Types of Rocks ...
Rocks are naturally occurring solid masses or aggregates of minerals, which make up the outer layer of earth, known as the earth’s crust. They are characterized and classified by their chemical composition, way of formation, grain size, texture, and the minerals they contain. There are three major types of rocks: Metamorphic, Igneous, and Sedimentary.
Metamorphic Rocks
Metamorphic rocks are the rocks formed from other rocks. They change substantially from their original form when subjected to high heat, pressure, hot mineral rich fluids or a combination of these.
Properties & Characteristics: Lightweight, glassy appearance; relatively hard, but still easy enough to break; layered or foliated structure made up of crystals of the same size.
Some Examples: Gneiss, phyllite, schist, marble, hornfels and quartzite and slate.
Image Source: BBC Bitesize
Igneous Rocks
Igneous rocks form when magma (molten rock) cools and crystallizes, either at volcanoes on the surface of the Earth or while the melted rock is still inside the crust.
Properties & Characteristics: Dense, hard, and crystalline; rocky streamlined flow structures with no layers; not affected by chemical corrosion.
Examples: Granite, gabbro, diorite, basalt, andesite, obsidian, pumice, rhyolite.
Image Source: BBC Bitesize
Sedimentary Rocks
Sedimentary rocks are formed by the accumulation of sediments, like fragments of older rocks, organic remains (shell, bones), and minerals that eventually compact and harden to rocks over a long period. Sediments accumulate in layers on earth’s surface or at the bottom of lakes, rivers and oceans. As the sediments lose water they cement and harden to create a rock.
Properties & Characteristics: Crumbles easily, can be scratched by fingernails; grainy texture due to layers of sand, silt or gravel; varying sizes of crystals.
Examples: Sandstone, conglomerate, limestone, coal and shale
Image Source: BBC Bitesize
Rock and its colour
Rocks can appear to display many different colours on their surface, depending on the specific mineral components of the rock. As well as this, impurities within those minerals, crystal structure defects, or even environmental factors like oxidation also contribute to the colour of the rock. Some of the most common minerals that undergo oxidisation are; iron (which shows as a reddish brown 'rust' colour), manganese (black or dark brown) and copper (green or red). In this EarthCache we will be observing examples of oxidisation of rocks.
Examples... Left to right: Iron, Manganese, Copper
Iron Oxide
When iron-bearing minerals in rocks are exposed to oxygen and water, they undergo a chemical reaction that transforms iron from its reduced form (Fe⊃2;⁺) to its oxidised form (Fe⊃3;⁺). Haematite and goethite, two iron oxide minerals produced by this process, frequently give rocks and soils a red, brown, or orange surface stain. Iron oxidation is the most common type of oxidation which gives rocks a distinctive 'rusty' look.
Manganese Oxide
Manganese in rocks can oxidise when it reacts with oxygen and water, forming manganese oxides such as pyrolusite (MnO₂) and manganite (MnO(OH)). These reactions often result in a branching pattern across bedding planes or fractures of the rock, and they usually generate coatings that are black or a dark brown colour. Manganese oxidation is less frequent than iron oxidation, however it can be very visually striking.
Copper Oxide
Minerals that include copper oxidise when exposed to oxygen and water, producing copper oxides as tenorite (CuO), which is typically black, and cuprite (Cu₂O), which is usually red/reddish-brown. These chemical changes show how copper minerals develop over time and produce distinctive, colourful weathering patterns on rock surfaces. Copper oxides are commonly localised to small areas of a rock and are typically hard and compact, often with a metallic like sheen to the surface.
TO LOG THIS CACHE
As this is an EarthCache there is no physical box at the location, instead you must visit the marked location and answer the following questions. Please send your answers to me via email or the Geocaching message centre.
1) Observe the rock at GZ... What type of rock do you think this is? Igneous, Sedimentary or Metamorphic?
2)You can observe evidence of mineral oxidisation on the rock at GZ...
a) What colour is the oxidisation?
b) What oxide do you think this is? How do you know?
c) How is this oxide created?
3) If you run your finger across the oxidised area, what (if anything) happens?
4) Photos are always nice to see on your logs, but they are entirely optional. If you do choose to upload photos, please try to refrain from revealing any answers.