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18-Geol-A1 Mineralogy and Petrology · May 2016

Question 10 of 13: Oceanic Crust — Cross Section, Age Contrast and Subduction

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2016-May. Closed book; no calculator permitted. Part 1 requires all six 10-mark short-answer questions (60 marks); Part 2 instructs "four of the seven ten-mark questions" (40 marks).

Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (mineral/silicate structural classification, ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, cumulates, metamorphic reactions and facies, AFM projections, volcanic processes, oceanic crust petrogenesis); Nesse, Introduction to Optical Mineralogy, 4th ed. (index-mineral optics); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sedimentary/pyroclastic textural context).

Question 10: Oceanic Crust — Cross Section, Age Contrast and Subduction (Part 2 – 10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Oceanic crust is built at a mid-ocean spreading ridge and forms a layered sequence, from the seafloor down to the Moho, whose thickness and internal proportions are remarkably uniform (typically ≈6–7 km) regardless of spreading rate.

Layered structure (ridge to Moho)

  1. Pelagic sediment Slowly-accumulating deep-sea sediment (clay, radiolarian/foraminiferal ooze); thickness increases with distance/age away from the ridge as more time has elapsed to deposit it.
  2. Layer 2A — pillow basalt Extrusive lava erupted at/near the ridge axis, quenched by seawater into characteristic pillow structures.
  3. Layer 2B — sheeted dike complex Vertical basaltic dikes that fed the overlying pillow lavas, representing the frozen magma-supply conduits of the ridge axis.
  4. Layer 3 — gabbro Coarser-grained plutonic rock crystallized slowly within the sub-ridge magma chamber itself.
  5. Moho The petrologic/seismic crust–mantle boundary (basalt-derivable gabbro above vs. residual mantle peridotite below).
  6. Upper mantle — peridotite Depleted harzburgite tectonite (the residue left after partial melting extracted the basaltic crust), locally overlain by a thin layered/cumulate ultramafic zone at the base of layer 3.
Young crust (near ridge) Old crust (far from ridge) thin sediment thick sediment 2A pillow basalt 2B sheeted dikes 3 gabbro Moho mantle peridotite mantle peridotite warm, buoyant, thin sediment cool, denser, thick sediment subducts readily (dense)
Oceanic crust layering is essentially the same thickness/sequence at any age; what changes with age is thermal state, density, sediment cover and, ultimately, subductability.

Old vs. young: how and why

Oceanic lithosphere cools and thickens with age as heat conducts away from the ridge (following the classic $\text{age-depth}\propto\sqrt{\text{age}}$ thermal-subsidence relation), so older crust sits on a cooler, thicker, denser thermal boundary layer and has subsided to greater water depth. It also carries a thicker blanket of pelagic sediment simply because more time has elapsed to deposit it, and it has undergone more extensive low-temperature hydrothermal alteration/serpentinization of its upper mantle and lower crust as circulating seawater has had longer to react with it.

Does it subduct the same?

No. As lithosphere cools and thickens with age it becomes progressively denser, eventually exceeding the density of the underlying asthenosphere, so old, cold oceanic lithosphere is strongly negatively buoyant and subducts readily, typically at a steep slab dip, pulling the plate down under its own weight (slab pull). Young, warm oceanic lithosphere is comparatively buoyant (still close to or even above asthenosphere density) and resists subduction — where young crust does enter a subduction zone (e.g. at a ridge–trench collision, or where a young back-arc basin closes) it tends to subduct at a shallower angle ("flat-slab" subduction) or can stall/jam the margin, sometimes leading to ridge subduction, ophiolite obduction, or a shift in the locus of arc magmatism, rather than the steady, steep, self-sustaining subduction typical of old Pacific-type lithosphere.