Question 1 of 7: Plate Tectonics, Earth Structure & Seismology

Question 1: Plate Tectonics, Earth Structure & Seismology (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.

(a) Region 10 sits in the western Pacific near Asia — the plate occupying that area on a standard world tectonic map is the Philippine Sea Plate. Region 12 spans the broad South Pacific, which is the Pacific Plate, the largest plate on Earth. Letters G (northern Eurasia/Arctic), M (North Atlantic) and O (South Atlantic, off southern Africa) all fall on mid-ocean ridge segments — the Gakkel Ridge, the Mid-Atlantic Ridge, and its southern extension respectively — so each marks a divergent (spreading) boundary, where new oceanic crust is created as plates move apart.

(b) Dense, siderophile elements — principally iron and nickel — sank during early planetary differentiation to form the earth's core. The thickest layer by volume is the mantle, composed of rocks that are mafic/ultramafic (silicate) in composition (peridotite). The rigid lithospheric plates ride on top of the hotter, weaker asthenosphere.

(c) (i) P wave — a primary (compressional) seismic body wave; particle motion is parallel to the direction of travel, it moves through solids, liquids and gases, and it is the fastest wave, arriving first at a seismic station. (ii) Long wave (L wave) — a surface wave that travels along the Earth's surface with a long period and the largest amplitude of the seismic wave train; it causes most earthquake damage because its energy is concentrated near the surface and decays slowly with distance. (iii) Earthquake intensity — a qualitative measure of the local shaking and damage produced by an earthquake at a given site (e.g. the Modified Mercalli Scale), as distinct from magnitude, which is a single quantitative measure of the energy released at the source. (iv) Epicenter — the point on the Earth's surface located directly above the focus (hypocenter), the point at depth where fault rupture begins. (v) Elastic rebound — Reid's theory that rock on either side of a fault deforms elastically as tectonic strain accumulates, storing energy, until the accumulated stress exceeds the rock's strength; the rock then ruptures and snaps back toward its unstrained shape, releasing the stored energy as an earthquake.

(d) (i) False — phaneritic describes a coarse-grained texture (crystals visible to the naked eye, from slow intrusive cooling); very fine-grained is aphanitic. (ii) False — in Bowen's reaction series the high-temperature assemblage is olivine, pyroxene and calcium-rich plagioclase; sodium-rich plagioclase crystallizes later, at lower temperature, in the continuous branch. (iii) False — assimilation is the incorporation and melting of surrounding country (wall) rock into a magma body, changing its bulk composition; resorption of early-formed crystals back into the melt is a separate process (reaction with the melt), not assimilation. (iv) False — granite bodies (batholiths) are commonly emplaced during mountain- building (orogenic, subduction- or collision-related) episodes, but granite is a coarse, quartz-and- feldspar-rich rock that is comparatively resistant to weathering and erosion (it typically forms durable peaks and domes, e.g. Yosemite), not easily weathered. (v) False — a batholith is a large (>100 km²) plutonic body that generally cuts across the structure/layering of the host rock, i.e. it is discordant, not concordant.