Types of Rocks
Sedimentary Rocks
The sedimentary rocks are composed of fragments of others, coming from weathering and erosion. these fragments are carried by ice, wind and / or water. They accumulate on plateaus in the Earth's crust. They cover about 75% of the Earth's surface and 90% of the seabed and correspond to 5% of the Earth's volume. Sedimentary rocks are important sources of fossil material. An example of a sedimentary rock is sand.
Metamorphic rocks
They derive from the deformation of other rocks, magmatic, sedimentary and other metamorphic, due to changes in environmental conditions, such as the temperature and the pressure to which they are subjected. Some examples are gnaisse (formed from granite); the slate (formed of the argillite); the marble (formed of the limestone) and the quartzite (formed of the sandstone).
Magmatic / Igneous Rock
The magma forms inside the Earth, in places where the conditions of pressure and of temperature allow the fusion of the rocks. Normally the magma is less dense than the encasing, enveloping rocks, and so it eventually ascends, approaching the surface. In doing so, it becomes and consolidates, forming the magmatic or igneous rocks.
When it consolidates at the surface, the resulting rock is called extrusive or volcanic magma. On the contrary, by solidifying in depth, it forms intrusive or plutonic igneous rocks. Examples are, respectively, basalt and granite.
Sand
The sand is abundant along the entire coast of Portugal and forms the characteristic sand of the beaches we know (ok, except for the beach of Antas, Esposende, which is made exclusively of pebbles).
It is a group of particles of weathered and eroded rocks, a material of mineral origin composed of silicon dioxide among other minerals that gives it the more or less dark color and the known diversities.
It forms at the surface of the earth by the fragmentation of rocks by erosion, by the action of wind or water. By means of sedimentation processes can be transformed into sandstone. Because of this, sand is also used in civil engineering, landfills, mortars, concretes and also in the manufacture of glass. The size of its grains has importance in the characteristics of the materials that use it as a component.
Grain Size Classification
According to the Wentworth scale, sand is the entire mineral or rock fragment variably and the size ranges from 0.06mm to 2mm.
However, within this category, it can be subdivided into 3 groups:
- Fine sand, if it varies between 0.06mm and 0.2mm;
- Average sand, if it varies between 0.2mm and 0.6mm;
- Thick sand, if it varies between 0.6mm and 2mm;
Drawings in the Sand - what are they?
In general, the sand presents a diversity of components, from minerals to shells, algae, among others. In some beaches, like the beach of Amorosa, the sand is also constituted by heavy minerals.
These heavy minerals, so called because of having greater density than the common minerals, are dark giving the black coloration to the sand of the beach. Among them are: magnetite (which is magnetic), ilmenite (with titanium in the composition and metallic luster), monazite (fluorescent in ultraviolet light due to the presence of radioactive elements), zircon (vitreous with zirconium) and rutile of nature titanium).
The waves of the sea, due to the transport of the sand, end up helping and creating these characteristic drawings. The waves separate and group the minerals according to their density: the lighter ones (and also read the less dense ones) are carried by the current towards the sea while the heavier ones (also the denser ones - like the heavy minerals) are left far from the sea. The drawings, the stripes of color, form strips that can extend along the coast.