles are a subduction zone still at work. The Bahamas are a limestone bank on the wrong plate. Every economy in the basin is arranged around which of the three an island happens to be.

Read: A subduction zone that is still working →

1973 · June to November · the volcanic arc

The machine that built the islands, drawn once and at size

The Atlantic plate bends down beneath the Caribbean plate along the arc's eastern edge. Melt rises through the overriding crust and reaches the surface as an island. Soufrière Hills on Montserrat has been erupting in phases since July 1995; La Soufrière on St Vincent erupted explosively on 9 April 2021.

0 KM50100 KM INNER ARC — ACTIVE VENT, SOUFRIÈRE HILLS, 1995 ONWARD TRADE WINDS, FROM THE EAST OUTER ARC — LOWER, LIMESTONE-CAPPED TRENCH NORTH AMERICAN PLATE, DESCENDING WEST CARIBBEAN SEA CARIBBEAN PLATE RISING MELT
  • Active vent, inner arc — Soufrière Hills, 1995 onward
  • Rising melt, from the descending plate
  • Sea level, seafloor, and the trench at the eastern edge
  • North American plate, descending west beneath the Caribbean plate
  • Trade winds from the east, and orographic rain on the windward flank
  • Outer arc — lower, limestone-capped; depth scale to 100 km

A schematic section across the arc: the descending plate to the right, the melt path in cochineal, the volcanic island at the crest. Vertical scale exaggerated.

Drawn for Carib Pro

The arc is a line you can draw. The rest of the basin is not.

From Anguilla to Grenada the islands sit in a curve, spaced along a plate boundary. North and west of them the geology is older and the outlines are accidents of continental break-up.

Geometry: real coordinates, projected

  • A marked place, and the guide it belongs to
  • Island in the basin
  • Continental mainland, for orientation only

The eight marked places