A Comprehensive Guide to Rock Features and Types

Posted on May 12, 2024 in Geology

Chapter 1: Features of Rocks

1. Adherence & Cohesion

This refers to the bonding force between particles. Rocks are classified based on their bond or cohesion:

  • Consistent: Require significant effort for grinding.
  • Friable: Crumble easily.
  • Loose: Grains formed by loose sand.

Rocks can also be classified based on the source of their adherence:

  • Rocks added: Adherence is achieved by the beads being attached to each other (e.g., granite).
  • Rocks agglomerated: Adherence is achieved by means of a cement (clastic).

2. Hardness

Hardness is the difficulty a rock offers to being scratched.

  • Soft stones: Easily cut with ordinary saw teeth (e.g., travertine, tuff, marl).
  • Semi-hard stones: Require special hard-tooth saws (e.g., limestone, marble, sandstone).
  • Hard stones: Need an arena saw for cutting (e.g., crystalline limestone, siliceous sandstone, serpentine, trachytes, diabase).
  • Very hard stones: Carborundum saws (black diamond) are used (e.g., granite, syenite, diorite, basalt).

Types of Rocks

1. Eruptive Rocks

These rocks originate from the consolidation of magma located deep within the lithosphere. They have come to the surface or solidified in areas close to a fluid state.

1.1 Depth or Metamorphic Rocks

Also called intrusive or hypogenic, these rocks have solidified slowly within the Earth’s crust without communication with the outside world. They have a holocrystalline structure, like granite.

The most important rocks of this group in construction are: granite, syenite, diorite, gabbro, peridotite, and serpentine.

1.1.1 Granite

Granite is a plutonic rock and the most abundant of all. It exists as crystal-like masses.

For use in construction, granite must be durable and free of imperfections such as cracks, hairs, fissures, and nodules that may compromise structural integrity. It should not contain accessory minerals that are reversible, which may endanger the physical and mechanical properties, or patches that alter its original appearance.

Granite should be easy to saw, carve, polish, and work with.

1.1.2 Rhyolites or Loparitas

Quartz porphyry with a predominance of alkali feldspar is called rhyolite due to its fluidal texture, common in vitreous varieties.

Rhyolite is similar in composition to granite. It is light-colored, beige, gray, and green.

In factory building, blocks of pumice, a type of rhyolite, are used for thermal and acoustic insulation. It is also used as an auxiliary material in the process of polishing natural stones in the honing phase.

1.1.3 Basalts

Basalt is the most widespread volcanic rock, and its mineralogical composition corresponds to gabbros. Basalts are sometimes blackish and take on dark red or dark green tones. They are generally subdivided into two main groups:

  • Tholeiitic basalts
  • Olivine basalts

2. Sedimentary Rocks

The process of sedimentary rock formation is as follows:

  1. Destruction of pre-existing rocks by the action of water, ice, weathering, earthquakes, etc.
  2. Transportation by water, ice, or air. Products of this destruction can be carried in suspension, washed away, or dissolved (in the case of water and ice), or suspended or dragged (with air).
  3. Sedimentation or deposit of materials, sometimes mechanically due to decreased drag from lack of speed or other reasons, and other times chemically by evaporation of the liquid carrier.
  4. Transformation of sediment into rock through consolidation, sometimes due to adherence established between particles by drying, compression of the sediment, or other causes, and other times by filling the gaps with a cement binder.

Types of sedimentary rocks:

  • Aggregates
    • Gravel: Larger sizes and coarse aggregate (fraction retained on the sieve of 4 mm) used for concrete, ballast, etc.
    • Sand: Grains are between 4-0.2 mm.
    • Powders: Grains are between 0.2-0.063 mm.
  • Clays

    Clays are friable masses with additions of clay, produced by the decomposition of aluminous silicates of igneous rocks.

    They come in different colors, from white to dark red or even yellow.

    They are greasy to the touch, exhale the smell of damp earth, and stick to the breath.

    They are the raw material for the manufacture of ceramic products and cement.


    Plasticity, is a physical property that only has the clay when soaked in water

    Compact rocks
    They are rocks from the loose rocks lapidificación (floods, pebbles and sand) by a natural cement. When materials are bonded sands are called sandstone and if they are larger CLUSTERS.

    · Sandstone
    Rocks are composed of quartz grains and some other minerals cemented with a natural cement of the same or different nature.

    1.1.4.Algez or gypsum
    It is a sedimentary rock salt into large masses has evaporated deposits with rock salt and anhydrite.
    It occurs in nature, as Plutonic and compact, fibrous, Iaminares with large crystals, fine-grained or sugary and chalky. The purer varieties are colorless and transparent, white, sometimes tinged with red or yellow from iron, and dark carbon.

    1.1.5.Calizas
    They are rocks formed by precipitation of dissolved bicarbonate or by double decomposition between ammonium sulfate and calcium carbonate.
    Types of limestone: pisolithic, tufa limestone, lithographic limestone, stalactites, oolites, limestone fibrous.

    1.1.6.Calizas
    They consist of shells of marine animals cemented with limestone, marble calling many of them, allowing a good polish.
    1.2.Gneis
    Rocks are more or less leafy, the same composition as granite and many accessory items.

    1.3.Pizarras
    Rocks are microcrystalline structure, caused by clay sediments termomecánicode metamorphism. Very rarely can come from basic hipogenic rocks, affected by an alteration process later.
    Shales: Also known as common roofing stone. They are rocks from clay sediments, slightly metamorphosed. Laminar structure have very compact, waterproof, oily to the touch, elastic, not hard, allowing ease of machining (cutting, sawing, drilling, etc.
    Crystalline slates: They are called also “crystalline schists

    1.4.Mármoles
    It is a limestone, metamorphic, crystalline, granular, compact criptogranular. It consists almost entirely of calcite grains grouped without guidance.