Abstract The basic serpentine structure is extremely simple. In spite of the simple crystal-chemical features involving the nearest neighbours (namely, the coordination polyhe-dra), complexity arises because of the different ways in which the basic polyhedra assemble together, forming flat layers in lizardite, curled layers in chrysotile, alternating layers in antigorite, flat kinked layers in polygonal serpentine, and flat geodesically kinked layers in polyhedral serpentine. Further complexity is derived from not-nearest-neighbour relations, such as polytyp-ism and polysomatism, that may occur as ordered and disordered distributions. A peculiar feature of chrysotile and polygonal serpentine is the presence of local fivefold symmetry. Chrysotile shares many nanoscale properties with synthetic nanotubes and nanowires. Serpentine minerals may be mutually associated, or interleaved with other layer silicates. Serpentinization and deserpentinization reactions have important implications for many extremely important large-scale processes occurring in the Earth’s crust and outer mantle. Due to their occurrence as tiny disordered crystals, meaningful structural study of serpentine minerals deals mostly with the nanoscale and may require alternative, unconventional methods. For this reason, electron microscopy techniques have long been used widely in the study of serpentine minerals, revealing a fascinating microstructural world.

In southern México, the subducting Cocos slab drastically changes its geometry: from a flat slab in central México to a ∼45° dip angle beneath Chiapas. Also, the currently active volcanic arc, the modern Chiapanecan volcanic arc, is oblique and situated far inland from the Middle America trench, where the slab depth is ∼200 km. In contrast, the Central America volcanic arc is parallel to the Middle America trench, and the slab depth is ∼100 km. A two-dimensional steady-state thermomechanical model explains the calc-alkaline volcanism by high temperature (∼1300 °C) in the mantle wedge just beneath the Central America volcanic arc and the strong dehydration (∼5 wt%) of the Cocos slab. In contrast, the thermal model for the modern Chiapanecan volcanic arc shows high P-T conditions beneath the coast where the extinct Miocene Chiapanecan arc is present, and is therefore unable to offer a reasonable explanation for the origin of the modern Chiapanecan volcanic arc. We propose a model in which the origin of the modern Chiapanecan volcanic arc is related to the space-time evolution of the Cocos slab in central México. The initiation of flat subduction in central México in the middle Miocene would have generated a hot mantle wedge inflow from NW to SE, generating the new modern Chiapanecan volcanic arc. Because of the contact between the hot mantle wedge beneath Chiapas and the proximity of a newly formed cold, flat slab, the previous hot mantle wedge in Chiapas became colder in time, finally leading to the extinction of the Miocene Chiapanecan volcanic arc. The position and the distinct K-alkaline volcanism at El Chichón volcano are proposed to be related to the arrival of the highly serpentinized Tehuantepec Ridge beneath the modern Chiapanecan volcanic arc. The deserpentinization of Tehuantepec Ridge would have released significant amounts of water into the overlying mantle, therefore favoring vigorous melting of the mantle wedge and probably of the slab.

Steinmann recognized the association of serpentinized peridotites, radiolarian cherts, and spilitic lavas 50 years ago in the Alps. Benson, 30 years ago, emphasized the world-wide nature of serpentines associated with alpine-type mountains. Nearly 20 years ago the present writer pointed out the relationship of serpentines to island arcs, adding weight to the hypothesis that island arcs represent an early stage in alpine-type mountain building. Serpentinized peridotites are probably intruded during the first great deformation of a mountain belt and do not recur in subsequent deformations of the same belt. They are typically found in two belts about 120 miles apart, one on either side of the central axis of most intense deformation, but may also occur irregularly through this zone. This enables one to date orogenies by dating the serpentines and to follow the axis of an ancient orogenic belt in some cases for thousands of miles. On this basis it was pointed out in 1937 that the great orogeny in the Appalachians was in the Ordovician, not at the end of the Paleozoic, and the Caledonian Revolution in Scotland was of the same age, and not Silurian. These views are now widely accepted. The concept that geosynclines (long narrow troughs containing a thick section of clastic sediments, commonly of shallow-water origin) localize mountain building was challenged on the basis that such a feature is not present in island arcs before the first deformation, but normally develops later because of that deformation. This idea has met with strong resistance, but the writer maintains his original stand. For most of this century a magnificent argument has gone on between field geologists who have worked on the peridotites of alpine mountains and laboratory investigators of their chemistry (particularly Bowen). The former stoutly maintains that the evidence indicates that they were intruded in a fluid state, as magmas; and the latter equally forcefully has proved to his own satisfaction that such magmas are not possible. The writer casts his vote with the field geologist and believes that the field evidence takes precedence. Probably there is some factor or constituent missing in the laboratory investigations. In recent years the idea that the serpentines were emplaced as solids has gained much favor. Some are unquestionably so emplaced, but many cannot be ( see Hiessleitner on Balkan occurrences), so that this concept fails as a general explanation. At the other extreme we find Bailey and McCallien suggesting that the serpentines of Turkey are submarine lava flows. In the last few years it has been demonstrated that peridotitic material occurs at shallow depth beneath the oceans (10–12 km). Placing it at such shallow depth beneath island arcs at the time of their initial deformation makes things appear much easier for the solid intrusionists. Were it not for the occurrence of concordant sills in the relatively little deformed flanks of the orogenic zone and the lack of an internal fabric suggesting solid flow, the hypothesis would be an attractive one. That peridotites occur on fault scarps on the Mid-Atlantic Ridge does not indicate that this feature is to be interpreted as an alpine type of mountain system. In this case the faults have merely brought the peridotitic substratum high enough to be exposed. In the oldest rocks serpentinized peridotites do not occur in belts but are ubiquitous throughout the whole terrane. The best example of this is MacGregor’s description of the Sebakwian of Southern Rhodesia, but it also seems to hold for the oldest rocks of the Canadian Shield. These rocks seem to represent something similar to the present oceanic crust strongly deformed. Finally it is suggested that many features of suboceanic topography might be the result of uplift caused by serpentinization of peridotite below the Mohorovičić discontinuity brought about by water leaking from the interior of the earth. The reaction is reversible inasmuch as increase in temperature could cause deserpentinization. This uplift and subsequent subsidence could be accounted for by this reaction.