The geology of St. Lucia can be described as composing three main areas. The oldest, 16–18 Ma, volcanic rocks are exposed from Castries northward and consist of eroded basalt and andesite centres. The middle, central highlands, portion of the island consists of dissected andesite centres, 10.4 to 1 Mya, while the lower southwest portion of the island contains recent activity from the Soufriere Volcanic Centre (SVC). This SVC, centred about the Qualibou depression, contains pyroclastic flow deposits, lava flows, domes, block and ash flow deposits, and explosion craters. This depression's perimeter includes the town of Soufriere, Mount Tabac, Mt. Gimie, Morne Bonin, and Gros Piton. At  in diameter, though the western portion is open towards the Grenada basin, the depression formed as recently as 100 kya. The depression is noted for its geothermal activity, especially at Sulphur Springs and Soufrière Estates, a phreatic eruption in 1776, and recent seismic activity (2000–2001).