Rocks

Definition

Rocks are naturally occurring solid aggregation of one or more minerals. They are inorganic substances (although there are some exceptions, such as rocks derived from living beings,  like coal and petrol) not made by humans.

According to this definition, a rock is composed of mineral grains. A rock always has the same mineral composition, but the proportion of these minerals can change. Due to this, rocks are not homogeneous and don't have a fixed chemical composition.

Classification

Rocks can be classified according to their composition or their origin.

According to their composition, rocks can be:

• Mono-mineral rocks: they are only made up of one mineral. Quartzite (made up of quartz) or limestone (made up of calcite) are two examples.
• Multi-mineral rocks: they are made up of two or more different minerals. Granite (made up of quartz, feldspar and muscovite) or basalt (made up of pyroxene and amphibole) are examples of multi-mineral rocks. 

According  to their origin, rocks can be:

• Igneous rocks: rocks formed from solidification of molten rocks (magma or lava).
• Metamorphic rocks: formed from transformations of other rocks due to high pressure or temperature.
• Sedimentary rocks: formed from deposition and compaction of tiny particles of rock called sediments.

Igneous Rocks

Igneous rocks are formed through the cooling and solidification of a magma or lava. Magma and lava are both rocks in liquid or semisolid state, but there are some differences between them. Magma is a mixture of molten rocks beneath the Earth surface. Lava is a mixture of molten rocks expelled by a volcano.

There are two different types of igneous rocks, according to where they are formed: intrusive and extrusive igneous rocks.

Intrusive rocks, also called plutonic rocks, are igneous rocks formed from a magma that solidifies in deep parts of the Earth. Due to the depth, the magma solidifies slowly. As a result, the minerals formed are made up of large crystals. Their mineral grains can usually be identified with the naked eye.

Granite is a typical plutonic rock. 

Granite.

Plutonic rocks are usually formed in some typical structures, such as batholiths, sills, dikes, laccoliths and lopolithes.

• Batholiths are large emplacements of magma enclosed in deep parts of the crust.
• Sills are tubular sheet intrusions, parallel to the ground.
• Dikes are also tubular sheet intrusions, but perpendicular to the ground.
• Laccoliths are mushroom shaped emplacements of magma that are frequently located close to the crust surface.
• Lopoliths are lenticular intrusions, similar to laccoliths but inverted, flat in the upper part and concave in the lower part.

Lopoliths and laccoliths are usually connected by dikes to batholiths. 

Extrusive rocks, also called volcanic rocks, are igneous rocks formed form a magma that solidifies at the Earth surface. The magma cools very quickly. As a result, the minerals formed are made up of small crystals. Due to this, their mineral grains can't be identifies with the naked eye.

Pumice is a typical volcanic rock. 

Pumice.

Metamorphic Rocks

Metamorphic rocks are formed due to the effect of high pressure and temperature. The pressure and temperature lead to chemical and physical changes of the rock, although all these processes must occur always in a solid state, without melting (if the rock changes its state and become liquid, the rock formed would be an igneous rock).

The physical and chemical processes change the mineral composition of the rocks. As a result, some minerals are transformed into others.

These extreme conditions can be found on deep zones of the crust or near a big masses of magma. Gneiss or marble are two examples of metamorphic. 

There are three types of metamorphic processes according to the conditions of formation: contact, dynamic and regional metamorphism.

Contact metamorphism: this process takes place due to the increase in temperature of a rock when it comes into contact with a large mass of magma.

Dynamic metamorphism: this process takes place when the friction in fault zones increases the pressure and temperature, resulting in metamorphism.

Regional metamorphism: this process is related to huge geological phenomena that cover  large areas, such as mountain ranges or convergent tectonic plates. This metamorphism is typical in boundaries between different tectonic plates.

Gneiss.

Sedimentary Rocks

These rocks are formed by accumulation and compaction of fragments of other rocks that have been eroded. These fragments are called sediments. 

The first phase of the sedimentation is carried out by two processes called weathering and erosion.

Several agents break down the rock into small particles. This processes are called weathering and meteorization. The small pieces of rocks can move from one place to another. The weathering added to the movement of the rock fragments is called erosion.

The natural agents that break down the rocks can be classified into three groups: atmospheric agents, water and living beings.

Conglomerate.

The most important atmospheric agents are wind, rain, snow, hail and changes of temperature. The water courses, rivers, streams, glaciers (courses of solid water, made of ice) or the waves of the sea are very relevant erosive agents. Living beings can also erode the rocks. The roots of many plants, for instance, that grow in rock fissures can break the rocks, or many microorganisms that transform the rocks due to chemical reactions.

