Carlsberg Ridge: Magnetic Stripes, Sulphide Vents and India’s ISA Contract
Carlsberg Ridge explained: the India-Somalia plate boundary, the survey behind Vine and Matthews, its vents and India's sulphide contract.
The Carlsberg Ridge is the mid-ocean ridge of the north-western Indian Ocean, the seam on the seabed where the Indian plate and the Somali plate move apart and new ocean floor is made. It runs from just north of the equator towards the Gulf of Aden and separates the Arabian Sea from the Somali Basin. The Carlsberg Ridge matters for two reasons: a 1962 magnetic survey across it gave Frederick Vine and Drummond Matthews their evidence for seafloor spreading, and since September 2025 India has held a 15-year contract to explore its sulphide minerals.
Two ideas about it need correcting. The first is that it’s a separate mountain chain with fixed ends; some studies treat it that way, while others count it as the northern part of the Central Indian Ridge. The second is that India’s contract makes the ridge Indian. It doesn’t. The contract area lies in the international seabed, beyond any country’s jurisdiction, and India holds a right to explore, not a title.
What is the Carlsberg Ridge and where is it?
The Carlsberg Ridge is a submarine ridge on the floor of the Arabian Sea and the Indian Ocean, and it is a spreading ridge, the crest of a divergent plate boundary. Britannica gives its extent from 2°07’N 66°30’E, where it connects with the Mid-Indian (Central Indian) Ridge, north-west to the Gulf of Aden. The Carlsberg Ridge location and its anchor facts sit in the table below.
| Fact | Detail |
|---|---|
| What it is | A mid-ocean ridge and divergent plate boundary in the north-western Indian Ocean |
| Extent | From 2°07’N 66°30’E, where it meets the Mid-Indian Ridge, north-west to the Gulf of Aden (Britannica) |
| Plates | Indian plate to the north-east; Somali plate, called African in older references, to the south-west |
| Separates | The Arabian Sea to the north-east from the Somali Basin to the south-west |
| Depth | Crest at a mean depth of 1,800 to 3,600 m, rising about 2,100 m above the surrounding floor (Britannica) |
| Spreading rate | Slow: about 3 cm a year, full rate (Murton and colleagues, 2006) |
| Name | After the Carlsberg Foundation, which funded the Danish Dana expedition of 1928-1930 |
| Landmark survey | HMS Owen, November 1962, used by Vine and Matthews in Nature, 7 September 1963 |
| India’s stake | ISA exploration contract for polymetallic sulphides over 10,000 sq km, dated 15 September 2025, for 15 years |
How the Carlsberg Ridge works as a plate boundary
The Indian plate moves away from the Somali plate along the Carlsberg Ridge, and hot mantle rock rises into the gap, partly melts and cools into fresh basalt crust. That’s what a divergent boundary means: a line where the seafloor is created rather than destroyed. The general mechanism is laid out in the note on plate tectonics theory, and the basalt itself is the same lava covered in the note on volcanism and its landforms.
Which plate lies on the western side depends on the reference. Britannica, like most general texts, names the African plate. Plate-motion studies such as DeMets and Merkouriev (2021) work with a separate Somali plate, the eastern block of Africa beyond the East African Rift, and they reconstruct India-Somalia motion from about 9,000 crossings of magnetic reversals on the Carlsberg and northern Central Indian ridges. Both labels describe the same seafloor. The Somali one is simply more precise, and it explains why the country note on Somalia and this ridge share a name for the plate.
The shape of the ridge
Britannica gives the ridge’s main dimensions and its northern course:
- the crest sits at a mean depth of about 1,800 to 3,600 m below the sea surface;
- it rises about 2,100 m above the seafloor on either side;
- it turns westward around the island of Socotra and then connects with the East African Rift System by way of the Gulf of Aden;
- it coincides with the Indian Ocean’s belt of earthquakes.
Along its axis runs a rift valley, a trough down the middle of the ridge where the plates actually separate. The John Murray Expedition on the Egyptian ship Mabahiss charted that axial valley in 1933-34, and such valleys are now known to be typical of divergent boundaries. Why a spreading boundary shakes is explained in the note on earthquake causes and types.
