The Six Geological Provinces and Continental Shield

The entire Earth has been divided into several Geological Provinces on the basis of their origin. A geologic or geomorphic province is an entity with common geologic or geomorphic attributes. The six Geological provinces include:

• Continental Shield
• Platform-which is a shield covered with sediment
• Orogen-which leads to development of mountains
• Structural Basins-which are geological depressions, and are the inverse of domes
• Large igneous provinces-which are extremely large (More than 100,000 Km² ) accumulation of igneous rocks—intrusive, extrusive, or both—in the earth’s crust. One example of large igneous province is India’s Deccan trap.
• Extended Crust.

Continental Shields

The first order of relief contains Earth’s continents and ocean basin, which were created by the movements of plates on the surface of the Earth. The lithospheric shell of the Earth is divided into large pieces called lithospheric plates. A single plate can be as large as a continent and can move independently of the plates that surround it. This is very much similar to a great slab of ice floating on the polar sea. The continents can be geologically derived into two types of regions viz.

• Active mountain-making belts and
• Inactive regions of old, stable rock.

The mountain ranges in the active belts grow through two major complex geologic processes.

• First of them is volcanism, in which massive accumulations of volcanic rock are formed by extrusion of magma.
• Second process is the tectonic activitye. the breaking and bending of the Earth’s crust under internal Earth forces. This tectonic activity usually occurs when great lithospheric plates come together in collision. Crustal masses that are raised by tectonic activity create mountains and plateaus. At some places, both volcanism and tectonic activity combine to produce a mountain range. Tectonic activity can not only form mountains but also lower crustal masses to form depressions.

Please note that the active mountain-making belts are narrow zones that are usually found along the margins of lithospheric plates. The rest of the Lithospheric plates are much older, comparatively inactive rocks. There are two types of stable structures— continental shields and mountain roots.

The continental shields are regions of low-lying igneous and metamorphic rocks. The shields may be exposed or covered by layers of sedimentary rock. The core areas of some shields are made of rock dating back to the Archean eon, 2.5 to 3.5 billion years ago. Thus, continental shields are formed on ancient metamorphic rocks such as granitic, batholiths, and dikes. The oldest rocks on Earth are found in the shields.

Mountain roots are mostly formed of Paleozoic and early Mesozoic sedimentary rocks that have been intensely bent and folded, and in some locations changed into metamorphic rocks. Thousands of meters of overlying rocks have been removed from these old tectonic belts, so that only the lowermost structures remain. Roots appear as chains of long, narrow ridges, rarely rising over a thousand meters above sea level.