Figure 7. Basalt dikes cut across the gabbro on many parts of the island. These may be related to the latest phase of gabbro emplacement, or may be due to unrelated more recent geological processes. Careful mapping and geochemical analysis may provide answers to these questions. Here, a fine-grained basalt dike in the upper two-thirds of the image cuts across the gabbro with glomeroporphyritic texture. The finer-grained and darker edge of the dike is a "chilled margin" where the warm magma of the dike was cooled rapidly by the cooler, older gabbro, allowing only small crystals to form. Toward the center of the dike, larger crystals could form as the magma cooled more slowly. Note a smaller dike cutting the coarse gabbro in a vertical orientation at the bottom of the image. This smaller dike is also cut by the larger dike, demonstrating several phases of dike emplacement.

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Last updated on May 13, 2010

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