TECTONICS AND CLIMATE

Tectonics and climate are both directly and indirectly related. The di rect connection is between uplift, atmospheric circulation, and the hy drologic cycle. The indirect links are via subduction, volcanism, the introduction of gasses into the atmosphere, and through erosion and co nsumption of atmospheric gases by chemical weathering. Rifting of cont inental blocks involves broad upwarping followed by subsidence of a ce ntral valley and uplift of marginal shoulders. The result is an evolvi ng regional climate which has been repeated many times in the Phaneroz oic: first a vapor-trapping arch, followed by a rift valley with fresh -water lakes, culminating in an arid rift bordered by mountains interc epting incoming precipitation. Convergence tectonics affects climate o n a larger scale. A mountain range is a barrier to atmospheric circula tion, especially if perpendicular to the circulation. It also traps wa ter vapor converting latent to sensible heat. Broad uplift results in a shorter path for both incoming and outgoing radiation resulting in s easonal climate extremes with reversals of atmospheric pressure and en hanced monsoonal circulation. Volcanism affects climate by introducing ash and aerosols into the atmosphere, but unless these are injected i nto the stratosphere, they have little effect. Stratospheric injection is most likely to oc cur at high latitudes, where the thickness of th e troposphere is minimal. Volcanoes introduce CO2, a greenhouse gas, i nto the atmosphere. Geochemical effects of tectonic uplift and unroofi ng relate to the weathering of silicate rocks, the means by which CO2 is removed from the atmosphere-ocean system on long-term time scales.