Most of these mineral deposits were created from magmatic hydrothermal activity. These processes occur primarily at magmatic arcs associated with subduction. Tremor is commonly associated with the underground movement of magmatic or hydrothermal fluids. Dikes can be either magmatic or sedimentary in origin. The second type is a complex of felsic magmatic rocks. Magmatic eruptions produce juvenile clasts during explosive decompression from gas release. All three main groups of rocks, magmatic, sedimentary and metamorphic, are found in the country. Vesicles are formed by the expansion of magmatic gas before the magma has solidified. They differ from exclusively magmatic eruptions and phreatic eruptions. Large crystals of quartz are found in magmatic pegmatites. During this period, four episodes of magmatic eruptive activity have been recently recognized. Ore deposits of chromite form as early magmatic differentiates. This is the last episode of undoubted magmatic activity preserved in the geologic record. The eruption was largely magmatic with lava fountains visible. Geologists can now identify characteristic patterns in seismic waves that indicate particular magmatic activity.