Question 2 of 7: Short (Paragraph) Answer

Question 2: Short (Paragraph) Answer (20 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.

1) Bowen’s Reaction Series

Bowen’s Reaction Series (N.L. Bowen, from experimental crystallization of basaltic magma) describes the order in which silicate minerals crystallize from a cooling melt, explaining why certain minerals commonly occur together while others rarely do. It has two branches merging at their low-temperature end:

Discontinuous (mafic/ferromagnesian) branch: minerals crystallize in a stepwise sequence, each reacting with the remaining melt to form the next mineral as temperature falls — olivine (isolated SiO₄ tetrahedra) → pyroxene (single chains) → amphibole (double chains) → biotite mica (sheets). Each step is a distinct mineral with a different crystal structure, hence “discontinuous.”

Continuous (plagioclase feldspar) branch: a single mineral family, plagioclase feldspar, changes composition smoothly as it crystallizes, from Ca-rich (anorthite) at high temperature through intermediate compositions to Na-rich (albite) at lower temperature, via continuous ionic substitution (Ca²⁺/Al³⁺ ↔ Na⁺/Si⁴⁺) without a change in crystal structure.

The two branches converge and continue as a single sequence at lower temperature: K-feldspar → muscovite mica → quartz, the last minerals to crystallize. The series predicts crystallization order in an igneous rock and, in reverse, weathering stability (Goldich’s weathering series): minerals crystallizing first at high temperature, furthest from surface conditions (olivine, Ca-plagioclase), are least stable and weather fastest, while quartz, crystallizing last, is the most weathering-resistant.

2) Structure of the Earth’s interior

The Earth is divided into concentric compositional and mechanical layers, established from seismic-wave travel times, density/moment-of-inertia constraints and meteorite (chondrite) composition analogues.

• Crust (compositional layer, ~5–70 km thick): the thin, brittle, low-density outermost rind. Oceanic crust (~5–10 km) is thin, dense, mafic (basalt/gabbro); continental crust (~30–70 km, thickest under mountain belts) is thicker, less dense, felsic-to-intermediate (granite/granodiorite, metamorphic and sedimentary cover).
• Mantle (~2,900 km thick, from the Moho to ~2,900 km depth): ultramafic peridotite (olivine + pyroxene dominated), by far the largest layer by volume (~84% of Earth’s volume). Mechanically subdivided into the rigid lithospheric mantle (uppermost, welded to the crust to form the plates), the weak, partially molten asthenosphere (~100–350 km, flows plastically over geological time and enables plate motion), and the stiffer mesosphere/lower mantle extending to the core-mantle boundary.
• Outer core (~2,200 km thick, ~2,900–5,150 km depth): liquid iron-nickel alloy with minor light elements (S, O, Si). Its vigorous convective/rotational flow, coupled with the Earth’s rotation, generates the geodynamo and the planet’s magnetic field; it is liquid because temperature there exceeds the iron-alloy melting point at that pressure.
• Inner core (~1,220 km radius, centre of the Earth): solid iron-nickel alloy, despite even higher temperature than the outer core, because the extreme pressure at the centre raises the melting point above the local temperature. It slowly grows as the Earth cools and the outer core progressively crystallizes onto it.