earth science

Lower oceanic crust

Lower oceanic crust 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 Lower oceanic crust rather than just read about it. In short: The lower oceanic crust is the lower part of the oceanic crust and represents the major part of it (the largest part by volume). It is generally located 4–8 km below the ocean floor and the major lithologies are mafic (ultramafic and gabbroic rocks) which derive from melts rising from the Earth's mantle.

Key takeaways

Lower oceanic crust 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 Lower oceanic crust to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Lower oceanic crust from memory before moving on to harder problems.

Reference excerpt

The lower oceanic crust is the lower part of the oceanic crust and represents the major part of it (the largest part by volume). It is generally located 4–8 km below the ocean floor and the major lithologies are mafic (ultramafic and gabbroic rocks) which derive from melts rising from the Earth's mantle. This part of the oceanic crust is an important zone for processes such as melt accumulation and melt modification (fractional crystallisation and crustal assimilation). The recycling of this part of the oceanic crust, together with the upper mantle has been suggested as a significant source component for tholeiitic magmas in Hawaiian volcanoes. Although the lower oceanic crust builds the link between the mantle and the MORB, and can't be neglected for the understanding of MORB evolution, the complex processes operating in this zone remain unclear and there is an ongoing debate in Earth Sciences about this. It is 6KM long.

Processes The lower oceanic crust connects the Earth's mantle with the MORB, where around 60% of the total magma production of the Earth happens. The three main processes happening in this region of the oceanic crust are partial melting of the Earth's mantle, melt accumulation at various depths and the chemical modification of this melts during ascent,. This three processes do not happen in a strict order but occur all simultaneously over a depth range of 4–18 km suggesting that these processes can occur already in the upper mantle. The mantle melts are most commonly modified by fractional crystallisation due to cooling and by assimilation of crustal rocks.

Spreading rates The most important parameter controlling the processes operating in the lower oceanic crust is the magma supply, this is further controlled by the spreading rate, and therefore, spreading rate is a critical variable in models for the formation of the lower oceanic crust. The rate at which plate divergence occurs at mid-ocean ridges is not the same for all ridge segments. Ridges with a spreading rate less than 3 cm/a are considered slow-spreading ridges, while those with a rate greater than 5 cm/a are considered fast-spreading ridges

Fast-spreading ridges Intensive search spanning over three decades of seismic imaging have shown that the ridge axis is underlain by a crystal mush containing a small percentage of melt, capped by a thin melt lens containing a generally high, but variable melt fraction. The completely liquid body is a thin and narrow sill-like lens (10 to 150 m [33 to 492 ft] thick and < 2 km [1.2 mi] wide). The lens is maintained by reinjection of primitive magma. The lack of any detectable large magma chamber and the common detection of small lens/mush zone at fast-spreading ridges emphasize the small magma chamber model. Modally and compositionally layered gabbroic rock is often found (or abundant) in the lower crustal sections of ophiolite. The layered lower crust is thus one of the key features of all models of fast-spreading lower crust. Nevertheless, distinct modal layering as observed in major ophiolites has rarely been observed or sampled on the ocean floor. The IODP expedition 345 was one of the first drilling project, which sampled a significant thickness of layered igneous rocks. A shallow melt can erupt through cool crust and produce sheeted dikes and volcanics, but the small chamber seems difficult to resolve with traditional ideas of fractional crystallization and crystal settling to form the thick sequence of layered gabbros and foliated gabbros and ultramafics. One proposed model is the so-called "gabbro glacier", where crystals settle in a shallow melt-dominated lens beneath the ridge axis. The weight of the accumulating crystals settling to the bottom of the magma lens induces a ductile flow and deformation within the gabbros, just like the ice in a glacier responds to accumulated snow. Nevertheless, the model fails to explain the layered variations in mineral types, the correlated layering in mineral compositional variations, and the apparently primary near-vertical fabrics in the upper gabbros that appear to represent subvertical melt conduits. Kelemen and co-workers concluded that most of the lower oceanic crust crystallized in place, and proposed "the sheeted sill" model. In the model the sills form when porous flow of rising basaltic liquids (or small melt-filled fractures) are stopped beneath permeability (earth sciences) barriers of earlier crystallized melts and pond to form the sills. Cooling rates are generally sufficiently slow that crystals and their interstitial liquids are in chemical equilibrium, as long as the liquid is immobile. However, buoyancy and/or compaction (geology) may induce liquid migration through the mush, resulting a significant compositional and microstructural modification.

Slow-spreading ridges Slow- and intermediate-spreading ridges form typically valleys about 30 to 50 km (19 to 31 mi) wide and 1 to 5 km (0.62 to 3.11 mi) deep, with step-like inward-facing scarps, similar to rift valleys on land. Compared to fast spreading-ridges, the magma supply and therefore the heat flow is low and can't maintain a persistent liquid magma chamber. Sinton and Detrick (1992) modelled a schematic cross section of an axial magma chamber beneath a slow-spreading ridge such as the Mid-Atlantic Ridge. Due to the reduced heat and magma supply, a steady-state eruptible magma lens is relinquished in favor of a sill-like mush zone and a smaller transition zone beneath the well-developed rift valley. Convection and mixing in the magma chamber is far less likely than at fast ridges. Thermal constrains led to the development of different models to reconstruct the accretion history. The "infinite leek" model suggests small magma batches, forming small "nested" intrusions. Another model proposed that crystallization could occur at depth, where temperatures are higher, the formed cumulates are then "dragged" up by mantle flow to form the lower oceanic crust. Today, a model intermediate between these two has become popular. This model is referred to as a "plum pudding", where the lower oceanic crust is constructed from a number of nested plutons that crystallize within the mantle or crust. Schwartz et al. (2005) describes another variant. He postulates that the lower crust is constructed both from the nested shallow-level plutons and from the products of deeper-seated crystallization

References

Worked examples

Example 1 — a first encounter with Lower oceanic crust

Start with the simplest possible case. Write down what Lower oceanic crust 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 Lower oceanic crust 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 Lower oceanic crust 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 Lower oceanic crust

In research: Lower oceanic crust 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 Lower oceanic crust 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: Lower oceanic crust is common in secondary-school and first-year university syllabi. It links to neighbouring topics Oceanographical terminology, Plate tectonics, Structure of the Earth, so understanding it makes those chapters shorter.
In everyday life: Look for Lower oceanic crust 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 Lower oceanic crust in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Lower oceanic crust 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.
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Frequently asked questions

What is Lower oceanic crust in simple terms?

Why does Lower oceanic crust 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 Lower oceanic crust?

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 Lower oceanic crust.