Tristan da Cunha is a 13-km-wide island volcano lying about 500 km east of the crest of the Mid-Atlantic Ridge just south of the latitudes of Buenos Aires and Cape Town.  The 2060-m-high shield volcano is bounded on most sides by high cliffs.  Lava flows dominate both the low-angle base and the steep upper flanks, although pyroclastic cones ringing the central cone are scattered around the lower flanks.  Eruptions have occurred from the 300-m-wide summit crater, Queen Mary's Peak, which contains a small lake, and from numerous flank vents, some of which occurred from radial fissures.  Radial dike swarms are prominently exposed on all sides of the island.  Numerous strombolian cinder cones occur on the flanks of the volcano along both concentric ring structures and NNW- and ENE-trending radial fissures.  The only historical eruption on Tristan da Cunha occurred during 1961 from a northshore vent and forced the evacuation of the island's only settlement.