http://geology.com/
Misalnya perbedaan antara Batu Apung (Pumice) dan Scoria :
A vesicular igneous rock that is very similar to scoria is pumice. There are a few differences that can be used to distinguish them. First is their color. Scoria is almost always black or dark gray to reddish brown while pumice is almost always white to light gray to light tan. This color difference is a result of their composition. Scoria forms from basaltic magmas while pumice forms from rhyolitic magmas - which usually contain a more gas.
Pumice has a much higher concentration of trapped bubbles - so many that the walls between them are very thin. The vesicles in pumice contain enough air that the rock will float on water. The thick walls of scoria make it heavy enough to sink.
Finally, when observed closely with a hand lens you can often see tiny mineral crystals in scoria. However, close observation of pumice reveals a "glassy" texture similar to obsidian. Pumice consists mainly of glass materials rather than mineral crystals. A "glass" is a noncrystalline substance. In the case of pumice it cooled so quickly that the atoms were unable to arrange themselves into ordered crystal structures.
(http://geology.com/)
