Geography · Ch 10 — Distribution of Oceans and Continents
Distribution of Earthquakes and Volcanoes
Distribution of Earthquakes and Volcanoes
The map is a world map drawn on a rectangular grid that runs from 90°N to 90°S and across the full range of longitudes, so every continent and ocean appears in a familiar flat layout. Its purpose is to show where earthquakes and volcanoes actually occur, and it does this with four distinct symbols and shaded zones, each explained in a legend.
The most striking feature is a shaded belt that marks the deep earthquake zones. This belt runs in two great arcs: one follows the Alpine-Himalayan system, sweeping from the Mediterranean through the Himalayas and into Southeast Asia; the other traces the entire rim of the Pacific Ocean. Along this Pacific rim, the map labels the region with arrows and calls it the "Ring of Fire," because it is also where active volcanoes cluster. These shaded belts are where earthquakes have deep foci — their origins lie far below the surface.
A second set of markings appears as small dots representing shallow earthquake centres. These dots form a clear line running down the middle of the Atlantic Ocean, roughly parallel to the coastlines on either side, and then continue into the Indian Ocean. South of the Indian subcontinent, this line splits into two branches: one moves westward into East Africa, and the other meets a similar line coming from Myanmar toward New Guinea. This dotted line coincides exactly with the mid-oceanic ridges, and the earthquakes here are shallow, unlike the deep ones in the shaded belts.
Volcanic eruptions are marked with filled dots, and their distribution mirrors the earthquake pattern closely — volcanoes concentrate along the same mid-oceanic ridges and around the Pacific rim. Finally, the map shows hot spots as larger circles, with examples such as Hawaii, Yellowstone, and the Galápagos. These are isolated volcanic centres that do not lie on plate boundaries, which makes them especially important for understanding what drives plate motion. …
The first thing to notice about earthquakes and volcanoes is that they are not scattered randomly across the globe. If you plot them on a map, they fall into distinct, narrow belts. This pattern is the single most important fact in this section, because it tells us that these events are concentrated along specific zones of the Earth's crust rather than happening everywhere.
Look at a map of seismic activity and you will see a line of dots running down the central part of the Atlantic Ocean, almost parallel to the coastlines on either side. This same line continues into the Indian Ocean. A little south of the Indian subcontinent, it splits into two branches — one branch moves into East Africa, while the other meets a similar line that runs from Myanmar towards New Guinea. This entire line of dots coincides exactly with the mid-oceanic ridges, the underwater mountain chains that run through the world's oceans.
A second, shaded belt of concentrated earthquake activity is also visible on the map. This belt coincides with two major surface features: the Alpine-Himalayan mountain system and the rim of the Pacific Ocean. So, in essence, earthquakes are concentrated in two great zones — the mid-oceanic ridges and the young fold mountain belts that encircle the Pacific and stretch across Eurasia.
The depth of earthquake foci is not uniform across these zones. In the areas of the mid-oceanic ridges, the foci of earthquakes are at shallow depths. In contrast, along the Alpine-Himalayan belt and the rim of the Pacific, the earthquakes are deep-seated ones, meaning their foci lie much deeper within the Earth. …