Day 21, Rocks


A rock is a compound formed by a group of minerals. There may be many minerals forming it or may be formed by one calling that mono mineral rock.

In The Motagua River there are many rocks, large, medium, small, and most are of metamorphic origin, because this is the Motagua fault and as seen before, when a fault is formed is exerted enormous pressure and temperature that make metamorphic rocks are formed. But not only find metamorphism there, we found igneous and sedimentary rocks.

The importance of knowing about the rocks

Knowing the types of rock that exist and how they formed is very important to have an idea or knowledge of what we see or work in the field, to better assess the chances of possible sources of gold in an area. The Motagua River know there but metamorphic rock type … which are metamorphic rocks? As they formed? What types of associated minerals as we distinguish and relate them to the gold? And so in each rock type.

rock-cycle-diagram-imIn general there are three types of rocks: igneous, metamorphic and sedimentary. And each has different properties, chemical composition, permeability, texture, etc.. And and no matter what type or rock it is, all are coming from the magma.

More about the rocks:

We can consider as a solid rock composed of more than one mineral or mineraloide and a single crystal is not considered as a rock, but the union of two crystals if, even if they are technically the same mineral is a rock. Mineraloids minerals or may be large enough to be easily identified, as in some spots, just as in a shale distinctive, or a mixture of microscopic granules as on a whiteboard.

A rock does not even have the need to present crystals, and which may have a non-crystalline solid, an amorphous mixture in which no chemicals were crystallized minerals like obsidian.
It is generally considered that the rocks are natural objects only, but sometimes as artificial substances include rocks.

As mentioned before, there are three main classes of rocks, classified according to how they originated, first find igneous rocks formed from the cooling of magma. Over time, the various processes of weathering and erosion of rocks resulting in particles that can settle in bed, then are compressed and cemented resulting sedimentary rocks. If these rocks are buried, heated and highly compressed, be transformed into metamorphic rocks. If these rocks remain heated and compressed to the point that they melt, the molten rock could then eventually form another igneous rock. This is known as the rock cycle. It forms a complete circle when a rock can be transformed into another. They may even lead to a different rock of their own class, a sedimentary rock, a sandstone can be eroded and degraded fragments could eventually become a shale.


Rocky incandescent flow, which is inside the terrestrial crust and is about 1,000 degrees centigrade. It is composed of silicates, oxides and volatile elements. Subsequent cooling and crystallization results in igneous rocks.

Geological Classification of Rocks and geological cycles

In Geology rocks are classified due to their origins in igneous, sedimentary and metamorphic rocks.

These classifications are given in turn by the rocks that are subject to constant changes by geological agent actions that take place inside and outside the Earth’s structure and are therefore called geological cycles.

Of these three types of rocks, the first originating are igneous rocks, which are formed inside the crust with a high pressure of dissolved gases, which generate an expulsion through the cracks to the terrestrial surface volcanic eruption shaped so that after reaching a cooling process where the material ejected magma and crystallize when contacted with water and carbon dioxide. These rocks are usually very hard, precisely because they are formed of the mass magma.

Due to these effects, these materials decompose and disintegrate thus calling process whereby upon weathering are transported and deposited by water, wind, the plant roots and generating ice forming erosions and solids concentrations or liquid in the bottom of rivers and lakes over the years accumulate fragments of material forming thick layers of sediment.

The sediments become rocks undergo changes so that at first the water is extracted from each layer. A change occurs when accumulated sediment layers, the innermost layer is crushed by the weight of the sand or mud above it. The grains are increasingly compressed, so that water is ejected upward and mixes with the sea or lake above it. Another change occurs when the material becomes becomes a kind of cement and over the years the water is around grains containing dissolved minerals. Losing water solutions of minerals crystallize in space forming sedimentary rock fragments.

rock_cycle_After igneous and sedimentary rocks are subjected to alteration and change in structure is solid by high temperatures and pressures and also by being in contact with a mass of magma, are leading to the formation of metamorphic rocks called because change shape. Additionally, this process creates other minerals.

Although the conformation of each previous rocks, the rocks experts say to classify either one igneous rock, sedimentary and metamorphic should take into account each of the elements which allow its operation and conformation, whenever their minerals vary according to the texture, shape and size.

