A mobile downwarping of the crust of the Earth, either elongate or basinlike, measured in scores of kilometers, in which sedimentary and volcanic rocks accumulate to thicknesses of thousands of meters. A geosyncline may form in part of a tectonic cycle, in which orogeny follows. Recognition of the plate structure of the lithosphere has led to appreciation that nearly all geosynclinal phenomena are related to ocean opening and closing. Compare: mobile belt geanticline.