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Our Home: Earth, a Unique Life Sustaining Planet - Soil, rocks, and minerals

Grade 8CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Earth is composed of three main layers: the Crust (lithosphere), the Mantle (mesosphere), and the Core (barysphere). The crust is the outermost solid shell where life exists.

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Minerals are naturally occurring inorganic substances with a definite chemical composition and physical properties. Common examples include Quartz (SiO2SiO_2) and Feldspar.

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Rocks are aggregates of one or more minerals. They are classified into three types: Igneous (formed from cooling magma/lava), Sedimentary (formed from accumulation of sediments), and Metamorphic (formed under high heat and pressure).

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The Rock Cycle describes the continuous transformation of rocks from one type to another due to geological processes like melting, weathering, and metamorphosis.

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Soil is formed through the process of weathering, where rocks break down over long periods. The soil profile consists of distinct layers called horizons: Horizon A (Topsoil), Horizon B (Subsoil), Horizon C (Parent Rock), and Horizon R (Bedrock).

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Soil contains various components including mineral particles (sand, silt, clay), organic matter (humus), water, and air. The acidity or alkalinity of soil is measured on the pHpH scale.

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Earth's uniqueness as a life-sustaining planet is due to the presence of the Biosphere, where the Lithosphere (land), Hydrosphere (water), and Atmosphere (air) interact.

📐Formulae

Soil Porosity (%)=(1−Bulk DensityParticle Density)×100Soil\ Porosity\ (\%) = \left( 1 - \frac{Bulk\ Density}{Particle\ Density} \right) \times 100

Density of Rock (ρ)=Mass (m)Volume (V)Density\ of\ Rock\ (\rho) = \frac{Mass\ (m)}{Volume\ (V)}

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

SiO2 (Chemical formula for Quartz)SiO_2 \text{ (Chemical formula for Quartz)}

CaCO3 (Chemical formula for Limestone/Calcite)CaCO_3 \text{ (Chemical formula for Limestone/Calcite)}

💡Examples

Problem 1:

A soil sample has a bulk density of 1.3 g/cm31.3\ g/cm^3 and a particle density of 2.6 g/cm32.6\ g/cm^3. Calculate the percentage of pore space (porosity) in the soil.

Solution:

Porosity (%)=(1−1.32.6)×100Porosity\ (\%) = \left( 1 - \frac{1.3}{2.6} \right) \times 100 Porosity (%)=(1−0.5)×100Porosity\ (\%) = \left( 1 - 0.5 \right) \times 100 Porosity (%)=0.5×100=50%Porosity\ (\%) = 0.5 \times 100 = 50\%

Explanation:

Porosity is the measure of void spaces in a material. By using the ratio of bulk density to particle density, we can determine the volume not occupied by solids.

Problem 2:

If a piece of Granite rock has a mass of 270 g270\ g and occupies a volume of 100 cm3100\ cm^3, find its density.

Solution:

Density=MassVolumeDensity = \frac{Mass}{Volume} Density=270100=2.7 g/cm3Density = \frac{270}{100} = 2.7\ g/cm^3

Explanation:

The density of a rock is the mass per unit volume, typically expressed in g/cm3g/cm^3 for geological samples.

Problem 3:

Identify the type of rock formed when limestone (CaCO3CaCO_3) is subjected to extreme heat and pressure over millions of years.

Solution:

The rock formed is Marble.

Explanation:

This is a metamorphic process. Sedimentary rocks like limestone undergo recrystallization under high temperature and pressure to become metamorphic rocks like marble.

Soil, rocks, and minerals Class 8 Notes & Examples