Understanding the Earth Chapter Notes | Footprints Class 7: Book Solutions, Notes & Worksheets PDF Download

Earth's Formation and Geology

• Earth's Age and Dynamic Nature

  We have previously learned that the Earth is estimated to be around four to five billion years old. Initially, various gases and space dust particles cooled over time, contributing to the formation of our planet. Since its inception, Earth has been undergoing continuous changes, both internally and externally, making it a constantly evolving entity often referred to as a 'dynamic planet'.

• Understanding Geology

  Geology is the scientific discipline that focuses on studying the Earth's structure. Geologists, who are experts in this field, delve into examining how the Earth's interior is formed and structured.

  • Geologists' Expertise

    Geologists possess specialized knowledge to explore and comprehend the intricate details of the Earth's composition and history.

  • Role of Geologists

    Geologists play a crucial role in unraveling the mysteries of Earth's past, present, and potentially its future by studying rocks, minerals, and various geological processes.

  • Field of Study

    Geology encompasses a broad spectrum of topics, including tectonics, mineralogy, petrology, and stratigraphy, to name a few.

Structure of the Earth

• Crust

  • The Earth's outermost layer is called the crust, which is composed of rocks and minerals.
  • The crust is divided into two layers:
    • The upper continental crust (sial) found in continental areas, consisting of lighter rocks rich in silica and alumina.
    • The lower oceanic crust (sima) comprised of minerals such as silica and magnesium.

• Mantle

  • The mantle is the layer beneath the crust, extending to a depth of about 2,900 km.
  • It contains molten magma and is divided into upper, middle, and lower sections.
  • The middle mantle, known as the asthenosphere, is partially molten due to intense heat and flows like a liquid.

• Core

  • The core is the innermost layer of the Earth, made up of heavy metals like nickel and iron.
  • It is about 3,500 km thick and reaches depths of 6,370 km.
  • The core's high temperatures and pressures contribute to Earth's magnetic field.

Rocks and Minerals

• A mineral is a naturally occurring inorganic substance with specific physical and chemical properties.
• Minerals like quartz, feldspar, and mica are common rock-forming minerals.
• Rocks are natural masses of mineral matter found in the Earth's crust.
• Rocks can be hard like granite or diamond, or loose like clay, mud, and sand.
• They vary in colors, sizes, and textures.
• Rocks rich in a particular metallic mineral are known as ores, like bauxite for aluminum and haematite for iron.

Classification of Rocks

• Geologists categorize rocks into three main groups: igneous, sedimentary, and metamorphic.

Igneous Rocks

• Igneous rocks form from the cooling and solidification of lava or magma.
• They can be extrusive (volcanic) or intrusive (plutonic).
• Extrusive rocks like basalt cool rapidly and have fine grains, while intrusive rocks like granite cool slowly and have coarse grains.
• Igneous rocks are crystalline, hard, impermeable to water, and lack fossils.

Sedimentary Rocks

• Sedimentary rocks form from the accumulation and compression of sediments over time.
• They are softer than igneous rocks and often contain fossils.
• Examples include sandstone, shale, and limestone.

Metamorphic Rocks

• Metamorphic rocks result from the transformation of existing rocks under heat and pressure.
• They are usually crystalline, hard, impermeable, and lack fossils.
• Examples include marble, gneiss, and quartzite.

The Rock Cycle

• Igneous or primary rocks are the first type of rocks formed.
• Through natural processes like weathering and erosion, igneous rocks are broken down into sediments.
• These sediments are then compacted to form sedimentary rocks.
• Under intense heat and pressure, igneous and sedimentary rocks transform into metamorphic rocks.
• Sedimentary rocks can be buried and melted to form magma.
• Metamorphic rocks can break down into sediments, which later form sedimentary rocks and can even be buried to form magma.
• Thus, a cyclical relationship exists between igneous, sedimentary, and metamorphic rocks, driven by the Sun's energy.

Uses of Rocks and Minerals

• Rocks consist of various minerals that hold significant value for humans.
• They are utilized in constructing roads and buildings.
• Building materials like granite, sandstone, and marble are derived from rocks.
• Minerals such as natural gas, coal, petroleum, and fossil fuels serve as energy sources.
• Rocks play a crucial role in agriculture by forming soil through decomposition.
• Mineral-rich rocks (ores) are essential in various industries like iron, aluminum, gold, and uranium.
• Chemicals present in rocks are employed as fertilizers.
• Many rocks contain fossils, offering valuable insights into past life forms.