science

Formation of rocks

Formation of rocks 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 Formation of rocks rather than just read about it. In short: In geology, terrestrial rocks are formed by three main mechanisms: Lithification: the gradual accumulation and compaction of sediments to form sedimentary rock. Fractional crystallisation: the solidification of melt or magma to form igneous rock.

Key takeaways

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

Reference excerpt

In geology, terrestrial rocks are formed by three main mechanisms:

Lithification: the gradual accumulation and compaction of sediments to form sedimentary rock. Fractional crystallisation: the solidification of melt or magma to form igneous rock. Metamorphism: the subjection of rock to varying degrees of pressure and heat within the Earth's crust, changing the composition and fabric to form metamorphic rock. Rock can also form in the absence of a substantial pressure gradient as material that condensed from a protoplanetary disk, without ever undergoing any transformations in the interior of a large object such as a planet or moon. Astrophysicists classify this as a fourth type of rock: primitive rock. This type is common in asteroids and meteorites.

Rock formation

19th-century efforts to synthesize rocks The synthetic investigation of rocks proceeds by experimental work that attempts to reproduce different rock types and to elucidate their origins and structures. In many cases no experiment is necessary. Every stage in the origin of clays, sands and gravels can be seen in process around us, but where these have been converted into coherent shales, sandstone and conglomerates, and still more where they have experienced some degree of metamorphism, there are many obscure points about their history upon which experiment may yet throw light. Attempts have been made to reproduce igneous rocks, by fusion of mixtures of crushed minerals or of chemicals in specially contrived furnaces. The earliest researches of this sort are those of Faujas St Fond and of de Saussure, but Sir James Hall really laid the foundations of this branch of petrology. He showed (1798) that the whinstones (diabases) of Edinburgh were fusible and if rapidly cooled yielded black vitreous masses closely resembling natural pitchstones and obsidians. If cooled more slowly they consolidated as crystalline rocks not unlike the whinstones themselves and containing olivine, augite and feldspar (the essential minerals of these rocks). Many years later Daubrée, Delesse and others carried on similar experiments, but the first notable advance was made in 1878, when Fouqué and Lévy began their researches. They succeeded in producing such rocks as porphyrite, leucite-tephrite, basalt and dolerite, and obtained also various structural modifications well known in igneous rocks, e.g. the porphyritic and the ophitic. Incidentally, they showed that while many basic rocks (basalts, etc.) could be perfectly imitated in the laboratory, the acid rocks could not, and advanced the explanation that for the crystallization of the latter the gases never absent in natural rock magmas were indispensable mineralizing agents. It has subsequently been proved that steam, or such volatile substances as certain borates, molybdates, chlorides, fluorides, assist in the formation of orthoclase, quartz and mica (the minerals of granite). Sir James Hall also made the first contribution to the experimental study of metamorphic rocks by converting chalk into marble by heating it in a closed gun-barrel, which prevented the escape of the carbonic acid at high temperatures. In 1901 Adams and Nicholson carried this a stage further by subjecting marble to great pressures in hydraulic presses and have shown how the foliated structures, frequent in natural marble, may be produced artificially.

Extraterrestrial rock Off-Earth, rock can also form in the absence of a substantial pressure gradient as material that condensed from a protoplanetary disk, without ever undergoing transformations in the interior of a large object such as planets and moons. Astrophysicists classify this as a fourth type of rock: Primitive rock. Primitive rocks "have never been heated much, although some of their constituents may have been quite hot early in the history of our Solar System. Primitive rocks are common on the surfaces of many asteroids, and the majority of meteorites are primitive rocks."

An example of a primitive rock is the achondritic iron-nickel octahedrite mineral seen in the Widmanstätten pattern that is found in a number of iron-rich meteorites. Consisting of kamacite and taenite and formed under extremely slow cooling conditions—about 100 to 10,000 °C/Myr, with total cooling times of 10 Myr or less—it will precipitate kamacite and grow kamacite plates along certain crystallographic planes in the taenite crystal lattice.

Illustrations

Worked examples

Example 1 — a first encounter with Formation of rocks

Start with the simplest possible case. Write down what Formation of rocks 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 Formation of rocks 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 Formation of rocks 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 Formation of rocks

In research: Formation of rocks 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 Formation of rocks 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: Formation of rocks 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 Formation of rocks 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 Formation of rocks in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Formation of rocks 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 Formation of rocks out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Formation of rocks in simple terms?

In geology, terrestrial rocks are formed by three main mechanisms: Lithification: the gradual accumulation and compaction of sediments to form sedimentary rock. Fractional crystallisation: the solidification of melt or magma to form igneous rock.

Why does Formation of rocks 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 Formation of rocks?

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 Formation of rocks.