For these reasons every rock requires special handling of examination in order to identify their chemical composition used for the definition and study of soil types. It has applications in agriculture, and is used to determine the degree of fertility of the land and its best use, in mining to determine the relationships between them and know with some precision where to find the source of a particular mineral like gold, in placer mining, besides studying the currents of the rivers, to determine if a river has the characteristics needed for its use.

Another aspect to check is your home, as mentioned above, in order to determine the formation and characteristics mostly.

In the classification of a rock must have a number of factors that have to do with getting a rock sample to study, define its order, its texture type and identify the group to which it belongs. Similarly for details of the rock, you must consult a table that would put the name of the rock and thus know which class belongs.

Igneous Rocks


Obsidian, is a naturally occurring volcanic glass formed as an extrusive igneous rock.

Igneous rocks (from Latin igneus-that comes from ignis-fire) or magmatic constitute the bulk of the solid portion of the Earth, at least in its outer region. Igneous rocks are formed from cooling and solidification of a molten silicate or magma (rock mass / viscose whose main elements are silicon and oxygen, in addition to potassium, sodium, calcium, magnesium, aluminum and iron). Magma solidification and subsequent crystallization may occur in the inner bark in both deep and shallow areas, or on the outer surface thereof.

If crystallization takes place in a deep zone of the cortex to the rocks so formed are called intrusive or plutonic rocks (Pluto, the god of the underworld in classical mythology). Conversely, if the magma solidification occurs in the Earth’s surface rocks are called extrusive or volcanic rocks (of Vulcan, god of fire in classical mythology that had its place under the volcano Etna). Finally, if the solidifying magma occurs near the earth’s surface, in a relatively quick and small tanks filled magma (eg dykes, veins, sills, laccoliths, etc.) To rock so formed the subvolcanic or hipoabisales called. These rocks are also called rock Philonian because usually are filling cracks or seams.

In igneous deposits commonly found concentrations of metals such as gold, platinum, uranium, copper, selenium, chromium, vanadium, and many other economic interest.

Igneous rocks consist mainly of:

  •      Feldspars 50%
  •      Quartz 20%
  •      Olivine, pyroxene and amphibole 17%
  •      Micas 7%
  •      Other minerals found in small amounts, such as gold.

Igneous rocks can be classified according to chemical parameters, or mineralogicalSiO2_-_Quarz_-_2D

According to silicon content igneous rocks are classified as follows: (siO2 formula of silicon)

  • Acid igneous rocks with high silica content, greater than 63%, SiO2 (rhyolite and dacite examples).
  • Intermediate igneous rock containing from 52 to 63% of SiO2 (example andesite).
  • Basic igneous rocks having low silica, 45 to 52%, and typically high iron content of magnesium (eg basalt).
  • Ultrabasic igneous rocks with less than 45% silicon. (Examples picrita and komatita).

Crystallization, minerals and chemicals

Igneous rocks are composed mainly of silicates, which are made ​​up primarily of silicon (Si) and oxygen (O). These two elements, together with the aluminum (Al), calcium (Ca), sodium (Na), potassium (K), magnesium (Mg) and iron (Fe) constitute more than 98% by weight of most magmas that form igneous rocks solidify. Furthermore magmas contain small amounts of many other elements such as sulfur (S), gold (Au), silver (Ag), uranium (U), rare earths, dissolved gases, etc..

The composition of igneous rock depend, therefore, the initial composition of the magma from which it is formed.

Different silicates are igneous rocks crystallize in a particular order, which is determined by the temperature. The series of crystallization of Bowen (Norman L.) (1928) shows the order of the various silicates crystallization temperature decreases as a magma.

In this evolutionary scheme we can distinguish three groups of minerals:

  •      The ferromagnesian well known for its high iron and magnesium (olivine, pyroxene, amphibole, biotite). Due to its mineral composition are darker colors. Crystallization form a series of   discontinuous change minerals as discrete steps, and crystallized in a high temperature range.
  •      The plagioclase. Forming a series of continuous crystallization, since the transformations occur progressively between albite and anorthite. Also crystallized in a high temperature range – mean.
  •     Non Ferromagnesian silicates  (quartz, muscovite and orthoclase). Are minerals that crystallize at lower temperatures. These minerals contain a higher proportion of aluminum (Al), potassium (K), calcium (Ca) and sodium (Na), which iron and magnesium.
  •      A rock with a high content of minerals are called mafic ferromagnesian (mafic, magnesium and Latin ferrum). They usually have a high color index (dark shades). And rocks high in ferromagnesian minerals (quartz, muscovite, K feldspar, plagioclase and feldespatoides) are called felsic (felsic, proviniente feldspar and silica). They are rocks with a low color index (light shades).

