4.2 Shifting ground
In Section 3 and in Section 4 so far, we have begun with the questions of how and why humans found their way to oceanic islands, and how other living things have come to make themselves at home on these same islands. The question we have yet to consider, the one that in a way underpins these other questions, is how there came to be isolated tracts of land in the middle of a vast ocean in the first place. To answer this, we need to turn to the insights of the earth sciences.
There are hints that have surfaced in previous sections of the unit about the formation of islands. As you may recall from Section 4.1, falling sea levels facilitated the accumulation of sand and gravel, and the coral growth that helped form the islands that are today's Tuvalu. However, in order for these processes to occur, there must already be a significant protrusion from the seabed, and it is the formation of such irregularities that directs our attention to some of the most powerful forces that have shaped, and continue to shape, our planet.
Defining earth science
Earth science (also known as ‘geoscience’) is an all-embracing term for the sciences related to the study of the origin, structure and physical phenomena of the planet earth. It includes the study of rocks, oceans and fresh water, ice and the atmosphere, as well as the dynamics that connect these parts of the planet together.
As most earth scientists now agree, it is the movement of the vast rigid plates that make up the earth's crust – the process of plate tectonics – that is behind the formation of the major peaks and ranges that rise up from the surface of the planet. The location of these plates, and their direction of movement, can be seen in Figure 10.
The molten rock (or magma) that creates these tectonic plates wells up from beneath the earth's crust along submarine ridges on the seabed. In turn, this creation of new crust pushes the existing plates sideways, where they collide with other plates. When plates converge in this way, one of them will be forced downwards, deep below the earth surface, in a process of plate destruction that balances out plate creation. The immense force of one plate being driven under the other melts the crust into magma. When this occurs on continental land masses, this magma tends to erupt into volcanic mountain ranges; when it occurs under the sea, the resulting eruptions tend to give rise to an arc of volcanoes which forms the basis of oceanic islands such as those found in the western Pacific (Colling et al., 1997, pp. 114–15). These processes are depicted below.
The action of plate tectonics, showing the formation of volcanic island arcs at the meeting point of plates
In the warmer reaches of the oceans, some of the work that brings the peak or ‘cone’ created by volcanic activity to the surface can be carried out by life itself. Coral, attracted to a submerged volcano, lives out its life just beneath sea level. The skeletons of coral polyps build up into solid structures of limestone over many generations, which may then be pushed above the waves by further tectonic activity to form an island or atoll.
Once a cone rises above the sea's surface, it begins to attract wandering life forms, as we saw in Section 3.2. Yet the same processes that produce the ground on which organisms can gain a foothold can also extinguish this life, reverting a verdant island to barrenness with a later coating of ash and lava, or annihilating the island altogether in a violent outburst or subsidence (Carson, 1953; Winchester, 2004).
The Indian Ocean tsunami of December 2004 was a reminder of the immense force of the ongoing process of plate tectonics. The waves were generated by an earthquake just north of the island of Sumatra, at a point along the juncture where the Indian Plate is being driven under at a rate of around 2.5 cm or so a year as a result of its convergence with the China Plate (often included as part of the larger Eurasian Plate). While a string of active volcanoes in the region usually provides a release for the energy generated by this ongoing collision, geologists believe that a sticking point in the convergence zone led to a gradual build-up of pressure. This in turn caused a 965-km-long section of plate to subside suddenly, displacing vast amounts of water and producing the massive swells that swept across the Indian Ocean with such devastating results.
Unlike the flows and fluctuations in global climate, which we now understand to be at least partially influenced by human activity, the geological processes that give rise to islands and other major protrusions on the earth's surface remain largely impervious to all the exertions of our species. Along with upwelling of magma, the shifting and shuddering of tectonic plates are evidence that even the ground beneath our feet is in flux. Furthermore, just as human migrations and excursions, the mobility of other living things, and the currents of air and water each constitute different kinds of flow, we might also conceive of these ongoing movements of the earth's crust as a particular kind of flow. Sometimes this flow is gradual, so slow as to be imperceptible, but at other times, as I intimated in Section 1, it manifests itself cataclysmically, in ways that can render our maps obsolete in minutes or seconds.
It may be that for most of us it is only at these occasional or exceptional moments of upheaval that the flow of the ground on which we construct our territories really makes itself felt. However, no less than the flows and circulations that comprise the earth's climate, the flux of plate tectonics is a fully global process in which different parts of the earth's crust work together on the planetary scale. An understanding of this geological volatility of the earth brings another dimension to the sense of territories as constantly in the making, which we have been exploring in various ways throughout this course. In a particularly powerful way, this volatility points to the contingency of any territory, and to the possibility for unmaking that is inseparable from the potential of making and remaking. There may be no better illustration of these contingencies than oceanic islands, which are forged from some of our planet's most convulsive processes, and brought to life by some of the most risky and opportunistic migrations ever undertaken.
What does this mean for the plight of islanders like the people of Tuvalu who find themselves in the front line of potentially momentous transformations of global flow? In the following section, we return to the question of human-induced climate change in relation to some of the other active processes and forces we have been examining.