Plate Boundaries: Divergent, Convergent, Transform

At a Glance

• Divergent Plates separate; new crust
• Convergent Plates collide; subduction or folding
• Transform Plates slide past; crust conserved
• Divergent example Mid-Atlantic Ridge, East African Rift
• Transform example San Andreas Fault
• Exam link UPSC Prelims, Geography: Plate Tectonics and Distribution of Continents and Oceans

Where Does It Fit?

This topic follows the statement of plate tectonics and describes what happens where plates meet. It is the topic that explains the distribution of earthquakes, volcanoes and mountain belts studied in the earlier macro. Previous topic: Plate Tectonics: Major and Minor Lithospheric Plates. Next topic: Rates and Forces of Plate Movement.

What Are the Types of Plate Boundaries?

At divergent boundaries the plates move apart and new crust is created. At a convergent boundary, a denser oceanic plate subducts below a lighter plate, and where two continental plates collide neither subducts and mountains rise. A transform boundary has no subduction and no new crust. The San Andreas Fault in California, which marks the boundary between the Pacific and North American plates, is the best-known transform fault.

How Does a Mid-Ocean Ridge Form?

The mid-ocean ridge is the longest mountain system on Earth, a chain of underwater mountains about 65,000 km long that winds through all the oceans. The Mid-Atlantic Ridge runs about 16,000 km from the Arctic to the far south Atlantic, midway between the continents, with a central rift valley along its crest where new crust forms. The ridge is offset along its length by transform faults, which are perpendicular to the ridge. Hot water rich in minerals, heated by the magma, escapes through hydrothermal vents, called black smokers, that support unique life forms based on chemical energy, not sunlight. In a few places the ridge rises above sea level: Iceland sits on the Mid-Atlantic Ridge, and its geysers, hot springs and volcanoes result from both the ridge and a hotspot. The Carlsberg and Central Indian ridges are the Indian Ocean parts of the system.

How Does a Rift Valley Form?

A rift valley forms when a continent is stretched, a process called continental rifting. The sequence is:

1. A mantle upwelling heats and domes the continental crust, and tension thins and stretches it.
2. The crust cracks along normal faults, and the central block drops down as a graben, forming a long, steep-sided valley.
3. Magma rises along the faults, producing volcanoes and lava plains; the valley floor holds lakes.
4. If stretching continues, the continent splits, seawater floods the rift and a new ocean basin forms, with a ridge in the centre.

The Red Sea, which began opening about 25 to 30 million years ago, shows the middle stage, and the Afar region of Ethiopia is a triple junction where the Red Sea, Gulf of Aden and East African rifts meet. The geomorphology of faults, including horst and graben, is explained in Endogenic Processes: Diastrophism, Orogenic and Epeirogenic Processes.

How Does an Island Arc Form?

• The subducting plate carries water in its sediments and minerals, and as it sinks to a depth of about 100 to 150 km, heat and pressure release this water.
• The water lowers the melting point of the overlying mantle, which partially melts and forms magma.
• The magma is lighter than the surrounding rock and rises through the overriding plate, erupting on the sea floor.
• Repeated eruptions build volcanic cones that rise above the sea as a chain of islands, curved in plan, because the plane of the subducting slab meets the sphere of the Earth in an arc. This is the island arc.

The curved shape and the position parallel to the trench are the signs of a volcanic island arc. The islands are made mostly of andesitic lava, with explosive eruptions because the magma is rich in gas and silica.

Which Are the Main Island Arcs and Trenches?

The deepest point in the oceans, the Challenger Deep, lies in the Mariana Trench at about 10,900 m below sea level. Many of these island arcs lie on the Ring of Fire, a topic developed in Distribution of Volcanoes: Pacific Ring of Fire, Mediterranean Volcanism.

How Were the Andes Formed?

The Andes are the longest continental mountain range above water, extending for about 7,000 km along the western coast of South America. They arose where the oceanic Nazca plate subducts beneath the South American plate at a speed of about 7 to 8 cm a year. The offshore trench is the Peru-Chile Trench, also called the Atacama Trench, with a depth of about 8,000 m. The highest peak is Aconcagua in Argentina, at about 6,961 m, which is the highest peak outside Asia. The range includes many active volcanoes, such as Ojos del Salado on the Chile-Argentina border, the highest active volcano in the world at about 6,893 m, and the Andean volcanic belt is part of the Ring of Fire. Powerful earthquakes occur: the 1960 Valdivia earthquake in Chile, of magnitude 9.5, is the largest ever recorded.

How Were the Himalayas Formed?

The Himalayas began to rise when the Indian plate collided with the Eurasian plate about 50 million years ago, after the Tethys Sea between them closed, as described in Plate Tectonics: Major and Minor Lithospheric Plates. The sediments of the Tethys were folded and thrust southward in a series of sheets, producing the three parallel ranges seen today. The Himalayas are about 2,400 km long and include the highest peaks on Earth, with Mount Everest at 8,848.86 m according to the joint measurement of Nepal and China announced in 2020. The mountains are still rising by a few millimetres a year, as the Indian plate continues to push north at about 5 cm a year.

The divisions of the range are described in Geological Regions of India: Deccan Plateau, The Himalayas, Other Peninsular Mountain Ranges, Indo-Gangetic-Brahmaputra Plain.

What Is a Transform Boundary?

At a transform boundary two plates slide past each other horizontally. Crust is neither created nor destroyed, so these are called conservative boundaries. They are common on the ocean floor, where they offset the ridges in short sections, and a few cross continents.

The term transform fault was coined by the Canadian geophysicist J. Tuzo Wilson in 1965. Because the plates are locked by friction, the stress builds and is released as shallow, strong earthquakes, without volcanoes.

Why Is This Topic Important?

• Unifying theory Explains most landforms
• Boundaries Core Prelims content
• Hazards Earthquakes and volcanoes

What Came Next?

The next topic explains the speed and the forces of plate movement. Next topic: Rates and Forces of Plate Movement.

Key People and Terms

• Tectonic plate Rigid slab of lithosphere
• Subduction Sinking of one plate under another
• Slab pull Pull of a sinking plate
• Ridge push Push from the elevated ridge
• Transform fault Plates slide past each other
• Divergent boundary Plates move apart

Exam Corner

Mains Angle

Questions ask for boundaries and features. These points add depth.

• Boundaries Three types with distinct landforms
• Forces Convection, slab pull, ridge push
• Hazards Boundaries match quake belts