Gold incrusted in quartz, quartz a mafic ferromagnesian mineral igneous type

The importance Bowen series is that the classification of igneous rocks, the composition is such series, rocks groups match the various steps of differentiation according to the series magma. From this correlation can be deduced that some of the terminology that relates to the composition of igneous rocks may indicate its genesis.


The texture reveals a lot about the history of the cooling of an igneous rock, it refers to the size of the grains of the minerals that make up the rock, the size depends on the rate of cooling and crystallization of magma. Like this:

  • A degree of slow cooling and crystallization originates large grains, easily visible and establishing few centers of crystallization.
  • A degree of rapid cooling and crystallization causes small grains, as many sets crystallization centers.

The following are the main textures of igneous rocks:

Phaneritic. (Granular, phaneros-visible Greek) is the rock in which the crystals can be recognized at a glance. This type of texture occurs in rocks that have undergone a slow cooling process, ie heat lost in a gradual and slow. It is typical of intrusive rocks (plutonic). We can distinguish various grain sizes in this group:

  •      Very coarse grain, when grains have a size greater than 30 mm
  •      Coarse, when the grains have sizes betweentool rock 30 and 5 mm
  •      Average grain when the grain sizes are between 5 and 2 mm
  •      Grained, when the grains have a size less than 2mm, but are visible

It is also known as holocrystalline comprises more than 90% by volume glass.

Aphanitic. (Microgranular, a-phaneros-not visible) is the rock in which the crystals can not be recognized by the naked eye and you need a magnifying glass or microscope. A texture aphanitic always indicates that the cooling process produced more or less rapid. This texture is typical of volcanic rocks and subvolcánicas.Se can distinguish two classes in this group:

  •      Microcrystalline when crystals are recognizable under the microscope
  •      Cryptocrystalline, when crystals are not recognizable under the microscope

Vitreous. It is a characteristic of extrusive rocks texture who have had a very rapid cooling on the surface, which determines the formation of glasses and crystals.

Porphyritic. Is a texture composed of two different grain sizes, reflecting an abrupt change in the cooling rate at the time was crystallizing magma.

Pegmatitic. A texture is formed of crystals of size greater than 3cm. In this group may also occur vesicular texture of cavities formed by gases from the rock in the cooling process.

Clasification of Igneous rocks by origin

Depending on how and where the magma cools are two major types of igneous rocks, the plutonic and volcanic or intrusive or extortion.

Plutonic or intrusive rocks

Plutonic or intrusive rocks are formed from solidified magma masses within the crust. The magma surrounded by preexisting rocks (rocks known as box), cooled slowly, allowing large crystals forming Minerals, visible to the naked eye, so are rocks “coarse”. Such is the case of granite or porphyry.

Igneous intrusions from which are formed are referred Plutos plutonic rocks, such as Batholiths, the laccoliths, sills and dikes. Plutonic rocks are only visible when the crust rises and erosion removes rocks covering the intrusion. When the dough is exposed rock outcrop called. The heart of the main mountain ranges consists of plutonic rocks that emerge when can cover huge areas of the earth’s surface.

volcanic rock

Recent volcanic activity forming volcanic rocks when magma gets cold, Pacaya Volcano, Guatemala

Volcanic rocks or extortion

The volcanic rocks are formed or extortion by the solidification of magma (lava) on the surface of the earth’s crust, usually after a volcanic eruption. Since cooling is much faster than in the case of intrusive rocks, minerals Ions can not be organized into large crystals and therefore volcanic rocks are fine grain (crystal invisible to the naked eye), such as basalt or completely amorphous (glass-like texture), like obsidian. In many volcanic rocks can be seen the holes left by the gas bubbles escaping during solidification of magma. The volume of extrusive rocks thrown by volcanoes annually dependent tectonic activity:

  •     Divergent Borders: 73% as mid-ocean ridges, Iceland and the East African Rift.
  •     Bordes convergent (subduction zones): 15%, as the Andes in Sudamerica, the Sierra Madre in Guatemala or Pacific island arcs.
  •     Hot spots (intraplate volcanism): 12% as Hawaii.

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