All these processes that transform the rocks into tiny particles are called fragmentation. These particles can be transported, completing the erosive process.

The transportation of particles can be carried out by different natural agents, such as the wind, streams or rivers. The fragments of rock move to low energetic places where they are built up.

These low energetic places where sediments tend to build up are called sedimentary basins or depositional environment. The process is called deposition.

The sediments are accumulated in parallel layers called strata. The depositional process can occur over thousands or even million years. Upper layers press lower layers, increasing their pressure. This high pressure brings about physical and chemical changes in the sediments of lower zones.

Physical and chemical changes undergone by sediments after their deposition are called diagenesis.

The diagenesis can be divided into three consecutive processes: compaction, cementation and lithification.

The compaction takes place when the weight of upper layers increases the pressure of lower layers, squeezing out the water and compressing the rock fragments. The release of water provokes chemical changes and precipitation of minerals in the pores between the sedimentary grains. The rock fragments bind together. This process is called cementation. All these chemical and physical changes lead to the lithification, that is the transformation of rocks fragments into sedimentary rocks. 

The sedimentary rocks can be classified according to their origin. There are three types of sedimentary rocks: clastic, chemical and biochemical.

Clastic rocks are formed by fragments transported by fluids in movement (water, wind or ice). These are the most typical sedimentary rocks. Conglomerates, sandstones and mudstones are three examples.

Chemical rocks are formed by precipitation of chemical substances dissolved in water. Evaporites, stalactites and stalagmites are three examples.

Biochemical rocks are formed by transformations of different parts of living beings. Limestone, oil and coal are three examples.

Rock Cycle

Rocks are not static structures, they change and evolve. Rocks are transformed into other different rocks. The process is called Rock Cycle. It is a continuous process, but takes place so slowly that human beings can hardly appreciate it.

Any rock can be transformed into an igneous rock if it heats, melts and then cools and solidifies. Any rock can be transformed into a metamorphic rock if its pressure and temperature rise causing chemical and physical changes without melting. Any rock can be transformed into a sedimentary rock if it is eroded, its fragments are deposited and then they suffer diagenesis.

Minerals

Components of the Geosphere

We define geosphere as the solid part of the planet. Nearly the whole solid components of the planet are minerals or rocks. Although there are some exceptions, such as the soil or the living beings, that are solid substances but are not minerals or rocks. The organic elements related to living things are a part of the Earth’s component called Biosphere.

Minerals and rocks are very important for human beings, not only because they are the main component of the planet, but also because they are the main source for raw materials. Lots of materials, such as the concrete used to build our house, the iron used to make a knife or the silicon used to make a computer come from different minerals or rocks.

Minerals

Definition

Minerals are solid, inorganic, naturally occurring but abiogenic, crystalline substances. They have to be homogeneous and have a representative, constant and fixed chemical composition.

The properties of minerals are inherent to this definition. As a result, all minerals have to satisfy all the criteria of the definition, and no substance can be named a mineral if it does not satisfy any point of the definition.

Let's analyse all the items of the definition.

• Minerals have to be solid substances. So they can't be liquid or gases.
• Minerals have to be inorganic. So they are always simple substances and are never made up of carbon chains.
• Minerals are naturally occurring. This fact means that they can't be produced by human beings. We can produce, for instance, artificial diamonds in a factory using carbon and high
  
  pressure machines, but they are not real minerals.
• Minerals are abiogenic. This means that they can't be made by living beings. Some inorganic substances produced by living things, such as the shells of some animals or  the hydroxyapatite in the bones of vertebrates are very similar to minerals, but they are not real minerals due to their origin.
• Minerals are crystalline. Their atoms must have an orderly repeated pattern. The atoms are, as a result, extremely ordered in tridimensional structures. This extremely ordered structures sometimes leads to the external shape of the mineral, so some minerals have a geometric shape. Although this is not a general rule and many minerals don't have a definite external shape and are amorphous.
• Minerals are homogeneous. This means that a mineral has the same properties and chemical composition in all its points.
• Minerals have definite chemical composition. A mineral must have the same chemical composition, wherever it is originated.

Characteristics

Minerals can be classified according to different characteristics. These characteristics can be divided into two groups: chemical and physical characteristics.

• Chemical characteristics.
• Chemical composition.
• Crystallisation (crystalline structure).
• Physical characteristics.
• Colour and brightness.
• Streak.
• Shape and cleavage.
• Hardness.
• Density.

