Page 439 - Global Tectonics

IMPLICATIONS OF PLATE TECTONICS 421

Not only can plate tectonics create the habitat of sinking of these regions preserves the organic layers
hydrocarbon deposits, it can also explain why certain by progressive burial.

regions are particularly rich in these deposits. A large The process of coalification refers to the physical
proportion of the Earth’s hydrocarbon reserves are and chemical changes experienced by the organic matter
located in the Middle East, and the evolution and pres- after burial in response to rising temperature and pres-
ervation of these deposits has been a consequence of a sure. On compression, water and volatiles are expelled

specific pattern of plate interactions (Irving et al., and the deposit becomes enriched in carbon. The degree

1974). of coalification is reflected in coal of different ranks,
During Mesozoic and early Cenozoic times two varying from the low rank lignite to high rank
large embayments existed on the continental shelf of anthracite.
the Afro-Arabian continent on the southern side of the Plate tectonics affects coal formation in that it con-
Tethys Ocean (Figs 13.5, 13.6). Such embayments trols the latitude of a region (Section 3.4) and creates
around the Tethys, which also included the Gulf of the environments necessary for the preservation of
México and the Persian Gulf, may have been connected organic matter, of which the most important are passive
via the proto-Mediterranean Sea, or Tethys-Atlantic continental margins (Section 7.7). Deltas formed on
seaway, which was situated at low latitudes. At about such margins produce the most favorable conditions for
100 Ma the rate of spreading of the seaway increased, coal formation, and swamps can develop on a regional
maximizing the development of hydrocarbon source scale. Present day examples include the Niger, Amazon,
rocks because of the formation of extensive, warm, and Mississippi deltas, and ancient examples the Car-
shallow seas to which were supplied large quantities of boniferous coals of North America and northwest
nutrients from the spreading center. When the seaway Europe. Intracratonic deltas, such as the Rhine, are
subsequently began to close following the development similarly productive and are likely to be preserved due
of a subduction zone at its north margin, the geometry to their stable surroundings. Coal deposits are also
of the plate movements was such as to protect the found in aulacogens and ensialic backarc basins. The
Persian Gulf from major tectonism. This arose because tectonism associated with collisional orogens provides
the rapid northerly motion of the Indian Plate absorbed an environment whereby coals increase in grade by
most of the energy associated with the collision with high-pressure metamorphism.
the Eurasian Plate. The Gulf of México was similarly
protected by northeastward motion of the Greater
Antilles. 13.2.4 Deposits related
Coal is a combustible sedimentary rock containing
in excess of 50% by weight of carbonaceous material. to climate
It is formed by the decomposition, compaction, and
diagenesis of an accumulation of terrestrial and fresh- It has already been stated that the formation of hydro-
water plant debris. Coals thus appear in the geological carbon and coal deposits is dependent upon both climate
record from Devonian times when the fi rst plants and special conditions of sedimentation. There are
appeared. certain deposits, however, which appear to be related
In order to prevent the total destruction of the solely to climate. Since climate is largely dependent
vegetable matter by biochemical decomposition, very upon latitude, north–south plate motion can be consid-
wet conditions are required to stop the decay by the ered as controlling the formation of such deposits. They
accumulation of toxic waste products. The conditions include laterites and evaporites.
under which this process occurs are controlled by The most important lateritic deposit is nickel later-
climate and topography. Normally a warm, wet climate ite, which results from the extreme weathering of the
is required to promote luxuriant growth, and this ultramafic parts of ophiolite bodies under tropical con-

should be under the condition of constant standing ditions. The original nickel content of fresh peridotite
water. Although, in regions of high rainfall, peat forms becomes enriched by a factor of about seven under the

in upland areas, it is rarely preserved due to the erosion influence of such weathering by percolating ground
experienced in this environment. The prime conditions water. These deposits are becoming increasingly impor-
for coal formation are those of flat, low-lying ground tant sources of nickel, and are exploited in the south-

invaded by swamps with stagnant water. The slow western Pacific and the northern Caribbean.