Ancient Europe, Geological History (4600 mya-Present): Introduction

Earth’s structure consists of three spherical shells surrounding an iron core. The outermost layer is the crust. Oceanic crust is a thin layer of heavy rock, chiefly basalt, under ocean floors; continental crust is a thicker layer of less dense rock, chiefly granite and related rocks, under continents.

Below the crust is the lithosphere, a layer of brittle rock broken into detached pieces, the tectonic plates. Below the lithosphere is the asthenosphere, a layer of partly molten rock, which acts as a lubricating layer, enabling the tectonic plates to slide, or ‘drift’ over lt. Below the asthenosphere is the mantle, a layer of silicate rock. The iron core consists of a liquid spherical shell encasing a solid inner core.

A continent consists of exposed Precambrian rock called a shield with a platform extending outwards beneath more recent rock. The shield and platform together form a craton, which is the stable core of the continent. A terrane is a crust fragment formed on a tectonic plate (or broken off from it) and accreted or sutured to a crust lying on another plate.

Tectonic plate movement has three categories: divergent, convergent and transform. At divergent boundaries the two plates gradually move apart. Ocean lithosphere is created and added to the edges of the plates as they move away from each other. Any continents attached to the plates move with them.

At convergent boundaries the two plates move towards each other and the impact can cause the edges of one or both plates to buckle up into mountain ranges, or the heavier denser oceanic crust to be subducted beneath the lighter less dense continental crust. The subduction process causes major earthquakes and volcanic activity. At transform boundaries the edges of the two plates grind past each other in opposite directions. Here too there is a high incidence of earthquakes.

Earth’s tectonic plate movement over geological time has carried its landmasses into a cycle of continental aggregation and dispersal. Because subduction destroys oceanic crust and the magnetic record that it carries, deducing the actions of events earlier than those of Pangaea (336-175 mya) relies on the interpretation of the present-day geology.

Seafloor spreading is a process during volcanic eruptions where magma (molten rock) oozes through rifts in mid-ocean ridges pushing the existing rock away to form new, younger rock. It helps to explain the mechanism for continental drift.