Tectonic Landforms

Abstract

Why is the Earth so restless and relentlessly changing? And what forces can tilt, bend or fracture rocks that seem so rigid and strong into wild and amazing patterns of folds, faults or fractures? Early geologists who understood all too well that most sedimentary rocks had been laid down as soft, horizontal layers have wrestled with questions such as these for centuries. Most educated Europeans during the medieval age until the 1700s were convinced that a biblical flood played a major role in shaping the Earth’s surface and geologic change is caused by a series of catastrophes. This way of thinking was known as “catastrophism” until James Hutton, a Scottish geologist, proposed a new way of thinking in 1785 centered on the “Uniformitarian Principle” which stated simply and elegantly: “The present is the key to the past”. This concept would become the spark that ignited a new way of viewing the Earth. It assumes that the geologic forces and processes – gradual as well as catastrophic – acting on the Earth today are the same as those that have acted in the geologic past. Pivotal in this paradigm change was the observation of tectonic forces that can indeed tilt ancient sediments. At Siccar Point (today famous in the history of geology), Hutton observed an angular unconformity where the gently sloping strata of the 345 million year old Devonian Old Red Sandstone overlie near vertical layers of 425 million year old Silurian greywacke. This chapter explores briefly the world of tectonic stress and its associated landforms of faults, folds, joints, domes and basins.