Lithology studies physical properties like color, texture, composition, or grain size of visible rock outcrop units, core, or hand samples, including with a low magnification microscope or land lens. However, it may also study unconsolidated surficial materials and soil.
Lithological studies, including maps, are vital in identifying and exploring minerals. Also, they help form correlations between different lithostratigraphic units, among other importance.
Today, we will define lithology, tell you why it is important, how it differs from petrology, and what description it gives. However, as we have it elsewhere, we will not look at lithological mapping and maps, color codes, patterns, etc.
What is lithology?
Lithology is a term that describes the physical properties of a visible outcrop, core, or hand samples of rock units, including with the aid of a low magnification microscope. These physical characteristics include composition, color, texture, grain size, etc.
Therefore, it is correct to say that the lithology of any rock unit describes the megascopic physical characteristics of a visible outcrop, hand, or core sample.
Lithologies of rocks can have a detailed description of the physical properties or a gross summary of a physical character. Considering the gross summary of physical characteristics, examples of lithologies include limestone, slate, basalt, chert, coal, claystone, shale, etc.
Lastly, in the petroleum or oil industry, lithology, more precisely known as mud logging (lithology logs), is a graphic depiction of the various geological formations as drilling is ongoing, drawn on mud logs (logs used for this reason). Experts sample and examine cuttings from boreholes under a 10X magnifying microscope or do chemical tests when necessary.
Why is lithology important?
Minerals and oil often occur in areas with certain lithologies. Therefore, explorers may use the lithologic information, including maps in exploration, i.e., target areas likely to have minerals or oil. That is not all. In the petroleum industry, lithology or mud logging will help predict and calculate resources and a good area.
Secondly, lithology may help show the correlation between different areas. Lithology knowledge is the basis stratigraphers use to subdivide rock sequences into various lithostratigraphic units. They then use these units for mapping and showing correlations.
Thirdly, lithology knowledge is a vital variable in various fields, including landscape evolution, river chemical composition, water pathways fluxes, isotope provenance, supply of matter to ecosystems, and lateral land ocean matter fluxes (Hartmann & Moosdorf, 2012).
Lastly, with lithologic knowledge, you can deduce things like porosity, water saturation, permeability, and other physical properties of rocks. Also, it helps determine how rocks respond to tectonic forces and metamorphism.
How does lithology differ from petrology?
At one point in time, lithology meant or was synonymous with petrology. However, the definition of these two terms has changed with time.
Let us start by looking at the meaning of the term petrology. It will help make things much clearer. Petrology comes from the Greek words pétros, meaning rock, and logía, which means the study of, meaning that petrology is the study of rocks.
How does it then differ from lithology? Petrology is a branch of geology that studies rock origin, composition, structure, and distribution. In contrast, lithology describes the rock’s macroscopic physical properties of the visible or outcrop, core, or hand samples. It doesn’t focus on microscopic details.
However, both divide rocks into three main categories: igneous, metamorphic, and sedimentary.
What does lithology description entail?
As noted, it describes rock’s physical properties visible to the unaided eye or using a microscope with low magnification. Some of the considerations are:
1. Rock type
Lithology names are essentially rock types, and they all fall into the three categories we know in petrology. You will have igneous (form from magma), sedimentary (lithified clasts or particles including minerals), or metamorphic (initially solid rock transformed under heat and or pressure).
Igneous rocks may be intrusive (have visible grains, i.e., phaneritic), extrusive (fine-grained/aphanitic or glassy), or pyroclastic (made of volcanic fragments). You can have further classification based on chemical composition.
Sedimentary rocks are categorized as carbonate and are further classified using Folk, Dunham, or siliciclastic. Siliciclastic can further fall into various categories according to the relative amount of quartz, lithic fragments, feldspar, and grain size distribution).
Lastly, metamorphic rock naming depends on mineral composition, texture, protolith (original un-metamorphosized rock), or metamorphic facies. Also, there are other special classifications, including when dominated by a single mineral.
2. Mineralogy
Lithologies need a record of minerals if the rock has minerals large enough to see with your naked eye or a hand lens. Also, you can conduct a hydrochloric acid to check for fizziness if you suspect calcite, especially in carbonates or those cemented by calcite.
Rock minerals are important in identifying and classifying rocks in ultramafic, carbonatites, or even QAPF. Also, mineral phases can help tell the degree of metamorphism (heat and temperature).
3. Grain sizes
In igneous rocks, grain size is the measure of mineral crystal size present, indicating where or the cooling rate. Larger grains indicate cooling occurred deep in the Earth’s crust and is in intrusive igneous rocks. Smaller grains show faster cooling on or near the Earth’s surface, while very rapid cooling will result in volcanic glass (an amorphous structure with no grains).
