21. Bird, P. (1986) Tectonics of the terrestrial planets, in: M. G. Kivelson (Ed.), The Solar System: Observations and Interpretations, Rubey Volume 4, Prentice Hall, Englewood Cliffs, New Jersey, 176- 206.

Abstract. Because geologic events are not reproducible, the unraveling of past tectonic events is always difficult and controversial. This is especially so for other planets, where the available data provide even less complete knowledge than we had of the Earth in 1930. Because of these limitations, we cannot study planetary tectonics empirically, so we are limited to testing for the presence of particular styles suggested by theory and intuition. The major possibilities are nonuniform contraction, despinning, volcanism, isostatic adjustment, and several varieties of convection, including homogeneous, layered, plate-tectonic, delaminating, and plumose.

All of the five planets considered show volcanism and some isostatic adjustment in their early histories, supporting the concepts of hot accretion, early differentiation, and gradual cooling. On the smaller planets (the Moon and Mercury), a global lithosphere formed early and remained unbroken, or was only slightly cracked, by the small strains of nonuniform contraction and perhaps despinning. On Mars, the plains and highland provinces may record a phase of homogeneous convection, with drift of blocks of primary crust. Now there is a global lithosphere, disrupted by the Tharsis plume, which thinned it from below, and fed volcanism, which loaded the thick lithosphere until it was bent and cracked on a global scale. The Earth also has had a history of waning convection and growing plates, with two important changes in style. After the first eon, continents became too big and buoyant to be recycled through zones of downward flow. After the Archean Era, the convection probably changed from the symmetrical homogeneous to the asymmetrical plate-tectonic form. The retention of a hydrosphere here is responsible for most of the present volcanism, the formation of continental crust, the thickness of oceanic crust, their neat separation at two different elevation levels, and the asymmetry of subduction. Our expectation that higher surface temperatures on Venus should cause even greater tectonic activity is confounded by radar images showing circular features like ancient impact craters. This observation seems to require a Martian model with a global lithosphere, where the plateaus known as "terrae" are underlain by plumes. Such a model can be reconciled with rock mechanics if Venus has no significant granitic crust. However, if future data show large areas free from craters, and a granitic composition in the terrae, then Venus would more probably be a unique example of layered convection, with complementary overturning cells in both the crust and mantle. The lithosphere might then be thinner than the crust, and consist of a set of thin, brittle "rafts" which preserve some ancient surface without impeding the convection below.