Lecture: 2 Geology and Earth crust

Geology is the study of the earth and it also known as ‘Geo-science’ or ‘Earth science’. Geology involves studying the materials that make up the earth, the features and structures found on Earth as well as the processes that act upon them. Typically, geology is divided into two categories: physical geology and historical geology. Physical geology deals with the study of the physical features of the earth and the processes acting on them. This includes volcanoes, earthquakes, rocks, mountains and the oceans; just about any feature of the earth. Historical geology is the study of the history of the earth. Historical geologists focus on what's happened to Earth since its formation.

 Mineralogy: Study of minerals

 Petrology: Study of rocks 

Palaeontology: Deals with mode of preservation of plant and animal fossils

 Interior of the Earth 

The Earth Ball consists of 3 concentric rings namely Crust, Mantle and Core.

 Crust:-

 Earth crust consist of 5 to 56 km. Density of rocks is 2.6 to 3.0 g cc-1. Distance varies from 5 to 11 km in oceans and 35 to 56 km in the continents. Dominant elements are silica and aluminium. 

Mantle

 A massive solid to semi solid layer below the crust; 2900 km in thickness, comprises mixed metals and silicate and basic rocks with density of 3.0 to 4.5 gcc-1. Dominant elements are silica and magnesium. 

Core

 Innermost portion of Earth, 3500 km in thickness, contains molten metals like Nickel, iron; average density: 9.0 to 12.0 g cc-1. Dominant elements are nickel and iron.

Exterior of the Earth

Geographically EARTH constitutes of three spheres corresponding to three states of

matter (Solid, Liquid & Gas). The solid zone is Lithosphere. The incomplete covering

of water forming seas and oceans (liquid spheres) is Hydrosphere. The gaseous

envelop over the earth surface is Atmosphere.

Atmosphere –

 1. The atmosphere forms an envelope, 322 Km over the earth’s surface. 

 2. This envelops of air that covers both lithosphere and hydrosphere is called atmosphere.

3. It contains water modules and dust, which act as nuclei for the condensation of water vapour to form cloud or fog.

4. Atmosphere contributes only 0.03 % weight to the earth.

 Hydrosphere

 1. It covers almost 3/4th of earth crust. 2. The Hydrosphere is 10 Km deep and occupies 70 % of earth’s surface. 3. Hydrosphere makes only 6.91 % of the earth mass 4. It contains absorbed air and carries particles of rocks as sediments. 5. Most of it lies within the ocean basins and also appears on surface of land in the form of rivers, ponds, lakes and as ground waters.

6. The quality of different waters, varies from place to place. Lithosphere 1. It is the solid sphere. It consists of continents, ocean basins, plains, plateaus and mountains, valleys, sand dunes, lava flows and fault scraps.

2. The interior of earth consists of rocks and minerals. 3. It is covered by gaseous and watery envelops. It amounts to 93.06 % of earth’s mass.

4. It having 95% Igneous rock and 5 % sedimentary rock

 Composition of Earth’s Crust

 The Earth’s crust is principally compassed of mineral matter. This mineral matter is made up of various elements combined together to form compounds. Each element is in combination with one or more other elements to form definite chemical compounds known as minerals. Many of these minerals in turn combine together to form aggregates, which we know as rocks. Almost all the mineral mater is present in the form of rocks in the earth’s crust. Rock is composed of elements, which in turn are made up of atoms.

Out of 106 elements known eight are sufficiently abundant as to constitute 98.6 % (by weight) of the earth crust (up to 16m). Out of eight elements, two are in great abundance and comprise ¾ the total composition of the crust. The other six elements are metals.