Unit 5: Lithosphere

1. Definition, Composition, and Importance

Definition: Lithosphere

The Lithosphere is the rigid, rocky outer layer of the Earth. It consists of the crust and the solid, upper part of the mantle.

The lithosphere is what is broken into the tectonic plates that move and "float" on the semi-molten asthenosphere below.

Composition

The lithosphere is composed of rocks, which are in turn made of minerals. The most abundant minerals are silicates (e.g., quartz, feldspar), and the most abundant elements are Oxygen (O) and Silicon (Si).

Importance of Lithosphere

• Habitat for Life: It provides the physical foundation (land) for most terrestrial life.
• Source of Soil: The soil, which is the basis for all agriculture and terrestrial ecosystems, is formed from the weathering of the lithosphere's rocks.
• Source of Resources: It is the source of all our mineral resources (e.g., iron, aluminum) and energy resources (e.g., coal, oil, natural gas, which are stored within its rocks).
• Plate Tectonics: The movement of the lithosphere (plate tectonics) drives the rock cycle, forms mountains, and causes earthquakes and volcanoes.

2. Formation and Composition of Core, Mantle, and Crust

During the Earth's early molten state, it underwent differentiation, where heavy elements sank to the center and light elements floated to the top. This created the Earth's layered structure.

Core

• Formation: Formed as the heaviest elements (iron and nickel) sank to the center.
• Composition: Primarily Iron (Fe) and Nickel (Ni).
• Structure: It is divided into two parts:
  • Inner Core: A solid sphere (solid due to immense pressure).
  • Outer Core: A liquid layer. The flow of this liquid metal generates Earth's magnetic field.

Mantle

• Formation: Formed from the "middle-weight" elements.
• Composition: Composed of hot, dense, silicate rocks rich in iron and magnesium (e.g., peridotite).
• Structure: It is a "plastic solid" that flows very slowly over geologic time (like thick tar). This slow, circular movement is called mantle convection and is what drives plate tectonics.

Crust

• Formation: The lightest, least-dense materials floated to the top and cooled.
• Composition: Rich in silicon and oxygen (silicates).
• Structure: It is the thin, brittle outer "skin." There are two types:
  • Continental Crust: Thicker, less dense, granitic. (Forms continents).
  • Oceanic Crust: Thinner, more dense, basaltic. (Forms ocean floor).

3. Composition and Formation of Soil

Definition: Soil

Soil is the thin upper layer of the lithosphere, at the interface of the atmosphere, hydrosphere, and biosphere. It is a complex mixture of weathered rock, organic matter, water, and air that is capable of supporting plant life.

Composition of a Typical Soil

• ~45% Mineral Matter: Weathered rock particles (sand, silt, clay).
• ~5% Organic Matter (Humus): Decomposed plant and animal material. This is vital for fertility and water retention.
• ~25% Water: Held in pore spaces.
• ~25% Air: Held in pore spaces.

Formation of Soil (Pedogenesis)

Soil forms very slowly from the "top down." The process is governed by five key factors (CLORPT):

1. CL - Climate: The most important factor. Temperature and precipitation determine the speed and type of weathering (e.g., warm, wet climates form thick soils quickly).
2. O - Organisms (Biota): Plants, animals, bacteria, and fungi add organic matter (humus) and help weather rock.
3. R - Relief (Topography): The shape of the land. Steep slopes have thin, poorly developed soils due to high erosion. Flat valleys have thick, well-developed soils.
4. P - Parent Material: The type of rock the soil is forming from. This determines the soil's initial texture and chemical composition (e.g., limestone parent rock leads to an alkaline soil).
5. T - Time: Soil formation is a very slow process, taking hundreds to thousands of years to form just a few centimeters.

4. Physical Properties of Soil

• Soil Texture: Refers to the particle size of the mineral components. It is the relative proportion of sand (largest particle), silt (medium), and clay (smallest particle).
  • Loam: A soil with a balanced mixture of sand, silt, and clay, often considered ideal for agriculture.
  • Texture is critical because it determines porosity and permeability.
• Soil Structure: The way soil particles (sand, silt, clay) clump together to form aggregates (or "peds"). Good structure (e.g., granular) allows for good water infiltration and root growth.
• Soil Porosity: The total amount of pore space (empty space) within a soil. Clay soils have high porosity but tiny pores.
• Soil Permeability: The ability of water and air to move *through* the soil. Sandy soils have high permeability. Clay soils have very low permeability.
• Soil Colour: Can indicate its properties. Dark soils are rich in organic matter (humus). Red/yellow soils indicate iron oxidation.

5. Soil Profile

A soil profile is a vertical cross-section of the soil from the surface down to the underlying parent material. It shows distinct layers, called horizons, that have formed over time.

• O Horizon (Organic): The surface layer of fresh and decomposing organic matter (leaves, litter).
• A Horizon (Topsoil): A mix of mineral particles and dark, rich humus. This is the most biologically active layer and is crucial for plant growth.
• B Horizon (Subsoil):Please do not use LaTeX formatting for mathematical and scientific notations. Instead, write them in plain text. For example: Write E = mc² instead of $E = mc^2$. Write λ = h / p instead of $\lambda = \frac{h}{p}$. Write (Δx)(Δp) ≥ h / 4π instead of $\Delta x \cdot \Delta p \ge \frac{h}{4\pi}$. Usually lighter in color, with less humus. It's a "zone of accumulation" where minerals and clay particles (leached from the A horizon by water) are deposited.
• C Horizon (Regolith): Composed of partially weathered parent material (broken rock). It is the first stage in soil formation.
• R Horizon (Bedrock): The solid, unweathered parent rock at the base.

6. Soil Water

Water held in the pore spaces of the soil. It is crucial for making nutrients available to plants (nutrients must be dissolved in water to be absorbed by roots).

There are three main types of soil water:

1. Gravitational Water: Water that fills the large pore spaces after a heavy rain and is rapidly pulled downward by gravity. It drains quickly and is not available to plants for long.
2. Capillary Water: Water held in the small "micropores" of the soil against the pull of gravity (by adhesion and cohesion). This is the primary form of water that is available for plants to absorb.
3. Hygroscopic Water: A very thin film of water bound so tightly to the surface of soil particles that it cannot be removed by plant roots. This water is unavailable to plants.

Field Capacity: The amount of water left in a soil after the gravitational water has drained away. (Capillary + Hygroscopic water).
Wilting Point: The point at which all capillary water is used up, and only hygroscopic water remains. Plants will wilt and die.

7. Soil Conservation

Soil erosion is the removal of topsoil by the agents of wind and water, a process greatly accelerated by human activities like deforestation, overgrazing, and improper agriculture.

Soil conservation is the set of practices and strategies used to protect the soil from erosion and maintain its fertility.

Key Conservation Methods:

• Aforestation and Reforestation: Planting trees. Tree roots bind the soil, and the canopy protects the soil from rain and wind.
• Contour Ploughing: Ploughing *across* the slope of a hill instead of up and down. Each furrow acts like a small dam, trapping water and preventing it from washing soil downhill.
• Terracing: Building a series of "steps" or terraces on steep hillsides. This slows down water runoff and creates flat areas for farming.
• Strip Cropping: Planting alternating strips of different crops in the same field (e.g., a strip of corn, then a strip of hay). The cover crop (hay) traps soil that may erode from the row crop (corn).
• Cover Crops / Mulching: Planting crops like clover during the off-season, or covering the soil with mulch (like straw), so that the soil is never left bare and exposed to wind and rain.
• No-Till Farming: A method where farmers plant new seeds directly into the residue of the previous crop, without ploughing the soil. This drastically reduces erosion.