Two periods of volcanic activity produced the major features of the island. The first about 6.9 million years (Mya) ago produced the northern and central hills, while the younger and highly eroded Mount Gower and Mount Lidgbird were produced about 6.3 Mya by successive basalt (an extrusive igneous rock) lava flows that once filled a large volcanic caldera (crater) and can now be seen as horizontal basalt strata on mountain cliffs (at Malabar and Mount Gower) occasionally interspersed with dikes (vertical lava intrusions). Geological pyroclastic remnants of volcanic eruption can be seen on the Roach Island (where the oldest rocks occur) and Boat Harbour as tuff (ash), breccia (with angular blocks), and agglomerate (rounded 'bombs'). Offshore on the Lord Howe Rise, water depths reach falling to to the west of the rise. From the dimensions of the rock on which the island stands, the island has been calculated to erode to 1/40th of its original size.