3.3 Mountain building and continental formation

Mountain building shapes Earth's surface through tectonic collisions, creating folded and faulted rocks. This process involves uplift, metamorphism, and isostatic adjustment, forming iconic ranges like the Himalayas and Andes.

Continents grow through accretion, adding new material at plate boundaries. Stable cratons form continental cores, while terranes and mountain belts like cordilleras contribute to landmass expansion. These processes sculpt Earth's ever-changing landscape.

Orogenic Processes

Formation and Deformation of Mountain Ranges

Orogeny is the process of mountain building driven by tectonic plate collisions and convergence. It involves uplift, folding, faulting, and metamorphism of rocks. The Himalayas (formed by the India-Eurasia collision) and the Andes (formed by oceanic-continental subduction) are two classic examples, but they represent very different types of orogeny.

When tectonic forces compress rock layers, the rocks respond in two main ways: folding and faulting.

Folding occurs when rock layers are compressed and bent into wave-like shapes:

• Anticlines are upward-arching folds, where the oldest rock layers sit in the center of the fold. You can see exposed anticline structures at places like the Grand Staircase-Escalante region.
• Synclines are downward-curving folds, where the youngest rock layers sit in the center. The Sideling Hill road cut in Maryland is a textbook exposure of a syncline.

Faulting involves the fracturing and displacement of rock along a fault plane. The type of fault depends on the direction of stress:

• Normal faults form under extensional (pulling-apart) stress. The hanging wall moves down relative to the footwall. The Basin and Range Province in the western U.S. is a landscape defined by repeated normal faulting.
• Reverse faults form under compressional stress. The hanging wall moves up relative to the footwall. Thrust faults (low-angle reverse faults) helped build the Rocky Mountains.
• Strike-slip faults involve horizontal movement along the fault plane, driven by shear stress. The San Andreas Fault in California is the most famous example.

Metamorphism and Isostatic Adjustment

Metamorphism is the transformation of rock under high temperature and pressure without melting. (If it melts, it becomes igneous rock instead.) Two types matter most for mountain building:

• Regional metamorphism occurs over large areas during orogenic events, as entire rock sequences get buried and squeezed. Barrovian metamorphism in the Scottish Highlands is the classic example, where geologists first mapped out distinct metamorphic zones based on increasing temperature and pressure.
• Contact metamorphism happens locally when magma intrudes into surrounding rock and "bakes" it. The zone of altered rock around the Skiddaw Granite in England's Lake District illustrates this well.

Isostasy is the gravitational equilibrium between Earth's crust and the denser mantle beneath it. Think of it like a block of wood floating in water: thicker blocks float higher, thinner blocks sink lower.

• When mountains form, the crust thickens and becomes more buoyant, pushing upward. The Tibetan Plateau sits at ~5,000 m elevation partly because its crust is roughly twice the normal continental thickness (~70 km vs. ~35 km).
• As mountains erode, the crust thins and loses buoyancy, causing gradual subsidence. The Appalachian Mountains were once Himalaya-scale but have worn down over hundreds of millions of years, with isostatic adjustment contributing to their slow sinking.

Continental Growth

Accretion and Terrane Addition

Continents don't just sit there unchanged. They grow over time through accretion, the process of adding new crustal material at convergent plate boundaries.

This happens at subduction zones and during continental collisions, where pieces of crust get swept up and welded onto the edge of a continent. Much of western North America grew this way, as island arcs and microcontinents collided with and stuck to the continental margin over the past few hundred million years.

Terranes are the building blocks of accretion. Each terrane is a fault-bounded crustal block with its own distinct geologic history, meaning it formed somewhere else before being carried to its current location by plate motion. Terranes can include:

• Island arcs (chains of volcanic islands from subduction)
• Seamounts (underwater volcanoes scraped off a subducting plate)
• Continental fragments
• Accretionary wedges (sediment piled up at a subduction zone)

The Wrangellia Terrane in western North America is one of the best-studied examples. It originated as a volcanic plateau in the Pacific and was carried thousands of kilometers before accreting to the continent.

Cratons and Continental Stability

Cratons are the stable, ancient cores of continents. They've remained relatively undeformed for billions of years, making them the oldest surviving pieces of continental crust.

• They're composed of Precambrian crystalline basement rock (often over 2 billion years old), frequently covered by a thin veneer of younger sedimentary layers.
• Major examples include the Canadian Shield, the Siberian Craton, and the Kaapvaal Craton in South Africa (one of the oldest, at ~3.6 billion years).

Cratons form the nucleus around which continents grow. Younger terranes and sedimentary basins accumulate around their edges over geologic time. Because cratons are rigid and strong, they resist deformation during later tectonic events and instead redirect stress into the weaker, younger rocks surrounding them.

Mountain Belts

Cordilleran Orogens

A cordillera is a long, linear chain of mountains formed along a convergent plate boundary, typically where an oceanic plate subducts beneath a continental plate. Cordilleran mountain belts are characterized by parallel mountain ranges, volcanic arcs, and associated sedimentary basins.

The North American Cordillera stretches from Alaska to Mexico and includes the Rocky Mountains, the Sierra Nevada, and the Cascade Range. The Andes in South America are another prime example.

Cordilleran orogens develop through several interconnected processes:

1. Subduction of oceanic crust beneath a continental margin generates magma, building a volcanic arc at the surface and forming an accretionary wedge of scraped-off sediment at the trench. The Andes are the type example of this process in action today.
2. Terrane accretion adds exotic crustal blocks to the continental margin over time, widening the mountain belt. Much of the North American Cordillera was built this way.
3. Collision of oceanic plateaus or microcontinents with the margin can produce more dramatic deformation, including the obduction (overthrusting) of ophiolites (slices of oceanic crust and mantle pushed onto land) and high-pressure metamorphism. The Coast Mountains of British Columbia preserve evidence of such collisions.

These processes can overlap in time and space, which is why cordilleran mountain belts tend to be wide, complex, and geologically diverse.