Chapter 2: Inside Our Earth

Interior Of The Earth

Our planet Earth is a dynamic sphere that is constantly changing both internally and externally. Similar to an onion, the Earth is composed of several concentric layers, one inside the other. Understanding these layers is key to understanding geological processes like earthquakes and volcanoes.

The Crust

The crust is the outermost and thinnest layer of the Earth. Its thickness varies depending on its location:

• Continental Crust: This forms the landmasses (continents) and is relatively thick, about 35 km on average. It is primarily composed of minerals rich in silica and alumina, which is why it's often referred to as sial (si-lica + al-umina).
• Oceanic Crust: This lies beneath the oceans and is much thinner, only about 5 km thick. Its main mineral constituents are silica and magnesium, giving it the name sima (si-lica + ma-gnesium).

The Mantle

Located directly beneath the crust is the mantle. This layer is significantly thicker than the crust, extending to a depth of about 2900 km. It accounts for about 84% of the Earth's volume. The mantle is composed of dense, hot rock and behaves like a semi-molten plastic, allowing the tectonic plates of the crust to move slowly over it.

The Core

The core is the innermost layer of the Earth, with a radius of approximately 3500 km. It is characterized by extremely high temperatures and immense pressure. The core is mainly composed of nickel and iron (its chemical symbol is Fe, from the Latin ferrum), which is why it is called nife (ni-ckel + fe-rrous). The core is divided into two parts:

• Outer Core: A layer of molten, liquid metal.
• Inner Core: A solid, dense ball, which remains solid despite the incredible heat because of the immense pressure at the center of the Earth.

Rocks And Minerals

The Earth's crust is built from different types of rocks. A rock is defined as any natural mass of mineral matter that makes up the Earth's crust. Rocks vary in colour, size, and texture. They are composed of one or more minerals, which are naturally occurring substances with a definite chemical composition and specific physical properties.

Minerals are vital to human life and industry. They are used as fuels (coal, petroleum), in industries (iron, aluminum, gold), and even in medicine and fertilizers.

There are three major categories of rocks:

1. Igneous Rocks

Formed from the cooling and solidification of molten magma (molten rock from the Earth's interior). The word Igneous comes from the Latin word Ignis, meaning "fire." They are also called primary rocks because they are the first rocks to be formed.

• Extrusive Igneous Rocks: These form when molten magma, known as lava once it reaches the surface, erupts from a volcano and cools down rapidly. This rapid cooling results in rocks with a very fine-grained structure. A common example is basalt, which makes up the Deccan Plateau in India.
• Intrusive Igneous Rocks: These form when molten magma cools down slowly deep inside the Earth's crust. The slow cooling allows for the formation of large mineral grains. Granite is a prime example, often used to make grinding stones for spices.

2. Sedimentary Rocks

These rocks are formed from the accumulation of sediments. The word Sedimentary comes from the Latin word sedimentum, meaning "to settle down." The process involves:

1. Weathering and erosion of existing rocks (igneous, metamorphic, or other sedimentary rocks) into small fragments.
2. Transportation of these sediments by agents like wind, water, or ice.
3. Deposition of sediments in layers.
4. Compaction and cementation of these loose layers over millions of years to form solid rock.

Sandstone (made from grains of sand) is a classic example. A unique feature of sedimentary rocks is that they often contain fossils—the preserved remains or traces of ancient plants and animals trapped within their layers.

3. Metamorphic Rocks

These rocks are formed when existing igneous and sedimentary rocks are changed by intense heat and pressure. The word Metamorphic comes from the Greek word metamorphose, meaning "change of form." The original rock's mineral composition and texture are altered without melting.

Examples of metamorphosis include:

• Clay (a sedimentary rock) changing into slate.
• Limestone (a sedimentary rock) changing into marble.

The Rock Cycle

The three rock types are interconnected through a continuous process called the rock cycle. Rocks are constantly being created, destroyed, and transformed from one type to another in a cyclical manner.

• Molten magma cools to form igneous rocks.
• Igneous rocks are broken down into sediments, which form sedimentary rocks.
• When igneous and sedimentary rocks are subjected to heat and pressure, they become metamorphic rocks.
• Under even greater heat and pressure, metamorphic rocks can melt back into magma, and the cycle begins again.