earth science

Pillow lava

Pillow lava is a earth 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 Pillow lava rather than just read about it. In short: Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one meter in diameter.

Key takeaways

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

Reference excerpt

Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or subaqueous extrusion. Pillow lavas in volcanic rock are characterized by thick sequences of discontinuous pillow-shaped masses, commonly up to one meter in diameter. They form the upper part of Layer 2 of normal oceanic crust.

Composition

Pillow lavas are commonly of basaltic composition, although pillows formed of komatiite, picrite, boninite, basaltic andesite, andesite, dacite or even rhyolite are known. In general, the more felsic the composition (richer in silica - resulting in an Intermediate composition), the larger the pillows, due to the increase in viscosity of the erupting lava.

Occurrence They occur wherever lava is extruded underwater, such as along marine hotspot volcano chains and the constructive plate boundaries of mid-ocean ridges. As new oceanic crust is formed, thick sequences of pillow lavas are erupted at the spreading center fed by dykes from the underlying magma chamber. Pillow lavas and the related sheeted dyke complexes form part of a classic ophiolite sequence (when a segment of oceanic crust is thrust over the continental crust, thus exposing the oceanic segment above sea level). The presence of pillow lavas in the oldest preserved volcanic sequences on the planet, the Isua and Barberton greenstone belts, confirms the presence of large bodies of water on the Earth's surface early in the Archean Eon. Pillow lavas are used generally to confirm subaqueous volcanism in metamorphic belts. Pillow lavas are also found associated with some subglacial volcanoes at an early stage of an eruption.

Formation They are created when magma reaches the surface but, as there is a large difference in temperature between the lava and the water, the surface of the emergent tongue cools very quickly, forming a skin. The tongue continues to lengthen and inflate with more lava, forming a lobe, until the pressure of the magma becomes sufficient to rupture the skin and start the formation of a new eruption point nearer the vent. This process produces a series of interconnecting lobate shapes that are pillow-like in cross-section. The skin cools much faster than the inside of the pillow, so it is very fine-grained, with a glassy texture. The magma inside the pillow cools slowly, so it is slightly coarser-grained than the skin, but it is still classified as fine grained.

Use as a way-up criterion Pillow lavas can be used as a way-up indicator in geology; that is, study of their shape reveals the attitude, or position, in which they were originally formed. Pillow lava shows it is still in its original orientation when:

Vesicles are found towards the top of a pillow (because the gas trapped as part of the rock is less dense than its solid surroundings). The pillow structures show a convex (rounded) upper surface. The pillows might have a tapered base downwards, as they may have molded themselves to any underlying pillows during their formation.

Gallery

See also Pillow basalt Spilite, a fine-grained igneous rock, resulting particularly from the alteration of oceanic basalt

References

External links NeMO Explorer, NOAA - Contains link to video of pillows being formed

Illustrations

Worked examples

Example 1 — a first encounter with Pillow lava

Start with the simplest possible case. Write down what Pillow lava claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth 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 Pillow lava 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 Pillow lava 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 Pillow lava

In research: Pillow lava appears in earth 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 Pillow lava 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: Pillow lava is common in secondary-school and first-year university syllabi. It links to neighbouring topics Structural geology, Volcanology, so understanding it makes those chapters shorter.
In everyday life: Look for Pillow lava 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 Pillow lava in 20 minutes

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

Frequently asked questions

What is Pillow lava in simple terms?

Why does Pillow lava matter?

Because it connects several earth 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 Pillow lava?

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 Pillow lava.