Mountain Belts formed at Divergent and Convergent Boundaries

Mountains Formed at Divergent Boundaries

Divergent Plate Boundaries

Mid Ocean Ridges Continuous underwater mountain chains that extend up to 60,000 km around the globe. Generally around 1500km wide and have peaks around 3km high. Site of most volcanism on earth

How are mountains at mid ocean ridges formed Mid-ocean ridges result from convective upwelling of mantle beneath thin oceanic lithosphere. Hot bouyant material in the mantle rises causing the lithosphere above it to form a dome.

They are formed along structurally weak zones created where the ocean floor is being pulled apart lengthwise along the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. As the crust moves away from the ridge it gets older and becomes thicker and more dense, sinking deeper into the asthenosphere, therefore lowering elevation.

Rate of spreading of mid ocean ridges Slow spreading produces stepper flanks and prominent rift valleys Fast spreading produces gentle slopes and lack of prominent rift valley

Structure of oceanic lithosphere

Features Volcanic activity Basaltic magma Pillow basalt fissures Shallow earthquakes Faulting (normal) but no folding (rocks have been deformed by extension forces not compression) Abyssal hills

Shallow earthquakes (<10km deep) Notice that earthquakes also are found at transform faults across mid ocean ridges.

Normal faults are a result of tension forces

Transform faults near divergent boundaries Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes.

Continental Rifts How are they formed? Young rift zones occur within continental landmasses and are caused by convective upwelling of mantle beneath weak continental lithosphere. When continental crust stretches beyond its limits, tension cracks begin to appear on the Earth's surface. Magma rises and squeezes through the widening cracks, sometimes to erupt and form volcanoes.

Normal faults are a result of tension forces

Faults- normal faults. faulting produces large elongated down-dropped blocks (grabens) and uplift blocks (horsts)

Rock Types Magma - Basaltic Rock Types - Mostly basaltic. However continental crust is much richer in silica, therefore can get Ryolite at zones of continental rifting. Andersite is less common.

Old divergent continental margins are the largest stores of sediment on earth. Therefore are very rich in oil deposits etc.

Mountains at convergent boundaries

Ocean / ocean boundaries Mountains formed at ocean/ocean boundaries are of the volcanic island arc type. They form on an oceanic plate that has another oceanic plate subducting under it.

Ocean / ocean Accretionary wedge elongated mounds of ocean floor sediments that have been tightly folded and chaotically mixed in the trench by the faulting and folding caused as they are scraped from the down-going oceanic plate

Ocean / ocean Those formed of chains of explosive volcanoes. These volcanoes form from andesitic magmas that are generated as the subducted plate partially melts when it comes in contact with the hot asthenosphere. Steam and other volatile substances find paths upwards, creating vents for magma to reach the surface to create the violent volcanoes.

Main Features

Ocean / continent boundaries As an oceanic plate is subducted beneath a continent, the sediments on the upper surface of the lower plate will be scraped off to produce a wedge of sediment called an accretionary wedge. Where the accretionary wedge is forced directly against the leading edge of continental crust, the subducting plate will be forced down steeply into the asthenosphere where the plate will be partially melted. Steam produced in the process also partially melts the upper mantle. Andesitic magmas are produced from these processes. Mountains will be produced in the continental plate from the compression and uplift of the low density wedge sediments and the sediments and rocks of the continent, and from the intrusion of magma produced from the partial melting in the subduction zone.

Rock types Volcanic and plutonic rocks (rich in silica) Andersitic lava Granites and dolerites found in plutons (underground) Metamophic rocks (schists/ gneisses)

Features Trench Forarc basin Accretion wedge Violent volcanic activity Fold and thrusts Sedimentary basins

Continent / continent boundaries When two continents collide, the ocean between them has been subducted under one of them (consumed). The continents will have been flanked by accreted sediment from the ocean floor that was scraped off from the subduction. This sediment forms into a huge wedge as it is folded, compressed and uplifted.