Internal Structure of Earth and Rocks

Internal Structure of Earth and Rocks

Internal Structure of the Earth

Crust

• The Earth's Crust, the outermost solid cover or shell of the Earth, is known as the Earth's crust.

• The thickness of the crust is about 30-100 km.

• It is thicker in the region of the continents and thinner in the region of the ocean floors.

• The density of the rocks in the Earth's crust ranges from 2.7 to 3 g/c.c (grams per cubic centimeter).

• The upper part of the crust consists of silica and aluminium in greater proportions. That is why it is called 'SIAL'.

Mantle

• This layer lies below the crust.

• The lower part of the crust is called 'SIMA' because the proportion of silica and magnesium is higher in this part.

• Its thickness is about 2900 km, and the density of substances in the mantle ranges from 3.0 to 4.7.

Core

• The Earth's core lies below the mantle.

• Its thickness may be about 3,471 km.

• Its radius is 6,371-78 km, according to IUGG.

• It is divided into two parts — the outer core and the inner core.

  • The outer core is probably in a liquid state.

  • The inner core is in a solid state.

• The core mainly consists of Nickel and Ferrous (Iron). Hence, it is called 'NIFE'.

• After the mantle, the Earth's density increases rapidly towards its centre and finally is more than 13.

• The temperature of the central part of the Earth may be about 5000°C.

Rocks

• The solid parts of the Earth's crust are called rocks.

• Most of the rocks are made up of two or more minerals.

• In the same type of rocks, the proportions of minerals may be different in different areas.

• Rocks may not always be hard.

• Minerals are obtained from rocks.

Types of Rocks

Rocks are classified into three main types depending on the process of their formation:

(a) Igneous

(b) Sedimentary

(c) Metamorphic

Igneous Rocks

• Hot lava pours out at the time of volcanic eruptions and cools down later on, forming rocks.

• The molten materials known as magma sometimes cool down beneath the Earth's crust, again forming rocks.

• Both these types of rocks are known as Igneous rocks.

• When the Earth's surface first became solid after it cooled down from its hot liquid state, the original rocks of the Earth's crust were formed. They are the Primary Igneous rocks, which are usually not found today.

• Igneous rocks are generally harder and granular.

• There are no layers in Igneous rocks.

• Fossils are not found in Igneous rocks.

• The formation of Igneous rocks takes place beneath and above the surface of the Earth.

• Rocks formed by the cooling of molten matter beneath the Earth's surface are called intrusive Igneous rocks.

  • Examples: Granite and Gabbro.

  • The intrusive rocks are thus crystalline.

• Sometimes, the molten matter oozes out through cracks in the Earth's crust and spreads on the surface, forming extrusive Igneous rocks.

  • Examples: Gabbro, Obsidian, Basalt, etc.

• A very large area of the Deccan Plateau consists of basalt rocks.

• These rocks contain silica (40 to 80%), along with felspar, magnesium, and iron.

• Other examples of Igneous rocks are: Granite, Pumice, Basalt, and Gabbro.

Batholith

• A large body of igneous rock formed beneath the Earth's surface by the intrusion and solidification of magma.

• It is commonly composed of coarse-grained rocks (e.g., granite or granodiorite) with a surface exposure of 100 square km (40 square miles) or larger.

• A well-known batholith is located in the Sierra Nevada range of California, U.S.

• The Idaho batholith is a geologic formation that covers about 90% of the Boise National Forest.

• Massive granitic rocks of the Coast Range Batholith, North America, are bigger than the Idaho Batholith.

Stocks

• Igneous intrusion shaped plutons are called either stocks or batholiths, depending on their size.

• Plutons larger than 100 square kilometres in area are termed batholiths, while those of lesser size are called stocks.

Laccolith

• In geology, any of a type of igneous intrusion that has split apart two strata, resulting in a domelike structure; the floor of the structure is usually horizontal.

• A laccolith is often smaller than a stock, usually less than 16 km (10 miles) in diameter.

• The thickness of laccoliths ranges from hundreds of metres to a few thousand metres.

Lopolith

• Igneous intrusion associated with a structural basin, with contacts that are parallel to the bedding of the enclosing rocks.

• In an ideal example, the enclosing sediments above and below the lopolith dip inward from all sides toward the centre, so that the lopolith is concave upward.

Phacolith

• A concordant lens-shaped pluton that typically occupies the crest of an anticline or trough of a syncline.

Dike (or Dyke)

• A tabular or sheet-like igneous body that is often oriented vertically or steeply inclined to the bedding of pre-existing intruded rocks.

• Similar bodies oriented parallel to the bedding of the enclosing rocks are called sills.

Sill

• A relatively thin tabular concordant body intruded along bedding planes.

Sedimentary Rocks

• They are formed by the deposition, sedimentation, and lithification of sediments over a long period of time.

• As layers over layers get deposited, unified sedimentary rocks are formed due to the tremendous pressure exerted by the layers above.

• Sometimes the remains of plants, dead animals, etc., are found in the deposited material.

  • Such fossil-containing sedimentary rocks are useful for studying life on Earth.

• Examples: Sandstone, Limestone, Shale.

  • Limestone is white as well as black.

  • Sandstone is dull white, pink, bright red, or sometimes black.

Metamorphic Rocks

• The nature of igneous and sedimentary rocks changes due to tremendous heat or pressure, forming new transformed rocks called metamorphic rocks.

• Minerals in the rocks get restructured due to heat and pressure, changing the original formation of the rocks.

Some examples of metamorphic rocks formed from igneous and sedimentary rocks:

Final Thoughts

The internal structure of the Earth is a fascinating study that reveals how our planet is composed of distinct layers — the crust, mantle, and core — each with unique properties.

Understanding rocks and their types (igneous, sedimentary, and metamorphic) helps us trace the Earth's geological history and the processes that have shaped its surface. The concepts of formations like batholiths, laccoliths, and dikes further deepen our knowledge of volcanic and tectonic activities.

Fossils found in sedimentary rocks give us a glimpse into past life forms and evolution on Earth. Metamorphic rocks, formed by heat and pressure, showcase nature’s ability to transform existing materials into something new. Overall, this knowledge forms the foundation of geology and is crucial for students and enthusiasts to appreciate the dynamic nature of our planet.