........................................................... The rocks

Introduction. When we go to the mountains, what we see are not minerals, but rocks. This is because the most abundant crust minerals are not metallic, but the non-metallic minerals that form rocks, and those basically are silicates and carbonates. It is why is so important to know the different types of rocks. In addition, we need to know them because they are very necessary in house constructing, building roads, bridges and to obtain energy, how it's done with carbon and petroleum. All of this is explained in this chapter.


Activities to do. Read the explanation on the concept of rock and different types of rocks and do the "Multiple Choice Test 6". Then read the text on the rock cycle and its utilities and do the exercises "Identify rocks and rocks types 6" and "Crossword 6".

1 . The concept of rock. A rock is a natural, inert substance, that means is not alive, it is consistuted by two or more types of minerals or mineral states. For example, the rock called granite is composed of three types of minerals called quartz, feldspar and mica. In many rocks, the particles of each mineral are so small that they are indistinguishable from each other at the first glance, it seems that they are composed of a single type of mineral. But if we analyze or observe under the microscope it is seen that there are several types of minerals. This applies, for example for limestone, a rock which is constituted by the mineral calcite and a small proportion of minerals forming a clay. Some rocks are forms by mineraloid states, in case for example of carbons and petroleum, which are two rocks that are formed by organic matter, and a volcanic glass called obsidian, whose components have no crystal structure (therefore, they are not minerals).

2 . The classification of rocks. The rocks are classified according to the process of forming that occurred. There are three main groups of rocks: sedimentary, magmatic and methamorpic.

3 . The sedimenatry rocks. Are rocks formed by the union of loose particles, called sediment, which accumulate in the bottom of the valleys, seas and oceans. Given the origin of these particles, there are four types of sedimentary rocks called: detrital, chemical, biochemical and organic.

Detrital. Are rocks formed by the compaction of particles coming from erosion of the mountains, due to air, rain, rivers, glaciers, etc. These agents wear out the relief, carrying torn particles and finally placing them in the form of horizontal layers, in places more or less distant from their point of origin. Once deposited there, through a process that takes a millions of years, called diagenesis, the particles bind together and originate detrital sedimentary rock strata.

Diagenesis is produced by process of compaction (due to the weight that superior sediment exerts on the inferior) and of foundation (precipitation of a substance that binds the particles). If the particles have the same size as the sand, they are called sandstone, if they are thicker, conglomerate and if they are thinner, as an impalpable powder – clay.




• Chemical. Are rocks formed by the union of particles sprout in the water, as a result of a chemical reaction. This is how chemically precipitation limestone  and gypsum  are formed. Evaporating a portion of water, facilitates precipitation of the substances that form rocks by accumulation. Other rocks, such as rock salt or sylvite require almost complete evaporation of the water, and are called evaporitic. As was explained in the previous topic, gypsum, rock salt and sylvite are three minerals, but when associated with other minerals, as usually happens, form rocks that obtain the same name.   

Chemically precipitation limestone

Rock salt

Biochemical. Are rocks formed by the compaction and foundation of the inorganic residues (skeletons) of living, such as shells, coral fragments, carapaces of microscopic organisms, etc. All these organisms have made their skeletons from dissolved salts during the chemical reactions within the body. Examples of these rocks are the coral limestone (derived from corals), nummular limestone (comes from fossils called Nummulites) and lumaquela (derived from mollusk shells).

Coral limestone

• Organic. They are rocks formed from the accumulation of organic remains of living things. Examples of these organic remains are stems and leaves of plants that lived millions of years ago and were buried in the ground, which prevented decomposition. That eventually led to origin of mineral carbons.

The carbon minerals by having more or less percentage of carbon are called anthracite, coal, lignite and peat, respectively. Petroleum and natural gas are formed from large masses of microscopic marine organisms (plankton) which were also buried millions years ago.

4 . Magmatic rocks. Are those that have originated from a magma which when cooled, it is solidified. Given the conditions under which this solidification occurs, there are three types of magmatic rocks:

• Plutonic rocks. Are formed when cooling takes place at a great depth. The process is slow and therefore all crystallized minerals have a similar size. For example, white granite, pink granite and syenite (when the last one differs from pink granite by not containing quartz).

White granite

Pink granite


• Philonian or porphyritic rocks
. Are the ones formed when the magma is cooled inside cracks. The cooling is faster than in previous case and minerals generally obtain large crystals surrounded by small ones. Examples are granite porphyry and syentic porphyry.

