Question

The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. Discuss the processes by which mountain belts form at convergent boundaries, in continental collisions, and in continental rifts. (E)

Short answer

Mountain belts form due to three main processes: convergent boundaries, continental collisions, and continental rifts. At convergent boundaries, tectonic plates move toward each other, resulting in mountain formation through oceanic-oceanic, oceanic-continental, or continental-continental collisions. Continental collisions involve the subduction of oceanic crust between two continental plates, followed by folding, thrusting, metamorphism, and magmatism, leading to mountain belt formation. Lastly, continental rifts occur when the Earth's crust is pulled apart, causing crust thinning, uplift, and eventually the formation of a new oceanic crust and mountains.

Step-by-step solution

Introduction

Mountain belts form due to various processes happening at the Earth's crust, where the tectonic forces result in deformation. We will discuss three main processes: convergent boundaries, continental collisions, and continental rifts.

Convergent Boundaries

Convergent boundaries occur when two tectonic plates move toward each other. In this process, there are three types of convergent boundaries: 1. Oceanic-oceanic: When two oceanic plates collide, one plate subducts beneath the other, forming a deep ocean trench and often a volcanic arc, resulting in a mountain range. 2. Oceanic-continental: When an oceanic plate and a continental plate collide, the denser oceanic plate subducts beneath the lighter continental plate. This leads to the formation of a deep ocean trench, volcanic activity, and the formation of a mountain range along the edge of the continental plate. 3. Continental-continental: As two continental plates collide, they tend to crumple and fold due to the compressional forces, causing the formation of mountain ranges.

Continental Collisions

Continental collisions occur when two continental plates converge and collide with each other. This process leads to the formation of mountain belts in several stages: 1. Initial collision: An oceanic crust that separates two continental plates is completely subducted, bringing the continents closer together and forming a suture zone. 2. Folding and thrusting: The compressional forces in the continental crust cause deformation and lead to large-scale folding and thrusting which results in an uplift of the landmass, creating mountain belts. 3. Metamorphism and Magmatism: Due to the intense heat and pressure, rocks are subjected to metamorphism and magmatism, resulting in the formation of new types of rocks.

Continental Rifts

Continental rifts are areas where the Earth's crust is being pulled apart due to tensional forces. This process involves the following stages: 1. Extension and stretching: The crust thins due to stretching, and normal faults are formed due to the tensional forces. 2. Uplift: As the crust thins, the hot mantle material rises, causing the crust to lift and form elevated rift valleys. 3. Break up and formation of a new oceanic crust: Eventually, when the crust becomes thin enough, the rift will break apart and form a new oceanic crust between the diverging plates, leading to the formation of mid-ocean ridges and mountains. Understanding these processes provides insight into how various mountain belts around the world have formed and the role that tectonic forces play in shaping the Earth's surface.

Key concepts

Convergent Boundaries

Imagine the Earth's crust as a giant puzzle with pieces constantly moving and interacting. When these pieces, known as tectonic plates, travel towards each other, they form convergent boundaries. The collision can occur between different types of plates, leading to dramatic geological phenomena.

• Oceanic-oceanic convergence leads to one plate being thrust beneath another, creating deep sea trenches and volcanic mountain ranges.
• Oceanic-continental convergence involves an oceanic plate plunging below a continental plate, which results in volcanic activity and mountain belt formation along the continental margin.
• Continental-continental convergence sees the meeting of two landmasses, which, unable to subduct due to their buoyant nature, crumple and fold to form expansive mountain belts.

This interplay not only changes the face of our planet but also announces the birth of majestic mountain ranges such as the Himalayas or the Andes.

Continental Collisions

When the dance of the tectonic plates leads to an encounter between two continental plates, we witness a continental collision. This process kicks off a chain of events that engineers great mountain belts.

• It all starts with the union of continents as the separating oceanic crust gets completely subducted, leaving behind a suture zone where the two continents merge.
• Next, the tremendous pressure forces the plates to crumple, folding and thrusting upwards in a slow-motion struggle, rising to form mountainous terrains.
• Finally, the intense heat and pressure remodel the very fabric of rocks, causing metamorphism. This geologic alchemy, coupled with magmatism, can forge new rock types and further sculpt the mountain architecture.

The transformation from flatland to highlands during continental collisions results in awe-inspiring landscapes like the Appalachian or Ural Mountains.

Continental Rifts

While convergent boundaries are about coming together, continental rifts are Earth's crust spreading its wings. The land under our feet stretches and pulls apart, driven by tectonic forces pulling in opposite directions.

• The process initiates with the crust becoming thinner as it stretches, forming normal faults due to tension.
• Things then get lifted; as the crust extends, underlying mantle material rises, causing a buoyant uplift to occur, thus creating rift valleys.
• If the separation continues, the thinning crust eventually ruptures completely. The void left behind may fill with oceanic crust material, paving the way for new mid-ocean ridges and, unexpectedly, mountain ridges as well.

Such rifting can ultimately lead to the birth of new oceans and is how the Red Sea came to be, gradually widening from what was once a rift between tectonic plates.

Tectonic Plates

At the heart of these dramatic Earth-shaping processes are the tectonic plates, colossal slabs of the Earth's lithosphere. These rigid plates float atop the semi-fluid asthenosphere, and their interactions at plate boundaries shape our planet's landscape.

They move because of the convection currents in the mantle, which are driven by heat from the Earth's core. