For the description of rocks, external features that reflect the features of their structure are of great importance. Such features are divided into two groups: the first describe the structure of the rock, and the second, which we dwell on in more detail, relate to texture features.
The concept of the structure and texture of rocks
The structure reflects the state of the rock-forming mineral substance and is associated with the process of crystallization and destruction of minerals, that is, with a change in the substance during the formation of the rock. Structural features include rock characteristics such as crystallinity, and the absolute and relative size and shape of the grains composing the rock.
The texture of a rock is a set of features characterizing its heterogeneity - in other words, the way in which structural elements fill the space in the rock, how they are distributed and oriented relative to each other. The appearance of the texture is associated with the relative motion of the rock components in the process of its formation. The shape of the individual rocks is also important in describing the features of its composition.
Texture classification and rock genesis
Different types of rock textures are classified according to the following criteria:
- Mutual arrangement of grain grains. Distinguish between homogeneous (massive) and heterogeneous textures. The latter, in turn, are of several types: banded, gneiss, schlieren, fluid, etc.
- The degree of space filling. The texture can be dense or porous of one nature or another (slag, myarolite, almond-stone, spherical).
The texture of the rocks, as well as their structure, depends on the origin. By this criterion, the rocks are divided into igneous, sedimentary, and metamorphic. They differ in their chemical and mineralogical composition and formation conditions. Each of them has its own texture features. Therefore, in more detail we will consider the types of textures for each class of rocks separately.
Igneous rocks
The formation of rocks of this type occurs during solidification of magmatic melts. Depending on the conditions of this process, the forming rocks are divided into two types. The structures and textures of the igneous rocks belonging to them differ with a similar chemical and mineral composition.
- Intrusive rocks are formed as a result of slow crystallization of magma in the deep regions of the earth's crust.
- Effusive rocks are formed by the rapid cooling of lava - magma that has poured onto the surface and other volcanic products (ashes).
About half the volume of the crust of our planet consists of igneous rocks of both types.
How are igneous rocks
The texture of magmatites is a reflection of the dynamics of magma movement and the intensity of its physicochemical interaction with the host thickness.
If the rock textures are formed simultaneously with the solidification of the magmatic melt, they are spoken of as syngenetic, among which massive, spherical, directive, porous are distinguished. The spherical texture is characterized by the presence of spherical or ellipsoidal formations in the rock; directive - by the presence of subparallel oriented grains of a flattened or elongated configuration.
In cases where a change in the primary breed occurs, the texture that arises is called epigenetic. An example is almond-stone (occurs when bubbles and pores are filled with hydrothermal products) or breccia-like texture (formed when debris of irregular shape of another magmatite accumulates in the rock).
The origin of the texture can be endogenous, associated with the processes of crystallization of rocks proper, or exogenous - depending on the action of external factors.
Textural features of intrusive rocks
The most common textures characteristic of intrusions are:
- massive with uniform distribution and random orientation of grains (for example, dunites, syenites, diorites, sometimes granite, gabbro);
- schlieren with the presence in the rock of sections of a different mineralogical composition and structure;
- banded (gneiss or directive), characterized by the alternation of bands having a different structure or mineral composition (migmatites, sometimes granite, gabbro);
- myarol with the presence in the rock mass of cavities formed by the faces of crystalline grains.
Textures of igneous rocks of effusive origin
Volcanic rocks most often have such textures as:
- Porous, bubbly and pumiceous. Possess more or less numerous voids arising as a result of degassing of magma when leaving the bowels of the earth to the surface. So, in pumice (pumiceitis), the porosity can reach 80%.
- Almond-stone. Pores in an effusive rock can be filled with chalcedony, quartz, chlorite, and carbonates.
- Ball (typical for pillow lavas).
- Shale (found in schist igneous rocks).
- Fluid - a texture in the form of a stream in the direction of movement of the lava. Inherent to vitreous volcanic rocks.