Granite porphyry

• The volcanic rocks
. Are formed when magma comes out from volcanoes and cools off one the outside. Cooling process is very fast, so as a result, the components do not have sufficient time to crystallize and form minerals. Because of it they form an amorphous mass, as in volcanic glasses as obsidian and lava, or they crystallize only a few minerals and the rest of components are a formless mass, which happens with a rock called basalt. A comment has to be made, that the glasses that are manufactured, for example, to put in windows are not crystals, because they are not made of crystal matter but of amorphous matter. 


Basaltic lava


5. Metamorphic rocks
. Are formed from other already existing rocks, when those are subjected to such an increase of pressure or temperature, or both at once, that in consequence without being melted, their minerals are transformed into other mineral, which are able to withstand new conditions. As a result, the new rock has a different appearance and properties from the initial rock. The best known are:



Clay --------------------------> Slate

Sandstone ------------------------> Quartzite

Limestone ----------------------> Marble

Granite ---------------------------> Gneiss


Slate (argillaceous schist) with fossils


Multiple Choice Test
(Spanish activity. Activity in English is under construction)

6. The rock cycle
. All natural processes which causes a type of rock being transformed into other type of rock is called the “rock cycle”. In the following chart all of these processes are shown.

7. The profits from rocks. The rock are used mainly in the construction of buildings, roads and bridges. For example clay, sand, tape, gypsum, etc. Some rocks are used as energy sources, for example carbons, petroleum and gas.


• Clays. Clays are composed of very small particles. If water is added, it gets soaked and then converted into mud; which prevents water from penetrating further into the ground by forming puddles. They are forming an impervious surfaces. When clays are humid the can be easily modeled and form vases, bricks and tiles. To prevent the melting while being wet again, they must be put in special ovens at high temperatures. This is to achieve that its tiny particles are welded and not deformable.


• Sand and gravel. These two materials are only different in the size of its particles. They are used in road construction to make surfaces plain in order to put the asphalt, to mix with cement and to make the mortar, so that the builders use to bind the bricks. The quartz sand, because of the fusion and cooling process, may be used to manufacture glass. Sand and gravel can be obtained from beaches and river banks. They can also be obtained by crushing other rocks.


• Agglomerating materials. Are the material used in construction, which by adding water, subsequently consolidate (harden). They are gypsum and cement.


• Gypsum. It is used for finishing walls and ceilings. Se utiliza para el acabado de techos y paredes. As it is a fine material it allows a good finishing and as it is also softs, it permits embedding pipes, cables and plugs easily. Natural gypsum is obtained from heating in an oven until it loses water. If water is added later, after half an hour it solidifies (hardens).


• Cement. It is a basic matrial in constructions. It is a gray powder, to which sand and water are added in order to make mortar for the bricks. If gravel is also added, concrete is obtained. This material, when harden is very consistent. If steel bars and wire network are also included, it is called armed cement. The cement is obtained from the calcination of a mixture of clay and limestone, to which later gypsum is added.


• Rocks used in construction. Are used to make surfaces in kitchens, hallway floors and cladding of facades of some building. The rocks used for this purpose are marble, granite (white granite and pink granite) limestone, sandstone and slate. The last one, given the ease with which it breaks into films, is often used to coat the sloping roofs of the houses in rainy climates.


 • Salt. Common salt or sodium chloride is used in food industry as a flavor and preventing medium. It is obtained from native salt rock, which is in a certain degree purified between the sedimentary layers and from the evaporation of seawater in salt mines.


 • Carbons, petroleum and natural gas. All these substances are used to produce energy by combustion with oxygen in the air. This energy is the one that moves cars, trucks and boats, which keeps heat and generates electricity that comes to industries and cities. There are four types of carbon: peat, lignite, coal and anthracite. 


In Spain there are only few deposits of peat, the carbon which is also used as organic fertilizer. Instead there are numerous deposits of lignite (Teruel), coal and anthracite (Asturias and Leon). This is a very good quality carbons that are difficult to extract. This makes it only profitable when the price of petroleum is very high. In Spain there is almost no petroleum production. Only Amposta (Tarragona) can be mentioned. Besides the energy, also plastics are obtained from petroleum. Currently it is being extended to all of Spain, the large pipeline that distributes natural gas to houses from Algeria.

Identify rocks and rock types (Spanish activity. Activity in English is under construction)

Crosswords (Spanish activity. Activity in English is under construction)

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