Oceans: Floor, Temperature & Salinity – NDA Geography PYQs

Question 1 [NDA 2008-I]
What is the name given to an almost circular coral reef inside which there is a lagoon?
(a) Fringing reef
(b) Barrier reef
(c) Atoll
(d) Isthmus

Answer: (c) Atoll
An atoll is a roughly circular or ring-shaped coral reef enclosing a central lagoon of shallow water. It forms when a volcanic island subsides beneath the ocean surface while the coral reef continues to grow upward. The island disappears but the coral ring remains — with the lagoon occupying the space where the island once was.

---

Question 2 [NDA 2008-II]
Sunda Trench lies in:
(a) Atlantic Ocean
(b) Pacific Ocean
(c) Indian Ocean
(d) Antarctic Ocean

Answer: (c) Indian Ocean
The Sunda Trench (also called Java Trench) is in the Indian Ocean, south of the island of Java in Indonesia. It is the deepest point in the Indian Ocean at about 7,455 metres. Students often confuse it with the Pacific because Indonesia is in the Pacific region — but this particular trench is in the Indian Ocean.

Question 3 [NDA 2016-I]
Statement I: There is high salinity in the Red Sea.
Statement II: The rate of evaporation is high in the Red Sea.

(a) Both true and II explains I
(b) Both true but II does not explain I
(c) I true but II false
(d) I false but II true

Answer: (a) Both true and II explains I
The Red Sea has very high salinity — among the highest of any major sea. This is directly caused by the very high rate of evaporation in the hot, dry climate of the region. There is almost no river input of fresh water and the sea is largely enclosed — so evaporation concentrates the salts continuously. Statement II correctly explains Statement I.

---

Question 4 [NDA 2017-II]
Mariana Trench is located in the ocean floor of:
(a) Southern Atlantic Ocean
(b) Western Pacific Ocean
(c) Eastern Pacific Ocean
(d) Northern Atlantic Ocean

Answer: (b) Western Pacific Ocean
The Mariana Trench is in the western Pacific Ocean, near the Mariana Islands — east of the Philippines. It contains the deepest known point on Earth — Challenger Deep at approximately 11,034 metres. It is not in the Atlantic Ocean.

---

Question 5 [NDA 2018-II]
The zone of sharp salinity change in vertical section of an ocean is known as:
(a) Thermocline
(b) Halocline
(c) Photic zone
(d) Pycnocline

Answer: (b) Halocline
The halocline is the zone of rapid salinity change with depth in the ocean. The thermocline is the zone of rapid temperature change with depth. The pycnocline is the zone of rapid density change with depth. The photic zone is the sunlit upper layer where photosynthesis occurs.

Question 6 [NDA 2025-II]
Which one among the following volcanic island chains is NOT associated with mid-oceanic ridge?
(a) Azores Islands
(b) Ascension Islands
(c) Hawaiian Islands
(d) Tristan da Cunha

Answer: (c) Hawaiian Islands
The Hawaiian Islands formed over a hot spot — a stationary plume of hot mantle material over which the Pacific Plate is moving. They are not associated with a mid-oceanic ridge. The Azores, Ascension Islands, and Tristan da Cunha are all located on or near the Mid-Atlantic Ridge.

---

Question 7 [NDA 2025-II]
Which of the following statements regarding oceanic trenches is/are correct?

1. They are associated with active volcanoes and strong earthquakes.
2. They are significant in the study of plate movements.

(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Answer: (c) Both 1 and 2
Both statements are correct. Oceanic trenches form at subduction zones — convergent plate boundaries — where intense seismic and volcanic activity occurs. Because they are direct products of plate subduction, they are essential features for understanding plate tectonic movements and history.