Mineral Mondays 1: Rock types, for prospectors who love metal detecting and minerals.
Rocks are the most common material on earth and the first thing you learn about in a geology class. Rocks are naturally occurring aggregates of one or more minerals. Rock divisions occur in three major families based on how they formed: igneous, sedimentary, and metamorphic.
Igneous; they form from the cooling of magma deep inside the earth. They often have large crystals (you can see them with the naked eye).
Sedimentary; they are formed through the solidification of sediment. They can be based on organic remains (such as limestone), or just form from the cementing of other rocks.
Metamorphic; they are formed through the change (metamorphosis) of igneous and sedimentary rocks. They can form both underground and at the surface.
The Rock Cycle
The rock cycle is an illustration that is used to explain how the three rock types are related to each other and how earth processes change a rock from one type to another through geologic time. Plate tectonic movement is responsible for the recycling of rock materials and is the driving force of the rock cycle.
The key element for igneous rocks is magma; magma is a mixture of molten or semi-molten rock, as well as some gases and other volatile elements. when magma cools slowly below the Earth's surface Intrusive, or plutonic, igneous rocks forms. Most intrusive rocks have large, well-formed crystals. Examples include granite, gabbro, diorite, and dunite.
However, if the magma erupts or is cooled rapidly, then you get what is called an Extrusive or volcanic rock. Most extrusive (volcanic) rocks have small crystals. Examples include basalt, rhyolite, andesite, and obsidian. Basalt
There are over 700 hundred types of igneous rocks, and they are generally the hardest and heaviest (densest) of all rocks. However, volcanic rocks can be incredibly lightweight – pumice, for example, can even float, and was called by ancient sailors “the foam of the sea”. This happens when a volcano violently erupts, creating pockets of air in the rock.
2. Sedimentary Rocks
Sedimentary rocks are called that because they were once sediment. Sediment is a naturally occurring material that is broken down by processes of weathering and erosion and is subsequently naturally transported (or not). Any rock (igneous, sedimentary, or metamorphic) exposed at the Earth's surface can become a sedimentary rock. The forces of wind, rain, snow, and ice combine to break down or dissolve (weather), and carry away (transport) rocks exposed at the surface. These particles eventually come to rest (deposited) and become a hard rock (lithified). Sedimentary rocks tell us what the Earth's surface was like in the geologic past. They can contain fossils that tell us about the animals and plants or show the climate in an area. Sedimentary rocks are also important because they may contain water for drinking or oil and gas to run our cars and heat our homes.
Conglomerate: Sedimentary Rock
Types of Sedimentary Rocks:
Clastic Sedimentary Rocks
Clastic sedimentary rocks form by weathering processes which break down rocks into pebble, sand, or clay particles by exposure to wind, ice, and water. Clastic and nonclastic sedimentary rocks are the only members of the rock family that contain fossils as well as indicators of the climate (ripple marks, mud cracks and raindrops) that was present when the rock was formed. Clastic sedimentary rocks are named according to the grain size of the sediment particles.
Conglomerate = coarse (64 mm to >256 mm), rounded grains
Breecia= coarse (2mm to 64 mm), angular grains
Sandstone = grains ranging in size from 2mm to 1/16 mm
Shale = grains ranging in size from 1/16 mm to 1/256 mm
2. Nonclastic Sedimentary Rocks
Nonclastic sedimentary rocks form from chemical reactions, chiefly in the ocean. Nonclastic and clastic sedimentary rocks are the only members of the rock family that contain fossils as well as indicators of the climate that was present when the rock was formed. Nonclastic sedimentary rocks are named according to the mineral present.
Limestone = composed of the mineral calcite may contain marine fossils; formed by precipitation from water
Rock salt = composed of the mineral halite (salt); formed by evaporation
Rock gypsum = composed of the mineral gypsum; formed by evaporation
Chert = composed of microscopic mineral grains of quartz; very hard with a sharp edge.
3. Metamorphic Rocks
Any rock (igneous, sedimentary, or metamorphic) can become a metamorphic rock. If rocks are buried deep in the Earth at high temperatures and pressures, they form new minerals and textures all without melting. If melting occurs, magma is formed, starting the rock cycle all over again. The term "metamorphic" means "to change form." Changes in the temperature and pressure conditions cause the minerals in the rock to become unstable so they either reorient themselves into layers (foliation) or recrystallize into larger crystals, all without undergoing melting.
Types of Metamorphism (change) :
contact metamorphism (or thermal metamorphism); the rocks are so close to magma that they start to partially melt and change their properties. You can have recrystallization, fusing between crystals and a lot of chemical reactions. Temperature is the main driver here.
regional metamorphism (or dynamic metamorphism); this typically happens when rocks are deep underground and they are subjected to massive pressure – so much that they often become elongated and the original features are destroyed. Pressure (often times with temperature) is the main driver here.
Metamorphic rocks can be divided into many categories, but they are typically split into:
Foliated Metamorphic Rocks Non Foliated Metamorphic Rocks
Foliated Metamorphic Rocks
Foliated metamorphic rocks are formed within the Earth's interior under extremely high pressures that are unequal, occurring when the pressure is greater in one direction than in the others (directed pressure). This causes the minerals in the original rock to reorient themselves with the long and flat minerals aligning perpendicular to the greatest pressure direction. This reduces the overall pressure on the rock and gives it a striped look. Foliated metamorphic rocks are identified on the basis of their texture:
Slate = formed at very low temperatures and pressures, rock breaks along nearly perfect parallel planes; used in pool tables and as roofing material
Phyllite = low to intermediate temperatures and pressures; slightly more crystallized which gives the rock a shiny appearance; layers may also be wavy or crinkled
Schist = intermediate to high temperatures and pressures; crystals are larger with the grains aligned in parallel to subparallel layers
Gneiss = very high temperatures and pressures; the coarse-grained texture of alternating light and dark mineral bands.
Folded foliation in a metamorphic rock from near Geirangerfjord, Norway. Image via Wiki Commons.
Nonfoliated Metamorphic Rocks
Nonfoliated metamorphic rocks are formed around igneous intrusions where the temperatures are high but the pressures are relatively low and equal in all directions (confining pressure). The original minerals within the rock recrystallize into larger sizes and the atoms become more tightly packed together, increasing the density of the rock. Nonfoliated metamorphic rocks are identified on the basis of their composition:
Quartzite = composed of the mineral quartz; metamorphosed sandstone
Marble = composed of the mineral calcite; metamorphosed limestone.
About the Author
This article is the first in the "Mineral Monday Series". It is written by Exploration Geologist & Geophysicist, Azeem Khel. By profession, Azeem researches and facilitates the processes of mineral extraction. Azeem is a member of Destination Gold's Facebook community, and has agreed to share his knowledge, work and, experience in the "Mineral Mondays Series".
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