Tectonism, Climate, and Landscape
Antony R. Orme
Earth's physical landscape is primarily an expression of tectonism and climate functioning within a gravitational context. Tectonism, namely, earth movements and the rocks and structures involved therein, forms the physical framework of the continents and ocean basins, and the environment for subsequent erosion and sedimentation. Climate, the synthesis of weather, generates the surface processes that reshape this framework by sculpting the landscape and providing habitat for plants and animals, mainly through the agency of water in its various states. In geodynamic terms, this is also a distinction between endogenic forces within Earth's crust and exogenic forces at the surface. Tectonism and climate have played these roles from early in Earth history when the nascent crust began to reorganize itself beneath a soupy atmosphere and primordial ocean. Much later, when 90% of Earth time had passed, organic activity became sufficiently organized beneath the atmospheric umbrella to begin clothing continental landscapes with vascular plants, which in turn encouraged soil formation, further climate change, and expansion of land animals. Recently, during but a small fraction of Earth time, human beings have come to refashion the landscape, like ants scurrying industriously across the surface, manipulating its resources, and generating fresh suites of environmental consequences to augment those caused by natural processes.
Tectonism and climate are not wholly independent forces. Through its impact on the distribution and shape of continents and ocean basins, tectonism influences climate, for example, by enhancing precipitation against windward mountains while reducing it in their lee and by influencing ocean circulations so important to atmospheric processes. Over time, tectonism also influences climate change, by promoting uplift favorable to prolonged cooling and eventual glaciation, by opening or closing seaways to ocean circulation and by affecting the composition of the atmosphere through the generation and consumption of crustal rocks. Though more subtle, climate in turn may affect tectonism by redistributing continental mass through erosion and deposition, thereby generating isostatic adjustments to crustal loading and unloading. Apart from tectonism's influence on plant and animal distributions, climate and vegetation are also interlinked, through biogeochemical cycles and the effect of vegetation cover on such variables as albedo and greenhouse gases, and thus on the exchange of energy between the atmosphere and the ground.
This chapter examines the interactive roles of tectonism and climate in shaping the North American landscape, without dwelling on the mechanics of tectonism or on tectonic events of the distant past. Nor does this chapter focus specifically on weather dynamics and regional expressions of climate—these are discussed in later chapters. Instead, the chapter outlines the continent's evolving tectonic framework and then focuses on the relationship between tec