Divergent boundary

Also called: Constructive boundary, Spreading site · Topic: Continental Drift, Sea-floor Spreading and Plate Tectonics · NCERT: Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 4 "Distribution of Oceans and Continents"

Meaning

A divergent boundary is a line where two plates pull apart. Magma (hot molten rock from the mantle) rises into the gap, cools, and forms new crust. Under the sea, this new crust is oceanic crust. On land, the crust first stretches and cracks into a rift valley.

It matters because this is how new sea floor is made and how oceans are born. It explains the break-up of Pangaea and the widening of the Atlantic Ocean.

Explanation

How it works: cause → process → result

• Cause: two plates move away from each other.
• A plate is a large, rigid slab of the lithosphere. The lithosphere is the earth's hard outer shell: the crust plus the upper part of the mantle.

• As the plates pull apart, a gap opens along the line between them.

• Process: magma fills the gap.

• Magma rises from the mantle into the gap.
• It cools and hardens into new oceanic crust.

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

• Result: a mid-ocean ridge.

• A mid-ocean ridge is a long underwater mountain chain that forms along the line of spreading.
• The newest rocks lie on the crest of the ridge. Rocks get older the farther they are from the ridge.
• This age pattern was the main evidence for sea-floor spreading (Hess, 1960s).

Names and why they fit

• Spreading site: new sea floor is made here and spreads outward.
• Constructive margin (or constructive boundary): crust is created here, not destroyed.
• Divergent: the two plates diverge, which means they move apart.

Stages: from a land rift to an ocean

• Rift valley: a long valley with steep sides.
• It forms when the crust is stretched and cracks along parallel faults.

• The block between the faults then sinks.

• An ocean opens in three stages:

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

Features and activity

• Earthquakes: shallow ones.
• Volcanoes: basaltic volcanism (runny lava that forms dark basalt rock).
• Transform faults: these breaks run roughly at right angles to the ridge. They cut it into offset segments, so the ridge looks like a zig-zag on a map.
• Eruptions do not happen along the whole crest at the same time.

• So different blocks of a plate move away at different rates, and the ridge breaks into segments.

• Hotspot boost: a hotspot is a fixed spot in the mantle where very hot material rises and feeds extra magma. Where a hotspot sits under a ridge, enough rock can pile up for the ridge to rise above the sea, as in Iceland.

Where it occurs

World: the Mid-Atlantic Ridge (the classic example)

• It runs north to south down the middle of the Atlantic Ocean.
• North Atlantic: the North American plate is to the west and the Eurasian plate is to the east.
• South Atlantic: the South American plate is to the west and the African plate is to the east.
• It spreads at about 2.5 cm per year on average, or 25 km in a million years [2].
• Over 100–200 million years, sea-floor spreading has widened the Atlantic from a narrow channel between the continents into a huge ocean [2].
• Iceland is the part of the ridge that rises above sea level. It sits on the boundary between the North American and Eurasian plates [2]. A hotspot lies beneath it.
• Krafla volcano area (north Iceland): during rifting episodes from 1975 to 1984, ground cracks widened and new ones opened. In total the ground moved about 7 metres apart [2]. This shows that spreading is still happening today.

World: East Africa (divergence on land)

• East African Rift: about 6,000 km long. It runs from the Afar region (Ethiopia) through Kenya and Tanzania.
• Along it, the African plate is splitting into 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, the Arabian, Nubian and Somali plates are pulling away from one another [3]. It has three arms:
• Red Sea: formed when spreading separated Arabia from Africa [2].
• Gulf of Aden: another young ocean opening from the same junction.

• East African Rift: still a rift on land, with no ocean yet.

• Rift lakes: Tanganyika, Malawi and Turkana. These long, deep lakes fill the sunken blocks.

• Rift volcanoes: Kilimanjaro (Tanzania) and Nyiragongo (DR Congo). Magma rises through the thinned crust.
• 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].

India

• According to NCERT, the western margin of the Indian plate 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.

Don't confuse with

• Convergent (destructive) boundary: here plates move towards each other, and crust is destroyed, for example where one plate sinks under another (subduction). At a divergent boundary, crust is created.
• Transform (conservative) boundary: here plates slide sideways past each other, and crust is neither made nor destroyed. It has strong, shallow earthquakes but almost no volcanoes. The San Andreas Fault is a transform boundary, not a divergent one.
• Fracture zone: this is the inactive scar that continues past the ends of the ridge segments. Only the stretch of a transform fault between the two ridge ends is an active plate boundary.
• Lake Victoria vs rift lakes: Lake Victoria lies between the rift arms. It is not a rift lake. Tanganyika, Malawi and Turkana are rift lakes.

Prelims Hooks

• Divergent boundary = constructive margin = spreading site. New oceanic crust forms here, and the youngest rocks lie at the ridge crest.
• Mid-Atlantic Ridge: North American–Eurasian plates in the north, 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): the Arabian, Nubian and Somali plates meet here [3]. Its arms are the Red Sea, the Gulf of Aden and the East African Rift.
• Order of ocean opening, from youngest to oldest stage: East African Rift → Red Sea → Atlantic Ocean.
• Matching trap: rift lakes are Tanganyika, Malawi and Turkana, and rift volcanoes are Kilimanjaro and Nyiragongo. Lake Victoria is not a rift lake.

Mains Points

• How oceans are born (GS-I):
• A continent stretches and forms a rift valley (East Africa).
• Sea water then 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.

• Earthquakes and volcanoes (GS-I/GS-III):

• Divergent zones have shallow earthquakes and basaltic volcanism, as in Iceland and the East African Rift.
• This helps explain the world map of seismic and volcanic belts.

• It also shows hazard risk for growing rift-valley populations, for example near Nyiragongo.

• Resources (GS-III):

• Heat at divergent zones supports geothermal energy in Iceland and the Kenyan rift.
• Mid-ocean ridges hold hydrothermal sulphide deposits (metal-rich minerals laid down by hot water venting from the sea floor).
• These deposits matter to India's deep-sea exploration interests in the Indian Ocean ridge system, which borders the Indian plate.

Related concepts

• Mid-Atlantic Ridge
• East African Rift
• Transform boundary
• Transform fault

Read more

• Divergent and transform boundaries