above the glassy margin contains vesicles trapped as the rock was rapidly crystallizing. These have a distinctive appearance likened to a clay tobacco pipe stem, particularly as the vesicle is usually subsequently filled with calcite or other light-colored minerals that contrast with the surrounding dark basalt. Petrology At still smaller scales, the texture of flood basalts is aphanitic, consisting of tiny interlocking crystals. These interlocking crystals give trap rock its tremendous toughness and durability. Crystals of plagioclase are embedded in or wrapped around crystals of pyroxene and are randomly oriented. This indicates rapid emplacement so that the lava is no longer flowing rapidly when it begins to crystallize. Flood basalts are almost devoid of large phenocrysts, larger crystals present in the lava prior to its being erupted to the surface, which are often present in other extrusive igneous rocks. Phenocrysts are more abundant in the dikes that fed lava to the surface. Flood basalts are most often quartz tholeiites. Olivine tholeiite (the characteristic rock of mid-ocean ridges) occurs less commonly, and there are rare cases of alkali basalts. Regardless of composition, the flows are very homogeneous and rarely contain xenoliths, fragments of the surrounding rock (country rock) that have been entrained in the lava. Because the lavas are low in dissolved gases, pyroclastic rock is extremely rare. Except where the flows entered lakes and became pillow lava, the flows are massive (featureless). Occasionally, flood basalts are associated with very small volumes of dacite or rhyolite (much more silica-rich volcanic rock), which forms late in the development of a large igneous province and marks a shift to more centralized volcanism. Geochemistry Flood basalts show a considerable degree of chemical uniformity across geologic time, being mostly iron-rich tholeiitic basalts. Their major element chemistry is similar to mid-ocean ridge basalts (MORBs), while their trace element chemistry, particularly of the rare earth elements, resembles that of ocean island basalt. They typically have a silica content of around 52%. The magnesium number (the mol% of magnesium out of the total iron and magnesium content) is around 55, versus 60 for a typical MORB. The rare earth elements show abundance patterns suggesting that the original (primitive) magma formed from rock of the Earth's mantle that was nearly undepleted; that is, it was mantle rock rich in garnet and from which little magma had previously been extracted. The chemistry of plagioclase and olivine in flood basalts suggests