Slow spreading and the Owen transform
The Carlsberg Ridge is a slow-spreading ridge. Bramley Murton and colleagues, writing in Geophysical Research Letters in 2006, give its full rate as 3 cm a year. Full rate means the total widening of both plates together, so each flank moves away from the axis at about half that. Put it in numbers: 3 cm a year for a million years adds 30 km of new ocean floor, which is how a slow ridge still builds an ocean given geological time.
At its northern end the ridge doesn’t simply bend into the Gulf of Aden. A transform fault, a fracture where two plates slide past each other sideways, takes over. Janin and colleagues (EGU, 2021) describe the Owen transform fault as a 300-km structure connecting the Carlsberg spreading center with the Sheba Ridge of the Gulf of Aden, and they say it forms, together with the Carlsberg Ridge, the active plate boundary between India and Somalia. So the ridge, the fault and the Sheba Ridge are three links in one boundary, running up the western edge of the Arabian Sea.
How the Carlsberg Ridge got its name
The ridge is named after a foundation, not after a person or a place. The Danish scientist Johannes Schmidt led a round-the-world voyage on the research ship Dana in 1928-1930, and he named the ridge Carlsbergryggen, Danish for Carlsberg Ridge, after the Carlsberg Foundation that paid for the voyage. The Danish scientist Å. Vedel Tåning set this on record in Nature in October 1934, replying to a proposal that the ridge be renamed after Schmidt, who had died by then.
The name tells you something useful. The ridge was first charted by a scientific expedition, so its history runs straight into the history of ocean science. Three voyages mark the path:
- 1929: the Dana records the ridge during its circumnavigation.
- 1933-34: the Mabahiss charts its axial valley and dredges basalts that differ from the Deccan lavas of India, which scientists of the day read as evidence against an old land bridge between India and Africa.
- November 1962: the Royal Navy’s HMS Owen maps the magnetic field over a central part of the ridge for the International Indian Ocean Expedition.
Nobody on the Mabahiss could read those results as plate movement. Continental drift was out of favor in the 1930s, and the meaning of the valley and the basalts became clear only after plate tectonics arrived in the 1960s.
Why the Carlsberg Ridge matters for seafloor spreading
The Carlsberg Ridge supplied the survey with which Vine and Matthews tested seafloor spreading in 1963. Their paper, “Magnetic Anomalies Over Oceanic Ridges” (Nature, volume 199, 7 September 1963), builds its new argument on the HMS Owen data: an area of 50 by 40 nautical miles centered on 5°25’N 61°45’E, with depths of 900 to 2,200 fathoms.
Here is what the survey showed, step by step:
- The seafloor was mountainous, and its ridges ran parallel to the trend of the Carlsberg Ridge.
- The magnetic map showed the same grain even more sharply: a trough of negative anomalies over the median valley, flanked by two areas of positive anomalies.
- The standard explanation, a block of unusually strongly magnetized rock under the valley, left a question open: why there and nowhere else under the ridge?
Their answer used two facts. Earth’s magnetic field has reversed many times. And basalt erupting at a ridge contains magnetite, which aligns with the field of the day as the rock cools. If the seafloor spreads, each band of new crust carries the field of its time away from the axis, and the ocean floor becomes a strip of normal and reversed stripes, mirrored on either side of the ridge. Vine and Matthews drew crustal models of alternately normal and reversed blocks, one of them for the Carlsberg Ridge, and showed that the essential form of the observed anomalies followed.
The usual picture is a tape recorder, and it holds up well: the ridge writes the field’s history into rock that then moves away. It breaks in two places. A tape runs one way, while a ridge writes two copies traveling in opposite directions. And the recording is smudged, because later lava flows and volcanoes lay fresh, differently magnetized rock over the old bands, which is why real profiles look messier than the models.
The Geological Society calls the result the first scientific test of seafloor spreading, and it credits the Canadian geologist Lawrence Morley with reaching the same idea independently, which is why the idea is also called the Vine-Matthews-Morley hypothesis. Britannica’s account of the hypothesis opens with a 1961 survey off Oregon and California by Arthur Raff and Ronald Mason, so the Indian Ocean part of the story is easy to miss.
Hydrothermal vents and polymetallic sulphides on the Carlsberg Ridge
Researchers have found evidence of active hydrothermal venting on the Carlsberg Ridge, and its rocks carry sulphide minerals. That combination is why India wanted to explore it.
