Kinds of Rock

There are three basic kinds of rock, each of which comes from an entirely different process: igneous, sedimentary and metamorphic rock.

Igneous rocks

Inside the earth where it is very hot, the minerals there are found in a liquid state called magma. Rocks which have cooled from a liquid state are called igneous. There are two main types of igneous rock: granite and basalt.

Most of the continental crust (bedrock) on the continents around the world consists of granite. Granite is mainly light in colour and coarse grained. Scientists believe that much of this rock was formed under water and the more slowly it solidified, the larger are the crystals which make up the rock. Granite is not forming today.

The other igneous rock type is basalt. This rock tends to be dark grey with very fine texture. Large collections of basalt rock typically came from massive lava flows of the past. Of course, there are also volcanoes active today which have the capacity to produce basalt rock. The massive rock deposits, thousands of feet thick, came from ancient swarms of volcanoes. The basalt rocks in Iceland, the Deccan Plateau of western India and the Columbia River region in Washington State all came about in this way.

More than 90% of volcanic rocks are basalt. One interesting kind of basalt rocks is columnar jointing, vertical hexagonal columns. One especially famous location is the Giant’s Causeway near Antrim, Northern Ireland.  

Sedimentary rocks

Many of these rocks initially came from sediments (gravel, sand, loam, or clay) that were stripped from land surfaces by rushing water and were carried along in these energetic water currents. When the moving water slowed down, the sediments dropped out, settling on the bottom surfaces below. Sedimentary rock can also take the form of broken up parts of organisms already living in the sea (sea shells) or microscopic single cells with hard coverings.

Sedimentary rocks include limestone, sandstone, shale and chert.

Limestone is any sedimentary rock made up mainly of white chalk (calcium carbonate to be exact). This material could come from the broken shells of sea dwelling animals or hard white crust that can form underwater on plants and algae. This is typically quite soft rock but the addition of other sediments may make it more resistant to damage and more suitable for stone coverings for buildings.

Sandstone consists of obvious particles from smashed up rock. In size the particles may look like table salt crystals. The sand grains then become cemented together by other minerals present in addition to the mainly silicon minerals that makes up the sand grains. Sandstone may be quite hard in consistency, hard to penetrate or cut.

Shale comes from the very fine particles typical of clay. Normally we would expect these tiny particles to sink only very slowly once they are suspended in water. However, when the water is disturbed in violent fashion (storms), the fine grains clump together into little pellets which can sink quickly to the bottom. Shale consists of thin flat layers which were used extensively in England and other places for roofing shingles or paving stones.

Another sedimentary rock, chert, is a very hard substance composed of the discarded shells of some algae (diatoms) and/or single celled animals (radiolarians) which live inside beautifully decorated almost pure glass walls. These organisms may develop large populations in the open sea and then later when they die, sink out onto the bottom sediments.

Metamorphic rock

This material originally began as one of the other kinds or rock, but heat and pressure changed it into something that looks different (is harder) and maybe has lost some of the original minerals. The main kinds of metamorphic rock are gneiss, schist and marble.

Gneiss and schist are fairly coarse-grained rocks with bands of lighter and darker consistency. The thickness of the bands may be 1 mm thick or up to several centimetres.  These rocks may have come originally from granite. There is quite a lot of gneiss in the Muskoka region of Ontario.

Marble is metamorphosed limestone and is much harder than limestone itself. Many of the beautiful statues of Renaissance Italy (think MichaelAngelo’s famous statues) were carved out of famous Italian (Carrera) marble.

Of all the types of rock, it is evident that igneous rock has the potential to form quickly (maybe years, but certainly not millions of years). Radiometric dating techniques however have been used to suggest that some of these rocks are billions of years old. These dating techniques however depend on some assumptions which cannot be verified (cannot be established as true). (See radiometric dating discussion under HeadStart for reasons to be critical of radiometric dating that involves such old ages).

Sedimentary rocks, for their part, need to be examined more carefully. They could accumulate quickly or slowly, so we need to consider this issue more carefully. See Record in Sedimentary Rocks.