The rocks

Introduction
We see rocks everywhere. They have been on the earth over billions of years. The rock we see might have once been part of a hill, a mountain or a cave. An animal could have even stampedit! Rocks are an important way to discover earth’s past.
It is known that most rocks are made of minerals but all rocks are not the same. Every rock has its own story.
Rocks can be put into threebasic rock types: igneous, sedimentary, and metamorphic. How a rock is formed determines which group it is placed in. In the next pages you will find useful facts about a rock of each type

Theobsidian is an igneous rock. Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystalgrowth. Obsidian is black and glassy. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a highviscosity and polymerization degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Because of this lack of crystalstructure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile points and blades, and its modern use as surgical scalpel blades.

The sandstone is asedimentary rock. Sandstone is formed from grains of sand pressed tightly together. Sandstones are very common rocks. They are formed from the sand on beaches, in riverbeds, and sand dunes. Like sand,sandstone may be any color, but the most common colors are tan, brown, yellow (like the one in the picture), red, gray, pink, and white. Sandstones are formed from cemented grains that may either befragments of a pre-existing rock or be mono-minerallic crystals. The formation of sandstone involves two principal stages. First, a layer or layers of sand accumulates as the result of sedimentation,...