A hydrothermal vent is a hot spring on the seafloor. Seawater seeps down through cracks near the ridge axis, is heated by the magma below, dissolves metals from the rock and rises back out. When that hot, metal-rich fluid meets cold seawater, the metals drop out as sulphides. PIB describes polymetallic sulphides in short as precipitates formed by hot hydrothermal fluids from oceanic crust, carrying iron, copper, zinc, silver, gold and platinum.
The record on the Carlsberg Ridge builds in three steps:
- 1993: Banerjee and Iyer, in the Journal of the Geological Society of India, describe pillow basalts from 3°35’N on the ridge that contain disseminated chalcopyrite, pyrite and magnetite, the first two being copper-iron and iron sulphides.
- 2006: Murton, Baker, Sands and German report an event plume, a sudden large release of hydrothermal water, over the ridge. It was 70 km long and up to 4,540 cubic km in volume, and it was the first such plume detected above a slow-spreading ridge.
- 2012: a team from the CSIR-National Institute of Oceanography in Goa, working with NOAA scientists, reports water-column surveys from 2007 and 2009 that found evidence for two separate vent fields, one near 3°42’N 63°40’E and another near 3°41.5’N 63°50’E, at depths of about 3,600 m or shallower.
The 2012 paper also explains why the finds mattered. Indian Ocean ridges north of the Rodrigues Triple Junction were poorly explored, with only two active sites confirmed north of 25°S. Later Chinese cruises have described vent fields on the ridge as well, among them the Daxi Vent Field.
One limit needs stating plainly. A plume shows that a vent lies within a few kilometers; it doesn’t say how large the sulphide deposit is. None of the sources used here gives a tonnage for Carlsberg Ridge sulphides, and a figure quoted without a survey behind it is a guess. The wider debate over whether such deposits should be mined is covered in the note on deep-sea mining.
India and the Carlsberg Ridge today
Since 15 September 2025 India has held a 15-year contract from the International Seabed Authority (ISA) to explore polymetallic sulphides in 10,000 sq km of the Carlsberg Ridge. The ISA register lists it as valid until 14 September 2040. It is India’s third exploration contract with the ISA and its second for sulphides.
The steps are dated:
- 18 January 2024: India applies to the ISA for sulphide exploration in the “Indian Ocean Ridge (Carlsberg Ridge)”: 10,000 sq km made up of 100 blocks of 10 km by 10 km. The same day it applies for cobalt-rich ferromanganese crusts on the Afanasy-Nikitin Seamount in the Central Indian Ocean, 3,000 sq km in 150 blocks.
- 15 September 2025: ISA Secretary-General Leticia Carvalho and Ministry of Earth Sciences Secretary M. Ravichandran sign the contract in New Delhi.
- 20 September 2025: a PIB release from the Ministry of Earth Sciences announces that India is the first country to hold two sulphide exploration contracts with the ISA and holds the largest area allotted for them.
India’s two older contracts sit elsewhere in the Indian Ocean:
- polymetallic nodules in the Central Indian Ocean Basin, signed on 25 March 2002, extended in 2017 and 2022, and due to expire on 24 March 2027;
- polymetallic sulphides in the Central Indian and Southwest Indian ridges, signed on 26 September 2016 and due to expire on 25 September 2031.
The rules come from the law of the sea. The ISA describes itself as mandated by the United Nations Convention on the Law of the Sea (UNCLOS) to manage the mineral resources of the seabed beyond national jurisdiction, an area the Convention calls the Area, for the shared benefit of humankind. At the signing the Secretary-General said India’s work there falls within UNCLOS and the 1994 Implementation Agreement. The note on UNCLOS as the constitution of the oceans covers that framework. PIB names the ministry’s autonomous National Centre for Polar and Ocean Research (NCPOR), Goa, as the institute that signed the contract on India’s side.
The contract also has a policy home. Union Minister Jitendra Singh called it a significant step towards the vision of the Deep Ocean Mission, which he said focuses on seabed mineral exploration, mining technology and India’s Blue Economy initiatives. The mission itself is covered in the note on the Deep Ocean Mission and Samudrayaan. What the contract doesn’t do is make the ridge part of India’s waters. It lies outside the country’s exclusive economic zone, in the part of the wider Indian Ocean seabed that no single state owns.
