Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Define the lithosphere and its composition.
• Explain how mineral deposits form.
• Describe mineral extraction and processing in South Africa.
• Identify environmental impacts of mining and mitigation strategies.
• Explain the origin of fossil fuels and their impact.

Lesson summary

This week, we delve into the lithosphere – the Earth's rigid outer layer, encompassing the crust and the upper part of the mantle. We will specifically focus on the chemical systems within the lithosphere related to mining and energy resources, particularly relevant to South Africa's economic landscape. South Africa is richly endowed with mineral resources, and understanding their formation, extraction, and impact is crucial for informed citizenship and potential career paths in fields like geology, engineering, and environmental science.

Lesson notes

2.1 The Lithosphere: Composition and Structure The lithosphere is the Earth's outermost solid layer, composed of the crust and the uppermost part of the mantle. It is broken into tectonic plates that float on the semi-molten asthenosphere. The composition varies depending on location. The crust is primarily composed of silicate rocks, with continental crust being richer in silicon and aluminum (sial) and oceanic crust being richer in silicon and magnesium (sima). 2.2 Formation of Mineral Deposits Mineral deposits are concentrations of valuable minerals in the Earth's crust. They form through various geological processes: Magmatic Processes: As magma cools, different minerals crystallize at different temperatures. This process, known as fractional crystallization, can lead to the concentration of valuable minerals like platinum, chromium, and nickel in specific layers within igneous rocks (e.g., the Bushveld Igneous Complex in South Africa, a major source of platinum group metals). Another magmatic process is related to hydrothermal fluids circulating through fractured rocks, precipitating dissolved metals to form vein deposits.

Sedimentary Processes: Weathering and erosion break down rocks, and the resulting sediments are transported and deposited elsewhere. Heavy minerals like gold and diamonds can be concentrated in placer deposits by flowing water. Chemical precipitation from solutions can also form sedimentary mineral deposits, such as banded iron formations (BIFs), which are important sources of iron ore. Evaporite deposits form as water evaporates from shallow basins, leaving behind concentrated salts and minerals like gypsum and halite.

Metamorphic Processes: When rocks are subjected to high pressure and temperature, they undergo metamorphism, which can alter their mineral composition. Some metamorphic processes can concentrate valuable minerals. For example, graphite forms through the metamorphism of carbon-rich sediments.

Weathering Processes: Intense weathering can leach away unwanted materials leaving behind concentrated deposits of minerals, especially of aluminum (bauxite). 2.3 Extraction and Processing of Minerals Mining involves extracting minerals from the Earth's crust.

Common methods include: Surface Mining (Open-pit mining): Used for large, shallow deposits. Overburden (soil and rock covering the ore) is removed to expose the ore body. This method is cost-effective but has significant environmental impacts.

Underground Mining: Used for deep deposits. Tunnels and shafts are dug to access the ore. This method is more expensive and dangerous but has a smaller surface footprint.

Alluvial Mining: Used for placer deposits in riverbeds. Sediments are washed to separate the valuable minerals (e.g., gold, diamonds). Once extracted, the ore undergoes processing to separate the valuable minerals from the waste rock (gangue).

Common processing methods include: Crushing and Grinding: Reduces the ore to a smaller size for further processing.

Concentration: Separates the valuable minerals from the gangue. Methods include gravity separation, flotation (using chemicals to make certain minerals hydrophobic), and magnetic separation.

Smelting: Uses high temperatures to extract metals from their ores. This process often produces air pollutants.

Leaching: Uses chemicals to dissolve the valuable minerals from the ore. This is used for gold and copper. 2.4 Environmental Impacts of Mining Mining activities have significant environmental impacts: Habitat Destruction: Surface mining destroys natural habitats and ecosystems.

Water Pollution: Mine drainage can contaminate surface and groundwater with heavy metals and acids.

Air Pollution: Dust from mining operations and emissions from smelting plants can pollute the air.

Soil Degradation: Mining can lead to soil erosion and loss of fertility.

Land Subsidence: Underground mining can cause the ground to collapse.

Social Impacts: Displacement of communities and health problems for workers.

Mitigation strategies include: Rehabilitation of mined areas: Replanting vegetation and restoring the land to its original state.

Water treatment: Treating mine drainage to remove pollutants.

Dust control: Using water sprays and other measures to suppress dust.

Sustainable mining practices: Minimizing environmental impacts and maximizing resource recovery.

Environmental Impact Assessments (EIAs): Conducting thorough EIAs before starting mining operations. 2.5 Fossil Fuels: Origin and Extraction Fossil fuels (coal, oil, and natural gas) are formed from the remains of ancient plants and animals that have been subjected to high pressure and temperature over millions of years.

Coal: Formed from plant matter that accumulated in swamps and was buried and compressed over time. Coal is extracted through surface and underground mining.

Oil and Natural Gas: Formed from marine organisms that accumulated on the ocean floor and were buried and transformed into hydrocarbons.