Philip Heimowitz, age 12, of Allentown, Pennsylvania, for his question:
How do lava rocks get so many colors?
Lava rocks display a multitude of variations in both color and texture. Among 100 samples, no two may be exactly alike. Their colors are due mainly to their assorted minerals. Their smooth or gritty textures are due to dramatic events in their past histories. Some of the colors are modified by these events.
Lavas, of course, are igneous rocks brewed in the seething heat of volcanic fury. Their formation begins five to 10 miles below ground where temperatures are high enough to melt the minerals of the earth's crust to molten magna. When quantities of steamy water are present, the brew may become molten at 800 degrees Centigrade. When moisture is lacking, the buried magma may reach 1300 degrees Centi¬grade. The ingredients in the molten brew may include any of the chemical elements is the earth's crust. Naturally, the most abundant ingredients are the most plentiful crustal elements such as oxygen and silicon, aluminum and iron, calcium and sodium and potassium.
As erupted magma cools, either on the surface or between surface layers of the crust, these and traces of rarer elements unite to form chemical compounds. These minerals add the multitude of colors to the wide assortment of lava rocks. Lava rocks can be divided roughly into two groups, though experts use several systems for subdividing them into smaller groups. A11 the groups are rich in feldspars ¬silicates of aluminum with potassium and sodium, calcium or other compounds.
The rhyolite group is rich in orthoclase feldspar, or potash feldspar. This mineral is pale and adds white, pastel grays or pinks to the lava rock. When horn¬blende or sugite are present, they may pepper the mineral with dark green. Flecks of beryl may add brighter green and specks of zircon may add glints of lustrous brown. Muscovite beclouds the mineral with murky brown and sometimes fine flakes of mica add a sprinkling of star dust. When exposed to moist weathering, iron oxides often streak these minerals with pinks and rusty reds.
The basic or basalt group of laves is heavier and darker in color. Its main ingredient is plagioclase feldspar, an aluminum silicate with both potassium and calcium. This mineral is palish green. However, basalts also contain a ferrous oxide mineral called magnetite, which is black or almost black. When olivine is present, it adds a bottle green tinge to basalt lava. The abundance of iron and magnesium mask the pale feldspars and usually color them dark gray. Most basalt lavas are buried below the surface. Those exposed to dry air often form a whitish crust. When the air is moist, the iron content tinges them with rusty brown.
In some cases, the color of lava rock is modified as it cools. Slow, even cooling produces even sized crystals and the lava is peppered and salted with the assorted colors of the various minerals in the mixture. Frothy white pumice cools before its gaseous bubbles can escape. Black, glassy obsidian cools before its ingredients can separate into assorted crystals.