science

Lithology

Lithology is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Lithology rather than just read about it. In short: The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition.

Key takeaways

Lithology belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Lithology to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Lithology from memory before moving on to harder problems.

Reference excerpt

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone. Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7.

Rock type

The naming of a lithology is based on the rock type. The three major rock types are igneous, sedimentary, and metamorphic. Igneous rocks are formed directly from magma, which is a mixture of molten rock, dissolved gases, and solid crystals. Sedimentary rock is formed from mineral or organic particles that collect at the Earth's surface and become lithified. Metamorphic rock forms by recrystallization of existing solid rock under conditions of great heat or pressure. Igneous rocks are further broken into three broad categories. Igneous rock composed of broken rock fragments created directly by volcanic processes (tephra) are classified as pyroclastic rock. Pyroclastic rocks are further classified by average fragment (clast) size and whether the fragments are mostly individual mineral crystals, particles of volcanic glass, or rock fragments. Further classifications, such as by chemical composition, may also be applied. Igneous rocks that have visible mineral grains (phaneritic rocks) are classified as intrusive, while those that are glassy or very fine-grained (aphanitic) are classified as extrusive rock. Intrusive igneous rocks are usually classified using the QAPF classification, which is based on the relative content of quartz, alkali feldspar, plagioclase, and feldspathoid. Special classifications exist for igneous rock of unusual compositions, such as ultramafic rock or carbonatites. Where possible, extrusive igneous rocks are also classified by mineral content using the extrusive QAPF classification, but when determining the mineral composition is impractical, they may be classified chemically using the TAS classification. This is based on the total content of silica and alkali metal oxides and other chemical criteria. Sedimentary rocks are further classified by whether they are siliciclastic or carbonate. Siliciclastic sedimentary rocks are then subcategorized based on their grain size distribution and the relative proportions of quartz, feldspar, and lithic (rock) fragments. Carbonate rocks are classified with the Dunham or Folk classification schemes according to the constituents of the carbonate rock. Metamorphic rock naming can be based on protolith, mineral composition, texture, or metamorphic facies. Naming based on texture and a pelite (e.g., shale, mudrock) protolith can be used to define slate and phyllite. Texture-based names are schist and gneiss. These textures, from slate to gneiss, define a continually-increasing extent of metamorphism. Metamorphic facies are defined by the pressure-temperature fields in which particular minerals form. Additional metamorphic rock names exist, such as greenschist (metamorphosed basalt and other extrusive igneous rock) or quartzite (metamorphosed quartz sand).

Grain/clast size

In igneous and metamorphic rocks, grain size is a measure of the sizes of the crystals in the rock. In igneous rock, this is used to determine the rate at which the material cooled: large crystals typically indicate intrusive igneous rock, while small crystals indicate that the rock was extrusive. Metamorphism of rock composed of mostly a single mineral, such as quartzite or marble, may increase grain size (grain growth), while metamorphism of sheared rock may decrease grain size (syntectonic recrystallization). In clastic sedimentary rocks, grain size is the diameter of the grains and/or clasts that constitute the rock. These are used to determine which rock naming system to use (e.g., a conglomerate, sandstone, or mudstone). In the case of sandstones and conglomerates, which cover a wide range of grain sizes, a word describing the grain size range is added to the rock name. Examples are "pebble conglomerate" and "fine quartz arenite".

Mineralogy

In rocks in which mineral grains are large enough to be identified using a hand lens, the visible mineralogy is included as part of the description. In the case of sequences possibly including carbonates, calcite-cemented rocks or those with possible calcite veins, it is normal to test for the presence of calcite (or other forms of calcium carbonate) using dilute hydrochloric acid and looking for effervescence. The mineralogical composition of a rock is one of the major ways in which it is classified. Igneous rocks are classified by their mineral content whenever practical, using the QAPF classification or special ultramafic or carbonatite classifications. Likewise metamorphic facies, which show the degree to which a rock has been exposed to heat and pressure and are therefore important in classifying metamorphic rocks, are determined by observing the mineral phases that are present in a sample.

Colour The colour of a rock or its component parts is a distinctive characteristic of some rocks and is always recorded, sometimes against standard colour charts, such as that produced by the Rock-Color Chart Committee of the Geological Society of America based on the Munsell color system.

Fabric The fabric of a rock describes the spatial and geometric configuration of all the elements that make it up. In sedimentary rocks the main visible fabric is normally bedding, and the scale and degree of development of the bedding is normally recorded as part of the description. Metamorphic rocks (apart from those created by contact metamorphism), are characterised by well-developed planar and linear fabrics. Igneous rocks may also have fabrics as a result of flow or the settling out of particular mineral phases during crystallisation, forming cumulates.

Texture

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Lithology

Start with the simplest possible case. Write down what Lithology claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lithology before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lithology ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lithology

In research: Lithology appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Lithology in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Lithology is common in secondary-school and first-year university syllabi. It links to neighbouring topics Petrology, so understanding it makes those chapters shorter.
In everyday life: Look for Lithology outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Lithology in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Lithology means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Lithology out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Lithology in simple terms?

Why does Lithology matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Lithology?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Lithology.