Chemical composition

The chemical composition of a mineral is related to the chemical components of the mineral and the formulation. Minerals have, by definition, a definite chemical composition. They are always inorganic substances, and usually have a simple formula.

Two minerals can be different and have the same chemical composition. Diamond and graphite, for instance, have the composition: carbon. The final properties of a mineral depend not on their chemical composition, but also on other characteristics, such as the crystalline structure.

Some minerals can also have impurities. Impurities are little amounts of elements or chemical components that are not a part of the regular formula. Impurities can change some properties of the mineral, such as its colour or brightness.

Mineral can be classified according to their chemical composition. There are two big groups, named silicates and non-silicates.

Silicates are minerals with silicon (Si) in their composition. Silicon is, apart from oxygen the most abundant chemical element of the Earth's crust. Due to this, silicates are very abundant and there are many types of silicates. Quartz, muscovite, biotite or olivine are some examples.

Quartz

Olivine

Non-silicates are minerals without silicon in their formula. There are seven groups of non-silicate minerals.

• Native minerals: these minerals have only one chemical element in their composition. There are many examples, such as sulphur, gold or silver.
• Sulphides: these minerals have sulphur linked to metallic or semi-metallic elements. There are many examples, such as pyrite (FeS2), galena (PbS) or cinnabar (HgS).

Sulphur

Pyrite

• Oxides: minerals with oxygen in their composition. Corundum (Al2O3), hematite (Fe2O3) or cuprite (Cu2O) are some examples.
• Halides: minerals with halogen elements (fluorine, chlorine, bromine or iodine) in their composition. Fluorite (CaF2), halite (NaCl) and sylvite are three typical examples.

Corundum

Fluorite

• Carbonates: minerals with CO3 in their formula. The most typical ones are the calcite (CaCO3) and aragonite (also CaCO3).
• Sulphates: minerals with SO4 in their composition. Gypsum (CaSO4), barite (BaSO4) or celestine (SrSO4) are some examples.
• Phosphates: minerals with PO4 in their composition. Fluoroapatite (Ca5(PO4)3F) or hydroxyapatite (Ca5(PO4)3OH) are the most typical examples.

Calcite

Gypsum

Hydroxiapatite

Crystallisation

The crystallisation is the tridimensional distribution of the atoms. The atoms of the minerals are arranged in extremely ordered microscopic structures. The repeated geometric figures that the atoms form and their axis and mirror planes define crystalline structure of the mineral.

According to these geometric characteristics there are seven different crystalline groups: monoclinic, triclinic, orthorhombic, tetragonal, trigonal, hexagonal and cubic.

The crystalline structure is extremely important. Two minerals with the same chemical composition can have different properties because of their crystalline structure. Diamond and graphite, for instance, have the same chemical composition: carbon (they are native minerals). Their properties are, however, different: diamond is the hardest mineral, graphite is so soft that it can be scratched with our fingernail. The main difference between these two minerals is their crystalline structure: diamond 

Colour

The colour of a mineral depends on how it the absorbs or reflects light. Some minerals have only one colour, so we it can be said that the colour is characteristic for that mineral. Most of them, however, can have different colours.

This variability depends on several factors, such as the origin and formation of the mineral (temperature and pressure during the formation of the mineral) or the presence of impurities. We have studied that impurities are chemical components that contaminate the mineral, but are not a part of its regular composition. Some of these chemical components can cause changes in colour even in very low quantities.

Brightness and lustre

These characteristics refer to the quality and quantity of light reflected by the minerals. Minerals can be classified according to their brightness. The most typical types of brightness or lustre are:

• Metallic: typical in mineral with metallic elements.
• Vitreous: similar to the brightness of glass.
• Greasy: like being covered by a film of grease.
• Silky: similar to the brightness of silk.
• Earthy: like being made of sand or soil.
• Adamantine: similar to the brightness of a diamond. 

Silver: metallic brightness

Fluorite: vitreous brightness

Calcite: greasy brightness

Talc: silky brightness

Limonite: earthy brightness

Diamond: adamantine brightness

Streak

It is also called powder colour. It is the colour of the powder when the mineral is dragged across a hard surface, called streak plate. Streak plates are usually made of porcelain. This characteristic is more consistent than colour: although the colour of a mineral can change, its streak is nearly constant. 

The colour of the powder can be different to the colour of the mineral. Nearly all the metallic minerals, for instance, have black streak, although their colour is very variable.

Streak (hematite) by KarlaPanchuk

Shape and cleavage

Cleavage is the tendency of a mineral to split along definite planes. The shape of the mineral depends on these planes and how the mineral split.

