Jon Wunrow, age 9, of North Greece, New York, for his question:
How is basalt formed?
There are vast slabs of basalt in the mountains of New England and all through the eastern United States. This solid dark rock was formed about 300 million years ago. If we could travel backward through these ages, we would be amazed at what was going on. The mighty Appalachians were growing along the coast. Earthquakes shook the ground and fiery volcanoes erupted time after time. The busy volcanoes formed those massive layers of basalt and the earth used them to build the mighty mountains.
As a rule, rock hounds classify their specimens in three main groups. Water provides the fine fragments to form layers of sedimentary rocks. The restless earth supplies heat and pressure to remodel minerals into metamorphic rocks. Igneous rocks are formed by fiery volcanoes. And basalt is one of the lava type igneous rocks. It happens to be one of the most plentiful minerals in the earth's rocky crust. There is a lot of it out on the surface. But there are immense slabs of basalt under the sea beds and below the surface layers of the land.
The complicated recipe began in hotter than hot pockets deep underground. Assorted minerals, f umy gases and steamy vapor were melted and mixed together. This molten mixture is called magma. The earth made vast, underground pools of magma to build up the stately Appalachians and the tall western mountains. Volcanoes erupted time after time and carried this molten material to the surface. Some flowed in rivers of red hot lava and some spread in flat floods over the ground.
These fiery igneous mixtures soon cooled in the air and formed solid racks. They formed granites, pumice stone and glassy black obsidian. Some hardened in beds of dark brown basalt rock. These special lavas were mixtures of many minerals in fine particles, plus gases and steamy vapor. As the basalt cooled and set, the gases often escaped and the steam boiled away. This left pores and empty pockets in the hardened rock. For this reason, most basalt is lighter than it looks.
Basalt contains a long list of mineral chemicals and not many samples are exactly alike. As a rule, the fine particles of different chemicals are too small to be seen.
In most samples, about half the ingredients are types of feldspar and the others are compounds of iron and magnesium. These basic ingredients contain ;pany different chemicals and some of them combine and add still oth e~r mtnerals. In moist climates, the iron tends to coat the surface of the dark rock with rusty brown. In dry regions, basalt often gathers a crusty coat of limy white.
Through the ages, water may seep through Zayers of basalt and fill its porous pockets with fine layers of agate, crystals of amethyst or ether semi precious minerals. Some basalts gathered pockets full of useful metals such as copper. Other fields of basalt contain a magnetic mineral callad magnetite. When people travel in these regions, the deposits of magnetite basalt tend to upset their compasses.