How to study the Carlsberg Ridge for exams
The Carlsberg Ridge sits in GS Paper I physical geography, under the ocean floor, plate tectonics and seafloor spreading, with a bridge to GS Paper III through deep-sea minerals and the Deep Ocean Mission, and to GS Paper II through UNCLOS and the ISA. Mains tests the mechanism rather than the name: Mains 2018 GS Paper I asked “Define mantle plume and explain its role in plate tectonics.“, and the Carlsberg survey is the example that turns such an answer from theory into evidence. Prelims leans on location and on statement questions about India’s contracts; geography accounts for 82 of the 1,403 questions in the site’s Prelims question bank.
These facts are the ones worth revising:
- Location: north-western Indian Ocean, with the Arabian Sea to the north-east and the Somali Basin to the south-west.
- Boundary: divergent, between the Indian and Somali (African) plates, linked by the Owen transform fault to the Sheba Ridge.
- Rate: slow-spreading, about 3 cm a year full rate.
- Name: from the Carlsberg Foundation, which funded the Dana expedition of 1928-1930.
- 1962-63: the HMS Owen survey, used by Vine and Matthews in Nature in 1963.
- Vents: evidence for two vent fields near 3°42’N from NIO-led surveys in 2007 and 2009.
- ISA: applied on 18 January 2024, signed on 15 September 2025, 10,000 sq km for 15 years, India’s second sulphide contract.
Three confusions cost marks. The Carlsberg Ridge and the Central Indian Ridge aren’t rivals: many sources treat the first as the northern part of the second, so an answer should say so. Nodules and sulphides aren’t the same resource: nodules lie on the floor of the Central Indian Ocean Basin, while sulphides form at vents on ridges. And an exploration contract isn’t sovereignty; the area stays outside India’s jurisdiction.
The sibling features in India’s seabed story compare like this:
| Feature | What it is | India’s ISA link |
|---|---|---|
| Carlsberg Ridge | Slow-spreading ridge on the India-Somalia boundary | Sulphide exploration contract, 10,000 sq km, 2025 |
| Central and Southwest Indian ridges | Ridges further south in the Indian Ocean | Sulphide exploration contract, 2016 |
| Central Indian Ocean Basin | Deep basin floor | Nodule exploration contract, 2002 |
| Afanasy-Nikitin Seamount | Seamount in the Central Indian Ocean | Crust exploration application, January 2024 |
| Sheba Ridge | Spreading center in the Gulf of Aden, joined by the Owen transform | None |
For preparation, the Carlsberg Ridge is worth more than its size in the syllabus suggests. It gives one example that serves a geography answer, a science answer and a law-of-the-sea answer, and an aspirant who can explain the 1962 survey in five lines has the strongest evidence for seafloor spreading ready for any plate tectonics question.
Frequently Asked Questions
Where is the Carlsberg Ridge located?
The Carlsberg Ridge lies on the floor of the north-western Indian Ocean and the Arabian Sea. Britannica traces it from 2°07’N 66°30’E, where it meets the Mid-Indian Ridge, north-west to the Gulf of Aden. It separates the Arabian Sea to the north-east from the Somali Basin to the south-west.
Which plates does the Carlsberg Ridge separate?
It separates the Indian plate from the Somali plate, which older and general references call the African plate. The two plates move apart along the ridge, which makes it a divergent boundary. At its northern end the Owen transform fault links it to the Sheba Ridge in the Gulf of Aden.
Why is it called the Carlsberg Ridge?
It is named after the Carlsberg Foundation of Denmark, which funded Johannes Schmidt’s round-the-world voyage on the research ship Dana in 1928-1930. Schmidt named it Carlsbergryggen, Danish for Carlsberg Ridge. The Danish scientist Å. Vedel Tåning recorded the origin of the name in Nature in 1934.
How is the Carlsberg Ridge linked to seafloor spreading?
A magnetic survey over the ridge by HMS Owen in November 1962 was the data Frederick Vine and Drummond Matthews used in their 1963 Nature paper. They showed that bands of normally and reversely magnetized crust, formed at the ridge as Earth’s field flipped, explained the pattern. The Geological Society calls this the first scientific test of seafloor spreading.