The planes and the shape are related to how the atoms of the mineral are ordered. According to their shape, we can classify minerals as basal, cubic, octahedral, prismatic, dodecahedral or rhombohedral.

Hardness

Hardness is defined as the resistance of a mineral to scratching. The reference to measure the hardness of a mineral is a group or list of ten minerals called Mohs scale of mineral hardness.

Mohs scale

The ten minerals that form this scale are ordered and numbered from softest to hardest.

The hardness of a mineral depends on which minerals of the scale can be scratched with this mineral. A mineral can be scratched by a mineral with its same hardness value or higher.

According to this, a mineral with a hardness of 6, for instance, could be scratched by orthoclase (or harder minerals), but it couldn't be scratched by apatite (or softer minerals).

The hardness of a mineral can also be approximately measured using ordinary objects. Minerals with a hardness of 2 or lower can be scratched by our fingernails. Minerals with a hardness of 3 or 4 can’t be scratched by our fingernail and they are hard enough to scratch copper (a coin, for instance). But they can't scratch iron or steel.

Minerals with hardness bigger than 4 can scratch iron and they can scratch steel if their hardness is 6 or bigger (they can scratch the blade of a knife, for instance). Minerals with a hardness lower than 6 can’t scratch a piece of glass. Minerals with hardness bigger than 7 can scratch a piece of porcelain.

Density

Density is defined as the mass (or weight) of a substance per unit of volume.

The density of a mineral can easily be measured weighing the mineral to know its mass and, after that, immersing the mineral in a graduated cylinder with water in order to measure its volume (this shouldn’t be done with soluble minerals).

The density of a mineral is nearly constant, so it tis a good property to distinguish minerals that have other common physical or appearance. 

Classification of minerals

Minerals are usually classified according to their chemical composition. We have already studied the chemical groups of minerals.

They can also be classified according to their crystalline system.

Finally, minerals can also be classified according to their origin (although this classification is more typical for rocks):

• Igneous minerals: they come from molten minerals that solidify.
• Metamorphic minerals: they come from other minerals that change their chemical composition or crystallisation due to high temperature or pressure.
• Sedimentary minerals: they come from other minerals after processes of weathering, erosion and lithification.

Planet Earth: Geosphere

Geosphere: Definition

Geosphere is defined as the solid part of the planet. It includes not only the superficial rocks, but also all the materials inside the Earth.

The main components of the geosphere are minerals and rocks, although in some layers or regions the rocks can be molten. Liquid or semiliquid materials, such as magma, are also components of the geosphere, but are not proper minerals or rocks.

The most rigid part of the geosphere is located in the upper layers and is called lithosphere.

Geosphere: Layers.

The geosphere is made of three concentric layers. These layers have different properties and composition. The boundaries between these layers and sub-layers, called discontinuities, are also very important.

Earth's Layers

Geosphere layers:

• Crust.
• Continental Crust.
• Oceanic Crust.
• Mantle.
• Upper Mantle.
• Lower Mantle.
• Core.
• Outer Core.
• Inner Core. 

Crust

The crust is the outermost and thinnest layer of the planet. It is less than 30km thick on average and occupies less than 1% of the Earth total volume. The most abundant components of this layer are silicates.

Earth’s crust can be divided into two different parts: continental crust and oceanic crust.

The continental crust is the part of the crust that forms the continents. It includes the emerged surface of the planet and a shallow part of the submerged crust, adjacent to the continents, called continental shelf.

It is thicker than the oceanic crust, it is 30km to 50km thick, and it is thicker under the mountains than under the coast. We can say that mountains have a sort of roots made of crust that penetrate deeper into the Mantle. It is, besides, older than the oceanic crust.

The typical components are the sodium potassium aluminium silicate rocks, such as granite. Due to this, it is called FELSIC.

The oceanic crust is the part of the crust that forms the sea floor. It is thinner than the continental crust, it is from 5km to 10km thick on average. It is younger than the continental crust, merely because the oceanic crust is constantly being generated by the intense volcanic activity that occurs in the middle of the large oceans, in volcanic chains called ridges. 

Oceanic crust, besides, is destroyed in some deep regions of the oceans called oceanic trenches.

Summing up, new oceanic crust is constantly being formed in the ridges of the middle of the ocean and, at the same time, old oceanic crust is being destroyed in the oceanic trenches, that are adjacent to some continental shelves. 

The typical components of this part of the crust are the iron magnesium silicate rocks, such as basalt. Due to this, it is called MAFIC.

The boundary that separates the crust and the Mantle is called Mohorovicic discontinuity.

