Mineral Deposits Found on Continental Shelves and Slopes

Before diving into the types of mineral deposits, let’s revise continental shelves and slopes:

• Continental Shelf is the extended submerged edge of a continent, which gently slopes into the ocean. It’s like the shallow “porch” of the continent under the sea.
• Continental Slope follows this shelf and descends more steeply into the deep ocean floor.

These areas are rich in various marine mineral deposits, which are crucial for understanding resource distribution and economic geography.

Surface Mineral Deposits on Shelves and Slopes

• The surface deposits found here are usually mixed with marine sand.
• These minerals are often remnants of materials eroded and washed from the land to the ocean floor.

Marine Placer Deposits

Now comes a very important category—Marine Placer Deposits.

What are Marine Placer Deposits?

• These are accumulations of heavy, valuable minerals like gold, cassiterite (tin), zircon, monazite, etc.
• These minerals are weathered from their parent rocks on land, then eroded and transported by rivers to the coast.
• Since they are dense, they settle near the coast on continental shelves.

Why are they important?

• These are economically valuable due to the concentration of:
  • Cassiterite (tin)
  • Ilmenite and Rutile (titanium)
  • Zircon
  • Chromite (chromium)
  • Monazite (thorium)
  • Magnetite (iron)
  • Gold and Diamonds

Key Global and Indian Reserves:

• India’s western coast has zircon.
• Kerala’s beaches have 90% of the world’s monazite—used for thorium, critical for nuclear energy.
• Australia’s east and west coasts hold ~30% of rutile reserves.
• Placer diamonds are extracted from Namibia and South Africa.
• Gold placers are found along Alaska’s East Pacific coast.
• Cassiterite is found off the coasts of Southeast Asia.

Marine Hydrocarbon Deposits: Fuel Beneath the Ocean

Now shifting focus to marine hydrocarbons—primarily oil and natural gas.

How are they formed?

• They originate in organic-rich layers buried deep (1000–2000m below).
• These hydrocarbons migrate from source rocks into geological traps, where they accumulate.

Types of Geological Traps:

• Shales
• Salt domes (also called evaporites, rich in sulphur)
• Anticlinal folds—folded rock layers where oil/gas can get trapped between permeable and impermeable strata.

Location:

• Found mainly in subsurface layers of continental shelves.
• Abyssal plains (deep-sea areas) generally lack enough sediment thickness to support hydrocarbon formation.

Hydrocarbon Reserves (As of Early 21st Century)

• Total global oil reserves: ~1 trillion barrels
  • Marine share: 25%
• Total natural gas: ~4,000 trillion cubic feet
  • Marine share: 26%

Major Offshore Hydrocarbon Fields:

• 8 in the Persian Gulf
• 8 in the North Sea
• Others in:
  • Gulf of Mexico
  • South China Sea
  • South Asia
  • Western coast of India (promising zones)
  • Even submerged coal is reported near Maharashtra’s coast

Offshore Drilling

• This is the actual process of extracting oil and gas from beneath the seabed—mainly on continental shelves.

Legal Framework: The 1958 Geneva Convention

• This international treaty gave sovereign rights to countries over the continental shelf areas adjacent to their coastline.
• This enabled nations to legally explore and extract marine resources, especially oil and gas.

Environmental Concerns: At What Cost?

While offshore drilling brings energy and revenue, it comes with serious concerns:

• Oil Spills: Example – Deepwater Horizon Spill (2010) in the Gulf of Mexico.
• Water Pollution: Chemical leakage harms marine life.
• Noise Pollution: Affects whales and dolphins, who rely on sound to navigate and communicate.
• Habitat Destruction: Coral reefs and seagrasses are often damaged.
• Safety Risks: Risk of fires, explosions, accidents, even loss of life.

Marine Phosphorite Deposits

• Phosphorites are marine rocks rich in phosphates, used to make fertilisers.
• Found as nodules in shallow waters, particularly on shelves and slopes.
• Currently not mined, because land-based phosphate deposits are easier to access and more economical.