Divergent and transform boundaries

Detail

1. Divergent boundary: what it is

• Divergent boundary: a line where two plates pull apart from each other.

• Plate: a large, rigid slab of the lithosphere (the earth's hard outer shell of crust plus the upper part of the mantle).

• How new crust forms here:

• The plates move apart and a gap opens along the line.
• Magma (hot molten rock from the mantle) rises into the gap.
• The magma cools and hardens into new oceanic crust.

• The plates keep moving apart, and the new crust is carried away to both sides.

• Because new sea floor is made here, divergent boundaries are called spreading sites.

• Mid-ocean ridge: the long underwater mountain chain that forms where this happens. The newest rocks lie along the crest of the ridge. Rocks get older the farther they are from the ridge. This pattern was the main evidence for sea-floor spreading (Hess, 1960s).
• Constructive margin: another name for a divergent boundary, because crust is created here.

2. Mid-Atlantic Ridge: the classic example

• It runs north to south down the middle of the Atlantic Ocean.
• Plates on either side:

Part of the ridge	Plate to the west	Plate to the east
North Atlantic	North American plate	Eurasian plate
South Atlantic	South American plate	African plate

• Spreading rate: about 2.5 cm per year on average, or 25 km in a million years [2].
• Long-term effect: over 100–200 million years, sea-floor spreading has widened the Atlantic from a narrow channel between the continents into a huge ocean [2]. That narrow channel opened when Pangaea broke up.
• Iceland: this is the part of the Mid-Atlantic Ridge that rises above sea level.
• It sits on the boundary between the North American and Eurasian plates [2].
• There is a hotspot under it. A hotspot is a fixed spot in the mantle where unusually hot material rises and feeds extra magma. That is why so much rock has piled up that the ridge rises out of the sea.
• Krafla volcano area in north Iceland: ground cracks widened and new ones opened during rifting episodes between 1975 and 1984. In total the ground moved about 7 metres apart [2]. This shows that spreading is happening today.

3. Divergence on land: the East African Rift

• Rift valley: a long valley with steep sides. It forms when the crust is stretched, cracks along parallel faults, and the block between the faults sinks.
• The East African Rift:
• It is about 6,000 km long.
• It runs from the Afar region (Ethiopia) through Kenya and Tanzania.

• Along it, the African plate is splitting into two plates: the Nubian plate (west) and the Somali plate (east) [3].

• Afar triple junction: a triple junction is a point where three plates meet. At Afar, three plates are pulling away from one another: the Arabian plate, the Nubian plate and the Somali plate [3].

• Arm 1, Red Sea: formed when spreading separated Arabia from Africa [2].
• Arm 2, Gulf of Aden: another young ocean opening from the same junction.

• Arm 3, East African Rift: still a rift on land. No ocean has formed here yet.

• Features formed:

• Rift lakes (long, deep lakes that fill the sunken blocks): Tanganyika, Malawi, Turkana.

• Volcanoes (magma rises through the thinned crust): Kilimanjaro (Tanzania), Nyiragongo (DR Congo).

• Future: USGS says the East African Rift Zone may become the earth's next major ocean. If the plates separate fully, the Horn of Africa could become an island [2].

• Stages of opening an ocean, using real examples:

Stage	What happens	Example
Continental rift	Land stretches and cracks; rift valleys, lakes and volcanoes form	East African Rift
Young, narrow ocean	Sea water floods the rift; new oceanic crust begins to form	Red Sea, Gulf of Aden
Mature ocean	A wide ocean with a mid-ocean ridge in the middle	Atlantic Ocean, Mid-Atlantic Ridge

4. Transform boundary: what it is

• Transform boundary: a line where two plates slide sideways past each other, moving horizontally.
• Crust is neither made nor destroyed here. That is why it is also called a conservative margin.
• The plates lock, strain builds up, and then the strain is released suddenly. The result is strong, shallow earthquakes. There is usually little or no volcanic activity.

5. Transform faults on mid-ocean ridges

• Transform fault: a break (plane of separation) that runs roughly at right angles to a mid-ocean ridge. It cuts the ridge into a series of offset segments, so the ridge looks like a zig-zag on a map.
• Why the ridge gets offset:
• Eruptions do not happen along the whole crest of the ridge at the same time.
• So different blocks of a plate move away from the ridge axis at different rates.
• The earth's rotation also affects the separated blocks.

• Result: the ridge breaks into short segments joined by transform faults.

• Fracture zone: the long scar on the ocean floor that continues past the ends of the ridge segments. These offsets can stretch for hundreds to thousands of kilometres and reach about 8 km deep [2].

