Iceland is the only place in the world where a mid-oceanic ridge is exposed above sea level. Because of this, Iceland is an unusual place where we can see many types of basalt, a rock that makes up most of the world's ocean floor. More than ninety percent of all rocks in Iceland are some form of basalt. Basalt is a fine-grained, extrusive igneous rock with the same minerals as its intrusive equivalent, gabbro . Basalt is either aphanitic and black, or grey.
Many basalts contain phenocrysts .
This rock type is called a porphyritic basalt. The earth's crust in these areas is thinned, stretched, and eventually it breaks along long fractures called faults. As the plates pull apart a rift valley begins to form. As the Earth's crust is thinned, pressure is released on the underlying mantle. This release of pressure causes part of the mantle material to melt.