Sedimentary rocks
There are three sources of sedimentary rocks:
- redeposition of erosion products;
- precipitation from water;
- activity of various living organisms.
Accordingly, depending on the conditions and mechanism of formation, rocks of this type are divided into detrital, chemogenic and organogenic. There are also rocks of mixed origin.
The genesis of sedimentary rocks includes three stages:
- Diagenesis is the process of converting loose sediment into rock.
- Catagenesis is the stage at which a rock undergoes chemical, mineralogical, physical and structural changes. The result of catagenesis is dehydration, compaction and partial recrystallization of the rock.
- Metagenesis is a transitional stage to metamorphization. The maximum compaction of the rock occurs, the mineral composition and structure are transformed with further recrystallization until the remains of living organisms contained in the rock disappear.
The structure and texture of sedimentary rocks are determined by the primary factors that act during sedimentation (sedimentation), and secondary, which take effect at a particular stage of rock genesis.
Texture features of sedimentary rocks
This type of rock has inherent compositional features, grouped by two main features: intralayer and layer surface textures.
The relative position of the components of sedimentary rock inside the layer forms such types of textures as:
- erratic (characteristic, for example, of coarse-grained conglomerates);
- layered of various types: oblique, wavy, flysch, horizontal (most common);
- tubular or vacuolar, containing voids formed by decomposed plant debris (found in freshwater limestones);
- spotty texture of several varieties: streaky, zonal, flaky, scaly, etc .;
- patterned, characteristic of clays containing large mineral grains;
- fluid, or agitation texture with traces of disturbed primary orientation of structural elements.
The surface textures of the layer resulting from short-term changes in the sedimentation medium with subsequent rapid burial of the layer are imprints left by atmospheric precipitation or animals, ripple marks formed by winds, currents or waves of the water stream, drying cracks and other traces.
In general, sedimentary rock textures are extremely diverse due to the high variability of the conditions under which they form.
Metamorphic rocks
They are formed in the thickness of the earth's crust by changing magmatic and sedimentary rocks under the influence of physical (high pressures and temperatures) and chemical factors. The process of transforming a breed is called metamorphism; in the case of a significant change in the chemical composition, it is customary to talk about metasomatism.
The rocks of this class are grouped according to the so-called facies of metamorphism - aggregates within which they can have different composition, but form under certain similar conditions. The structure and texture of metamorphic rocks reflect the characteristics of the processes of recrystallization of the initial sedimentary or magmatic material.
Features of the addition of metamorphic rocks
The textures of the metamorphosed rocks are of the following types:
- massive (found, for example, in the deep zones of metamorphism and in metasomatic rocks of igneous origin that have retained the original texture);
- spotted - the result of contact-thermal metamorphism (spotted schists, hornfelses);
- almond-stone (slightly metamorphosed rocks, sometimes amphibolites);
- banded (gneiss) with different mineral composition of alternating strips;
- shale is the most common type of texture of metamorphic rocks.
Shale texture occurs under the influence of directional pressure. It has such varieties as plaited - in cases where shale is complicated by very small folds - and lenticular (or spectacle, with inclusions of quartz or feldspar) texture.
In addition, various types of deformational textures, such as boudage, are often inherent in metamorphic rocks.
On the distinction of concepts
It should be noted that a completely clear separation of interpretations of such closely related concepts as the structure and texture of rocks does not exist. In the structure of rocks, there are signs that can be qualified in two ways: for example, almond-rock composition of a rock is sometimes referred to as structural characteristics. Another example is oolitic limestones, for which it is difficult to distinguish features associated with the shape, size and structure of mineral grains - oolites.
The terminological ambiguity of these concepts is also manifested in the opposite use of the terms “structure” and “texture” in the English tradition. In international publications, as a rule, the generalized concept of “structural-textural features” is used without separating the features of the structure and composition of the rocks.
Nevertheless, a correct description of the texture of rocks is very important for solving many problems, for example, determining physical properties or elucidating the genesis of rocks and the dynamic conditions of their formation.