CompuServe Messages

Hi, Gerry– In case you have not had time to look it up, molten rock that erupts onto the surface is an extrusive (extruded from the Earth) igneous rock; if it does not make it to the surface, cooling slowly inside the Earth, it forms an intrusive igneous rock. Igneous rocks may be rich in silica (quartz and relatives), giving them a somewhat acidic nature in a technical sense, or may have instead of silica some minerals with more of an alkaline character (carbonates would be found in such rocks). Thus an "alkalic igneous intrusion" is a rock sort of like granite in that it cooled underground and thus has a similar texture, but unlike granite is short on silica, letting a different suite of minerals dominate. –Doug