(a) Explain what is ring of fire? How many plates and geographic areas are associated with ring of fire? (10 marks) (b) What is…

(a) Ring of Fire

The Ring of Fire is a horseshoe-shaped belt about 40,000 km long around the Pacific Ocean, where subduction of oceanic plates produces most of the world's volcanoes and earthquakes. It holds about three-quarters of the world's active and dormant volcanoes and about 90 per cent of earthquakes.

Plates involved: the Pacific plate is the main plate, subducting beneath or against the North American, South American, Eurasian, Philippine Sea and Indo-Australian plates; the Nazca, Cocos and Juan de Fuca plates subduct beneath the Americas; and the Antarctic plate borders it in the south. Plates in total are about ten major ones and a few microplates.

Geographic areas: the Andes (Chile, Peru), Central America and Mexico, the Cascades, Alaska and the Aleutian arc, Kamchatka and the Kuril arc, Japan, the Philippines, Indonesia, Papua New Guinea, Tonga–Kermadec and New Zealand, and Antarctica (Mount Erebus).

(b) Geographic Information System (GIS)

A GIS is a computer-based system to capture, store, manage, analyse and display spatially referenced data.

Concepts: georeferenced location on a coordinate system; layers of data, either vector (points, lines, polygons) or raster (grids); attributes linked to each feature; topology; and spatial analysis.

Components: hardware (computers, GPS, scanners, plotters), software (ArcGIS, QGIS), data (spatial and attribute), people (analysts), and methods or procedures.

Functions: input and digitising of maps and remote-sensing data; storage and database management; query and retrieval; analysis such as overlay, buffering, terrain and network analysis; and output as maps and reports. In geology it is used for mineral prospectivity maps, hazard zoning, structural and lineament analysis and groundwater studies.

(c) Stereoscopy and elements of photo interpretation

Stereoscopy is the viewing of two overlapping photographs of the same area taken from different positions, so that each eye sees one photo and the brain forms a three-dimensional model. Overlap is about 60 per cent along the flight line and 20–30 per cent between strips. A pocket or mirror stereoscope is used.

Advantages: three-dimensional view with exaggerated relief; recognition of landforms, drainage and slope; measurement of heights, slopes and dip of beds with a parallax bar; mapping of faults, folds, lineaments and rock units; and better discrimination of features than from a single photo.

Elements of photo interpretation: tone or colour, texture, pattern, shape, size, shadow, site, association and resolution.

(d) Stereographic projection in structural geology

Stereographic projection shows three-dimensional orientations of planes and lines on a two-dimensional net, using the lower hemisphere of a reference sphere.

Types: the equal-angle (Wulff) net, which keeps angles and is suited to constructions, and the equal-area (Schmidt or Lambert) net, which keeps equal areas and is suited to statistical plots of poles; a polar net is also used.

Nomenclature: primitive circle, great circles (planes through the centre), small circles, the pole of a plane, trend and plunge of a line, strike and dip, and rake or pitch.

Plotting: a plane is plotted as a great circle by rotating the tracing paper to its strike and counting the dip; the pole to a plane is plotted at 90° from the great circle; a line is a point given by trend and plunge; the intersection of two planes is the point where the great circles cross; and poles are contoured to find preferred orientations, as in a π-diagram for fold axes.

(e) Stress and strain ellipsoids

The stress ellipsoid represents the state of stress at a point by a three-axis ellipsoid whose semi-axes are the principal stresses σ1 ≥ σ2 ≥ σ3, acting on planes with no shear stress.

The strain ellipsoid is the shape into which a unit sphere is deformed by homogeneous strain. Its semi-axes are the principal stretches X ≥ Y ≥ Z, with X the greatest extension and Z the greatest shortening, so axes lie along the principal strain directions. For coaxial deformation of isotropic material, strain and stress axes coincide.

Shape is shown by Flinn's diagram, with a = X/Y and b = Y/Z and k = (a−1)/(b−1). If k > 1, the ellipsoid is prolate (cigar-shaped, constriction, L-tectonite); if k < 1, oblate (pancake, flattening, S-tectonite); if k = 1, plane strain. The ellipsoid shows how folds, boudins, foliations and lineations form.