Oceanic Relief and Islands
~8 min read
- Continental shelf: Shallow submerged extension of continent (avg 200 m deep). Rich in fish, oil, gas.
- Deep features: Continental slope → abyssal plain → trenches (deepest).
- Trenches: Mariana Trench (Pacific) = deepest point on Earth, ~11 km. Sunda Trench (Indian Ocean), Puerto Rico Trench (Atlantic).
The ocean floor is as varied as continents. From shallow shelves to deep trenches, mid-ocean ridges to volcanic islands — NDA tests these features and their names.
Ocean Floor Profile
| Zone | Depth | Note |
|---|---|---|
| Continental shelf | 0-200 m | Gentle slope; most fish; oil/gas reserves; widest off Siberia, narrow off west coast S America |
| Continental slope | 200-3000 m | Steeper drop; submarine canyons |
| Continental rise | 3000-4000 m | Gentle slope of sediment deposits |
| Abyssal plain | 3000-6000 m | Flattest place on Earth; covers most ocean floor |
| Oceanic trench | > 6000 m | Deepest: Mariana (Challenger Deep, ~11 km) |
| Mid-ocean ridge | Underwater mountain chain | Mid-Atlantic Ridge, East Pacific Rise; new crust forms here |
Types of Islands
- Continental: Part of continental shelf (Britain, Sri Lanka, New Guinea).
- Volcanic: Created by underwater volcanoes (Hawaii, Iceland, Andamans).
- Coral: Formed by reef-building corals (Maldives, Lakshadweep, Great Barrier Reef).
- Atolls: Ring-shaped coral islands enclosing a lagoon (Maldives, Marshall Islands).
India has two island groups: Andaman & Nicobar (volcanic, Bay of Bengal) and Lakshadweep (coral, Arabian Sea).
NDA PYQ Examples
Q: The deepest part of the ocean is the:
(a) Sunda Trench (b) Puerto Rico Trench (c) Mariana Trench (d) Java Trench
Answer: (c) Mariana Trench — Challenger Deep, ~11 km.
Q: Lakshadweep islands are:
(a) Volcanic (b) Coral (c) Continental (d) Tectonic
Answer: (b) Coral — atolls in the Arabian Sea.
Drill Oceanic Relief and Islands for NDA
NDA-pattern items on Oceanic Relief and Islands with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Where do new ocean crusts form?
At mid-ocean ridges, where two plates pull apart (divergent boundary). Magma rises and solidifies, adding new crust. Mid-Atlantic Ridge is the longest example.
How do atolls form?
Around a volcanic island, a fringing coral reef grows. Over time, the volcano subsides into the sea but the reef keeps growing upward → ends as a ring of coral around a lagoon.
Why is the continental shelf important?
Most marine life lives here (sunlit, nutrient-rich). It also holds the world's major offshore oil and gas reserves (North Sea, Persian Gulf, Mumbai High).
Which is India's only active volcano?
Barren Island in the Andaman Sea. Narcondam Island nearby has a dormant volcano.