Lord Howe Geology

Spectacular volcanic remnant

Sitting upon the Lord Howe Rise, Lord Howe forms part of a small group of islands that are the eroded remnant of a large shield volcano that erupted from the sea floor nearly seven million years ago.

65 kms long and 24 kms wide the Lord Howe Rise is a volcanic seamount that ascends from the ocean depths some 1800 m. and is the most southern part of a chain of mainly submerged seamounts that extend over 1000 kms. These mark the successive movement of the Australian tectonic plate over a ‘hot spot’ in the earth’s mantle below.

Starting approximately 6.9 million years ago, over a period of 500,000 years, constant eruptions produced a volcanic island estimated to be 40 times Lord Howe Island’s current land mass. Mt. Lidgbird, Mt. Gower and Balls Pyramid are believed to be the remnants of a caldera wall that rose 1200 m. above sea level.

There are four distinct strata. The oldest volcanic rocks are exposed in Lord Howe’s north-eastern tip and the Admiralty Islands and contain tuffs, breccia and basalts. The youngest volcanic rocks of less than 7 million years old are found in the southern mountains of Lidgbird and Gower where lava flows of up 30 m. thick exist. In the central lowlands volcanic breccia and marine-origin calcarenite (cemented coral sands) plus alluvium are found. Of excitement to paleontologists are discoveries of 130,000 year-old fossils, such as the extinct giant terrestrial horned turtle Meiloania platyceps, imbedded in the calcarenite rock.

Lord Howe Island’s present day size is the product of marine erosion, producing its spectacular sheer black basalt cliffs. Balls Pyramid to the south represents the near complete erosion of a volcanic island, with only a slither of basalt rock remaining that rises 562 m. skyward from the ocean surface. It's the world's tallest sea stack!