Is the Carlsberg Ridge a fast or slow spreading ridge?
It is a slow-spreading ridge. Murton and colleagues, writing in 2006, give its full spreading rate as about 3 cm a year, which means each side moves away from the axis at roughly half that. Until 2006 event plumes had been detected only above intermediate-rate ridges in the Pacific, which made the plume found over this ridge notable.
What minerals are found on the Carlsberg Ridge?
The ridge hosts polymetallic sulphides, which form when hot hydrothermal fluids from the oceanic crust meet cold seawater. PIB lists iron, copper, zinc, silver, gold and platinum among the metals they carry. Basalts dredged from the ridge also contain the sulphide minerals chalcopyrite and pyrite.
Does India own the Carlsberg Ridge?
No. The ridge lies in the international seabed beyond national jurisdiction, which the ISA manages under UNCLOS. India holds a 15-year contract, dated 15 September 2025, only to explore polymetallic sulphides in 10,000 sq km of it.
What is India’s contract on the Carlsberg Ridge?
It is an exploration contract with the International Seabed Authority for polymetallic sulphides over 10,000 sq km, signed in New Delhi in September 2025 and valid until 14 September 2040. It is India’s third ISA contract and its second for sulphides, making India the first country with two such contracts. The Ministry of Earth Sciences linked it to the Deep Ocean Mission.
Practice Questions
Prelims
1. Consider the following statements about the Carlsberg Ridge: 1. It separates the Arabian Sea from the Somali Basin. 2. It lies within India’s exclusive economic zone. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer: (a) The ridge divides the Arabian Sea from the Somali Basin, but it lies in the international seabed, not India’s EEZ.
2. The detailed magnetic survey of the Carlsberg Ridge that Vine and Matthews used in 1963 as evidence for seafloor spreading was made by
- (a) the research ship Dana
- (b) HMS Owen
- (c) the Mabahiss
- (d) the Challenger
Answer: (b) HMS Owen surveyed a central part of the ridge in November 1962 for the International Indian Ocean Expedition.
3. Consider the following statements about India’s contracts with the International Seabed Authority: 1. India holds two contracts for exploration of polymetallic sulphides. 2. India’s polymetallic nodule contract lies in the Central Indian Ocean Basin. 3. The Carlsberg Ridge contract allows India to extract minerals commercially. Which of the statements given above is/are correct?
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (a) The Carlsberg Ridge contract, India’s second for sulphides, is for exploration only; the nodule contract is in the Central Indian Ocean Basin.
4. The Carlsberg Ridge is named after
- (a) a Danish admiral who first sounded it
- (b) a foundation that funded an oceanographic expedition
- (c) a British survey ship
- (d) the scientist who proposed seafloor spreading
Answer: (b) Johannes Schmidt named it after the Carlsberg Foundation, which funded the Dana voyage of 1928-1930.
5. Polymetallic sulphides found on mid-ocean ridges such as the Carlsberg Ridge form mainly through
- (a) slow growth of metal layers around a nucleus on the abyssal plain
- (b) precipitation from hot hydrothermal fluids meeting cold seawater
- (c) evaporation of seawater in shallow basins
- (d) weathering of continental rocks carried by rivers
Answer: (b) Sulphides precipitate from hot, metal-rich hydrothermal fluids at vents; option (a) describes nodules.
Mains
- Explain how the magnetic survey over the Carlsberg Ridge helped establish the theory of seafloor spreading. (10 marks, 150 words)
- Describe the mid-ocean ridge system of the north-western Indian Ocean and its relationship with the India-Somalia plate boundary. (10 marks, 150 words)
- Slow-spreading ridges such as the Carlsberg Ridge were once thought to host little hydrothermal activity. Discuss how polymetallic sulphides form at such ridges and why they interest India. (15 marks, 250 words)
- India now holds three exploration contracts with the International Seabed Authority. Examine the strategic, economic and environmental dimensions of India’s deep-sea mineral exploration in the Indian Ocean. (15 marks, 250 words)
- An exploration contract in the international seabed confers no sovereignty. In the light of UNCLOS, discuss the legal status of India’s Carlsberg Ridge contract. (10 marks, 150 words)