Ascension Island (7° 57’S 14° 22’W) lies some 1,300 km north-west of St Helena and 1,054 km from the coast of Liberia. It is slightly smaller than its "neighbour", covering an area of 97km2. As befits its position closer to the Equator, Ascension has a warmer climate than St Helena with relatively little seasonal variation in temperature, usually between 25o and 32oC at sea-level. It is also considerably drier. The highest point, Green Mountain (859m) receives 600-700 mm rain annually at the summit but this declines rapidly to only some 150mm at sea level. Despite this, Ascension is occasionally subject to torrential downpours that can cause widespread flooding and have a dramatic effect on the vegetation.
Like St Helena, Ascension had a volcanic origin but is a much younger island. A total of 44 craters are scattered around the island. The oldest known surface rocks, from around Green Mountain, are only 1.5 million years old. The lava flows and ash cones that dominate the western two thirds of the island are younger still and it is suspected that there has been minor volcanic activity in the Sisters Peak area as recently as 500-700 years ago.
The western half of the island is fringed by several extensive sandy beaches, notably at Clarence Bay, South-West Bay and English Bay. The coastline of the eastern end of Ascension is, in contrast, dominated by cliffs, the highest being between South-East Point and Hummock Point. There are a number of offshore stacks, the largest of which is Boatswainbird Island with an area of 5 ha.
Below 350m the landscape consists of extensive plains covered by dark, relatively recent lava flows, many contorted into weird and dramatic shapes, and low cones of volcanic ash and cinders. This area is normally hot and very dry and, consequently, has a very sparse covering of vegetation. Some lava flows are completely barren. After rare cloudbursts, however, a lush growth of grasses and short-lived herbs can spring up in a remarkably short time.
Green Mountain is the only part of Ascension that supports substantial plant cover all year round. The vegetation is however almost entirely non-native, consisting largely of introduced shrubs and tall herbs but also including most of the island’s mature trees, including eucalyptus and figs. A farm was established below the summit during the 19th Century to provide fresh produce for the garrison and a small cultivated area still remains around the original farm buildings. The summit itself supports a stand of bamboo surrounding a dew-pond.