administrative subdivision is Tupua'i. Geology The Austral Islands form a chain of seven islands extending from southeast to northwest. They are the product of a hot spot beneath the still active Macdonald seamount, which rises 338 kilometers east-southeast of Bass Rocks (Marotori) to about 40 meters below sea level. The geologically oldest, more eroded and fractured islands are in the northwest, the younger ones in the southeast of the chain. The geological age of the basaltic rocks ranges from 28.6 million years on Rimatara in the northwest to 3.3 million years on Marotiri in the southeast. Marotiri, the youngest island, is merely a group of rugged volcanic rocks with no protective coral reef. Maria, on the other hand, the oldest, is an atoll whose volcanic central island sank into the sea long ago, leaving only four motus. The remaining islands are classic atolls with a central island, a coral reef and a fringe of coral islands in the various stages of development. Climate The northern Tupua'i islands have a tropical rainforest climate (Af according to the Köppen climate classification, Arab according to the Trewartha climate classification), but relatively cool compared to the Polynesian islands further north, as the Tupua'i islands are around the Tropic of Capricorn. Summers are hot and muggy, with an abundance of precipitation caused by thunderstorms from easterly winds. In addition, the islands can be affected by cyclones in the austral summer (between November and April). Winters are warm and humid, with easterly winds still creating high precipitation amounts. Far from any landmasses, the Tupua'i islands have a low absolute temperature range – a record high of 32.7 ºC (90.9 ºF) in March and a record low of 9.2 ºC (48.6 ºF) in August. The southern Bass Islands have a have a tropical rainforest climate (Af in the Köppen climate classification, Arab in the Trewartha climate classification), bordering on a very-mild winter humid subtropical climate (Cfa in the Köppen climate classification, Cfal in the Trewartha climate classification). Despite being situated south of the Tropic of Capricorn, the remote location in the middle of a tropical ocean enables the climate to be moderated in all seasons. Summers and winters are slightly cooler than in the Tupua'i Islands, and cyclones are rarer, as the islands are located near the Horse Latitudes at thirty degrees south. Precipitation is abundant in all seasons, and sunshine is uncommon every month of the