Mantle

The mantle is the intermediate layer, between the crust and the core. It is a very thick layer, about 2900km thick and constitutes about 84% of the Earth's total volume.

It is mainly made of solid rocks, although in some parts, above all in the upper part of the layer, the rocks have plastic properties and are not totally rigid. The most abundant components are silicates.

The mantle's temperature depends on the depth, rising from around 400°C in the upper boundary to 4000°C at the lower boundary. The pressure, logically, also rises depending on the depth due to the weight of the overlying rocks. This pressure is called lithostatic pressure. Summing up, the deeper the rocks are, the higher the temperature and pressure on those rocks. Rocks have different properties according to these two physical parameters.

The mantle has two different parts, called upper and lower mantle.

The upper mantle is adjacent to the crust and it is between 300km to 500km thick. It can also be divided into two parts. The rocks of the upper part, just beneath the crust, are rigid. The group of rigid rocks of the upper mantle and the crust form a rigid structure called lithosphere. The Earth's surface is made of pieces, like a jigsaw, called tectonic plates. Tectonic plates are solid and rigid structures made from the rocks of the crust and the upper part of the mantle.

In the lower part of the upper mantle, under the lithosphere, there is a region where rocks are partially molten. This region is called asthenosphere. The tectonic plates are, in some way, floating on the asthenosphere.

The lower mantle is just beneath this plastic region. It is very thick, about 2000km, and it is made of solid and rigid rocks. Its temperature is very high, above all at the lowest part, but rocks are not molten due to the enormous lithostatic pressure.

The boundary that separates the lower mantle of the core is called Gutenberg discontinuity.

Core

The core is the inner layer of the planet. It is around 3500km thick and extremely hot, with temperatures over 4000°C. In fact, the earth's core is hotter than the Sun surface. It is also extremely dense. The main chemical components are nickel and iron.

The core has two parts, the outer and the inner core.

The outer core is between the mantle and the inner core. It is 2200km thick. And the extremely high temperature, from 4000°C in the outer part to 6000°C near the inner core, melts the rocks, so this layer is liquid.

The inner core is the deepest layer of the Earth. Although its temperature is even higher than in the outer core, over 5000°C, the extremely high lithospheric pressure solidifies the rocks, so this layer is solid.

The movement of the nickel and iron fluid of the outer core around the solid nickel and iron of the inner core causes a dynamo effect, that is responsible for the magnetic field of the planet.

Earth’s Surface

As we studied, the Earth’s surface is made of large pieces called tectonic plates. Tectonic plates are like pieces of lithosphere than can move or crunch one another. Different plates are separated by different kind of boundaries. The most important ones are constructive boundaries, destructive boundaries and transforming boundaries.

Constructive boundaries are borders where new crust is formed. The most important ones are the oceanic ridges, volcanic ridges in the middle of the ocean where the volcanic activity creates new crust, causing the expansion of the oceanic floor.

Destructive boundaries are borders where the crust is destroyed. The crust passes to interior layers of the Earth in a process called subduction. The most important destructive boundaries are in the oceanic trenches.

Transforming boundaries are borders where crust is neither formed nor destroyed. They are faults that separate different tectonic plates. The friction between different plates can cause geological phenomena, such as earthquakes.

Continental features

When two tectonic plates collide some typical continental structures, like mountain ranges, are formed, due to the deformation of the crust in the zones where plates crush. The Andes and the Himalayas are two examples.

The oldest parts of the continental crust tend to be flat due to the effect of erosion, forming continental plains. Some deserts, such as The Sahara, are examples of great plains.

The part of the continental crust that is submerged under the sea is called the continental shelf. It is an extension of the continental crust, near the coastline, covered by water although it is always shallow, less than 200m deep. The continental shelves finish in the continental slopes, where the sea floor descends abruptly from 200m to more than 4000m on average.

Oceanic features

The friction of the tectonic plates in the subduction zones causes many geological phenomena. The friction tends to increase the temperature of the rocks, melting them and transforming them into magma. When this magma reaches the surface, it forms volcanoes. When the subduction is under the sea, the magma forms volcanic islands and archipelagos. Japan is a good example.

The oceanic trenches associated to the subduction zones are the deepest parts of the ocean.

As we studied, in the middle of the oceans there are constructive boundaries called oceanic ridges. They are long submarine chains of mountains with high volcanic activity. They are responsible for the formation of oceanic crust.

Due to this process, the sea floor tends to be flat. Abyssal plains are the largest plains of the planet. They are large extensions of submerged flat land, under more than 4000m of water on average.