It has been concluded through analytical work and modelling, that the felsic rocks on Ascension evolved almost entirely from the high Zr/Nb lavas through simple fractional crystallisation processes 3. These groups are defined as high, intermediate and low Zr/Nb based on trace element analyses of the lavas. Mafic lavas on the island have been sub-divided into three groups based on the ratio of Zr/Nb as these trace elements reflect heterogeneity in the magma source region 2. Where usually we see only the endmember rock types from mafic (basalt) to felsic (rhyolite) on many ocean islands, Ascension’s rocks display the entire range in SiO 2 content, along an alkaline trend from basalt – hawaiite – mugearite – benmoreite – trachyte – to rhyolite 7 (see figure 2).įigure 2: Total Alkali : Silica diagram (TAS) plotting all available Ascension whole rock compositional data (grey envelope) with stars to mark where the sodic rock types plot Adapted from Chamberlain et al., 2016 8 The range in rock composition observed on Ascension is atypical for an ocean island. More recent data from radiogenic isotopes indicate however, that magma production could be related to a portion of anomalously enriched mantle that was initially located at the MAR, initiating magma production and volcanism – only to be carried westward away from the ridge due to plate motion 6.įigure 1: map showing location of Ascension Island (red star, island not to scale), located 90km west of the Mid-Atlantic Ridge.Īscension exhibits multiple rock types that represent its varied volcanic history, with differences not only in composition, but also in eruptive style – from lava flows and domes to thick pumice and scoria deposits. It has been postulated that Ascension overlies a diverted shallow mantle plume similar to other oceanic islands not associated with subduction zone arc magmatism e.g. ![]() ![]() The question of the origins of Ascension Island and its volcanism are as of yet not fully resolved. Ascension’s highest point is Green Mountain, at 859m above sea level, it has been one of the island’s main centres of volcanic activity. At the ocean floor (3000 m deep) Ascension covers 2000 km 2, whereas only 88 km 2 is exposed above the surface of the waves 1 – corresponding to only the last million years or so of activity 2,3. Ascension Island is a volcanic island situated 90km west of the Mid-Atlantic Ridge – (MAR), where its formation began ~ 6-7 Myr ago 1.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |