Tectonic Landforms: Ridges, Trenches, Rifts, and Mountains

Tectonic landforms are large features of Earth’s surface created by the movement of plates: mid-ocean ridges (underwater mountain chains where new crust forms), ocean trenches (deep troughs where plates subduct), rift valleys (long depressions where the crust is pulled apart) and fold mountains (ranges formed where plates collide and the crust is compressed). In brief: the mid-ocean ridge system is the longest mountain chain on Earth (about 65,000 km); the deepest trench, the Mariana Trench, goes to about 10.9 km below sea level; the East African Rift is the best-known continental rift; and the Himalayas, formed by the collision of India and Eurasia, are the highest fold mountains. Each landform is linked to a type of plate boundary.

These landforms show where Earth is active and shape oceans and continents. This guide describes each of the four, how they form and where, compares them in a table, works through calculations of ocean-floor depth, uplift and erosion, outlines the history of their discovery and corrects common mistakes.

What are mid-ocean ridges?

A mid-ocean ridge is a long, broad, raised region of the sea floor at a divergent boundary, where magma rises and forms new oceanic crust. The ridge system winds around the globe for about 65,000 km, like the seam of a baseball; the Mid-Atlantic Ridge is about 16,000 km long. The crest is typically at a depth of about 2,500 m, 2–3 km above the surrounding abyssal plains, with a central rift valley on slow-spreading ridges, a smoother profile on fast-spreading ones, and offsets by transform faults. Features: basalt lava with pillow shapes, hydrothermal vents with unique ecosystems, and shallow earthquakes. The crust moves away, cools and contracts, so the sea floor gets deeper with age; this is why ridges are high and the older sea floor is deeper. A few ridge sections rise above the sea, e.g. Iceland and the Azores.

What are ocean trenches?

An ocean trench is a long, narrow, steep-sided depression in the sea floor at a subduction zone, where an oceanic plate bends down into the mantle. Trenches are the deepest parts of the oceans. The Mariana Trench in the western Pacific is about 2,550 km long and 69 km wide on average, and its deepest point, the Challenger Deep, is about 10,900 m below sea level; others include the Tonga, Kuril–Kamchatka, Philippine, Peru–Chile (Atacama) and Puerto Rico trenches. Most are around the Pacific (“Ring of Fire”). Trenches are filled with sediments in some places, and they are associated with volcanic arcs on the overriding plate and with the largest earthquakes and tsunamis (Chile 1960, Sumatra 2004, Japan 2011).

What are rift valleys?

A rift valley forms where the crust is stretched and thinned by divergent forces, so that a central block drops down between parallel normal faults (a graben), leaving steep walls on both sides. The East African Rift is a system over 3,000 km long running from the Afar region in Ethiopia to Mozambique, where the African Plate is splitting into the Nubian and Somali plates; it contains deep lakes (Tanganyika, Malawi), volcanoes (Kilimanjaro, Nyiragongo) and hot springs. Other rifts: the Rhine Graben in Europe, the Baikal Rift in Siberia, the Rio Grande Rift in North America and the Red Sea and Dead Sea rifts. If the rifting continues, the valley may be flooded by the sea, forming a young ocean, as the Red Sea did.

How do fold mountains form?

Fold mountains form where two plates collide, usually at convergent boundaries, and layers of sediment and rock are compressed, folded and thrust upward over millions of years. Rocks that were laid down in a sea between the plates are squeezed and uplifted into upfolds (anticlines) and downfolds (synclines); overthrusting along faults can stack the layers. The Himalayas, the highest range on Earth, formed from the collision of the Indian Plate with the Eurasian Plate that started about 50 million years ago and continues; India moves north at about 4–5 cm a year. Other examples are the Alps (African and Eurasian plates), the Andes (a fold mountain chain with volcanoes, where the oceanic Nazca Plate subducts), the Rockies and the Urals (an ancient, eroded collision belt). Fold mountains are young, high and jagged where they are recent, and lower and rounded when old.

Tectonic landforms compared

Worked example: depth, uplift and erosion

1. Sea-floor depth with age: an approximate empirical relation for the depth of the ocean floor younger than about 70 million years is d = 2,500 + 350√t meters, where t is the age in millions of years. At the ridge crest (t = 0), the depth is 2,500 m. At t = 4 Myr: 2,500 + 350 × 2 = 3,200 m. At t = 16 Myr: 2,500 + 350 × 4 = 3,900 m. At t = 36 Myr: 2,500 + 350 × 6 = 4,600 m.
2. Age from distance: at a half-spreading rate of 2 cm a year, a point 720 km from the ridge crest (72,000,000 cm) has an age of 72,000,000 ÷ 2 = 36 million years, so its depth would be about 4,600 m.
3. Trench versus mountain: the Challenger Deep (about 10,935 m below sea level) is deeper than Mount Everest (8,849 m) is high by 10,935 − 8,849 = 2,086 m, which means that if Everest were placed in the trench, its summit would be more than 2 km below the surface.
4. Himalayan uplift and erosion: if the range is uplifted at 5 mm a year and erosion removes 3 mm a year, the net rise is 2 mm a year, so in 1 million years the height increases by 2 mm × 1,000,000 = 2,000,000 mm = 2,000 m (an illustration, not a prediction, because rates change).
5. Ridge length: the ridge system of 65,000 km is 65,000 ÷ 40,075 = 1.6 times the circumference of Earth at the equator.
6. Collision speed: at 4.5 cm a year, India has moved 4.5 × 50,000,000 = 225,000,000 cm = 2,250 km during 50 million years of collision (some of which was absorbed in crumpling, thickening and extrusion of the Asian crust).

How were these landforms discovered and explained?

The HMS Challenger expedition (1872–1876) found the Mid-Atlantic Ridge and the first deep trench soundings. Sonar surveys in the 1950s allowed Marie Tharp and Bruce Heezen to draw the first maps of the sea floor, showing the continuous rift along the ridge. In 1960, the bathyscaphe Trieste dived to the Challenger Deep. The geosyncline theory of the 19th century explained fold mountains as the infilling and squeezing of long sedimentary basins, and it was replaced by plate tectonics in the 1960s. The Himalayan collision has been studied in detail with GPS and seismic methods.

Which mistakes are common?

Mistake 1: Thinking that the deepest ocean point lies in the center of the ocean

The deepest parts are trenches, near the edges of plates. The ridges in the middle are high.

Mistake 2: Confusing a rift valley with a river valley

A rift valley is formed by faulting, a river valley by erosion.

Mistake 3: Assuming that all mountains are formed by folding

Some mountains are volcanic, block-faulted or dome-shaped, and some are residual from erosion.

Mistake 4: Thinking that the Himalayas stopped rising

India is still moving north, and the range is rising by a few millimeters a year, partly offset by erosion.