Like igneous, grain size in metamorphic rocks indicates the size of crystals. In most cases, crystal size increases with the increase in metamorphic grade and vice versa.
Lastly, sedimentary rocks indicate the diameter of clasts that formed the rock and formed the basis for naming, e.g., conglomerate, siltstone, mudstone, etc.
4. Color
Rock color, or the color of its various components, is an important attribute in the lithology description of some rocks, making its recording necessary. It tells you more about the minerals, including those in small quantities or inclusions.
Sometimes, you can record the color against standard color charts like the one by the Rock-Color Chart Committee.
5. Texture
Texture describes grains (in sedimentary rocks) and crystals (in metamorphic and igneous rock size, shape, and arrangement).
Igneous rock texture depends on the cooling rates, affecting grain size, with some having huge or mixed sizes. Common igneous rock textures include pegmatitic, pyroclastic, glassy, aphanitic, phaneritic, and porphyritic. However, they include poikilitic, graphic, vesicular, spinifex, sub-ophitic trachytic, perthitic, etc.
Sedimentary texture may be clastic (fine to coarse-grained), made from pre-existing rock fragments, or non-clastic, which occurs in rocks that chemically precipitate from water or organic materials. Other important characteristics of sedimentary rock texture are its grading, roundness, and surface texture. These rocks can be subangular, subrounded, or well-rounded and sorting, i.e., well and poorly sorted.
Lastly, the metamorphic texture may be foliated or non-foliated, with grain size microcrystalline, fine, or coarse. Also, you can have textures with varying grain sizes, such as porphyroblast and porphyroblast.
6. Fabric
Fabric describes the spatial and geometrical arrangement of all elements that make up rocks or the rock pattern. It is another important description of lithology.
The primary fabric in sedimentary rock is bedding, which may sometimes tell you depositional direction. It would be best if you recorded the extent of its development.
The fabrics are well-developed linear and planar for metamorphic rock, except for those that form from contact metamorphism.
Lastly, in igneous rocks, fabrics are mainly flow characterized by banding and cumulate formation, where some minerals set out during crystallization.
7. Small-scale structures
Another important description in lithology is small-scale structures, as they can help a geologist make a clever guess on the origin and possible large-scale structures. We call them small-scale since they are smaller than the outcrop we study.
Sedimentary rocks will have ripple marks, sole markings, crossbedding, and mud cracks. All these features may help tell you depositional environment and paleocurrent direction information.
Asymmetric boudins and micro-folds in metamorphic rocks deeper in fault zones may help tell the sense of displacement that happened in the zone.
Lastly, igneous rocks will have small-scale structures in lavas like pahoehoe and ʻAʻā basaltic flows or in pillow lava, indicating that eruption occurred under ice or water.
8. Surficial and soil lithology
Unconsolidated surficial materials or soil can have a lithology. A description of the unconsolidated surficial materials includes their grain size, texture, thickness, composition, internal structure, and deposition environment, coupled with an interpretation of formation.
Lacustrine, fluvial, aeolian, glacial, lacustrine, coastal, and recent volcanic deposits can all have surficial lithologies.
Some lithological soil types include alluvial, basaltic, boulder clay, chalk, diorite, calcareous, and glacial till. Others are granite, loess, sandy, schist, shale, serpentine, fluvial, gabbro, and volcanic.
Frequently Asked Questions (FAQs)
Lithological discontinuity is an abrupt or clear change in mineral composition or particle size in a soil profile or column. It occurs when two or more parental materials are stacked vertically and may result from past depositional or geologic processes. Lithologic discontinuity is important in showing sedimentologic and geologic processes in the past, including age differences.
Lithologic correlation is a procedure that helps decide which geological cross-section located in different places still or once belonged to the same geological body. Stratigraphers consider physical characteristics and mineral composition with Steno’s principles to establish a relationship.
References
- Boggs, Jr, S. (2009). Sedimentary textures. In Petrology of Sedimentary Rocks (pp. 21-62). Cambridge: Cambridge University Press. doi:10.1017/CBO9780511626487.003
- Lithology. (2023, February 04). In Wikipedia.https://en.wikipedia.org/w/index.php?title=Lithology&oldid=1137326608
- Ahr, S. W., Nordt, L. C., & Schaetzl, R. J. (2017). Lithologic discontinuities in soils. International Encyclopedia of Geography: People, the Earth, Environment and Technology, 1–8. https://doi.org/10.1002/9781118786352.wbieg0816
- Hartmann, J., & Moosdorf, N. (2012). The new global lithological map database glim: A representation of rock properties at the Earth’s Surface. Geochemistry, Geophysics, Geosystems, 13(12). https://doi.org/10.1029/2012gc004370