• Key point: only the stretch of a transform fault between the two ridge ends is an active plate boundary. Beyond the ridge ends, both sides of the fracture zone belong to the same plate and move in the same direction, so there is no sideways slip.

6. Transform boundaries on land

Fault	Location	Plates or blocks	Notes
San Andreas Fault	California, USA	Pacific plate (west) slides north-west past the North American plate (east)	About 1,300 km long. Active for about 10 million years at an average 5 cm per year [2]. Near San Francisco the fault itself takes about 20.8 mm per year of slip [4]
North Anatolian Fault and East Anatolian Fault	Turkey	The Anatolian block is squeezed westward between the Eurasian, Arabian and African plates	The 2023 Turkey earthquake happened in this fault system
Sagaing Fault	Myanmar, running north to south past Mandalay	Burma microplate slides past the Sunda plate, at the eastern edge of the Indian plate's collision zone	Site of the 2025 Myanmar earthquake

7. Link to India

• In NCERT's description of the Indian plate, its western margin follows the Kirthar Mountains of Pakistan and the Makran coast. It then joins the spreading site that runs from the Red Sea rift south-eastward along the Chagos Archipelago.
• So India's own plate is bordered by a divergent ridge system in the Arabian Sea and the Indian Ocean.

8. Diagram for the note

            Fracture zone           ACTIVE transform        Fracture zone
            (inactive scar)         (plate boundary)        (inactive scar)
  ------------------------------|=====================|------------------------------
          <--  <--              |    <--         -->  |              -->  -->
                                |  ridge          ridge
                                |  segment A      segment B
           Plate X moves west   |                     |    Plate Y moves east

• Draw two ridge segments (double lines) offset sideways.
• Join their ends with a transform fault.
• Draw arrows in opposite directions only on the stretch between the ridge ends. This is the active boundary.
• Draw arrows in the same direction on the fracture-zone extensions. These are inactive.

Prelims Hooks

• Divergent boundary = constructive margin = spreading site. Transform boundary = conservative margin, where crust is neither created nor destroyed.
• Mid-Atlantic Ridge: North American–Eurasian plates in the north and South American–African plates in the south. Average spreading is about 2.5 cm per year [2].
• Iceland is the part of the Mid-Atlantic Ridge above sea level, with a hotspot beneath it. Rifting at Krafla moved the ground about 7 m apart in 1975–84 [2].
• Afar triple junction (Ethiopia) is where the Arabian, Nubian and Somali plates meet [3]. Its three arms are the Red Sea, the Gulf of Aden and the East African Rift.
• Rift lakes are Tanganyika, Malawi and Turkana. Rift volcanoes include Kilimanjaro and Nyiragongo. Trap: Lake Victoria lies between the rift arms. It is not a rift lake.
• Transform faults run roughly at right angles to mid-ocean ridges. Only the segment between the ridge ends is active. Fracture zones are inactive.
• San Andreas Fault (California) is a transform boundary between the Pacific and North American plates, about 1,300 km long [2]. It is not a subduction zone.
• Strike-slip and transform faults in recent news: Sagaing (Myanmar, 2025), North and East Anatolian (Turkey, 2023).
• Order from youngest to oldest stage of ocean opening: East African Rift → Red Sea → Atlantic Ocean.

Mains Points

• How oceans are born (GS-I): a continent stretches and forms a rift valley (East Africa) → sea water enters and a narrow sea forms (Red Sea, Gulf of Aden) → the sea widens into an ocean with a central ridge (Atlantic, about 2.5 cm per year [2]). This explains the break-up of Pangaea and the present shape of the continents.
• Distribution of earthquakes and volcanoes: divergent zones have shallow earthquakes and basaltic volcanism, for example in Iceland and the East African Rift. Transform zones have strong, shallow earthquakes but almost no volcanoes, for example San Andreas, Anatolia and Sagaing. This helps explain the world map of seismic belts.
• Disaster risk (GS-III): shallow quakes on transform faults strike close to densely populated cities: Mandalay in 2025, south-east Turkey in 2023, and the San Francisco–Los Angeles corridor. Lessons for India include seismic zoning, earthquake-resistant building codes and NDMA-style preparedness, especially for the north-east, which lies near the Indo-Burma plate edge.
• Resources and opportunities: heat at divergent zones supports geothermal energy (Iceland, the Kenyan rift). Mid-ocean ridges host hydrothermal sulphide deposits, which are relevant to India's deep-sea exploration interests in the Indian Ocean ridge system.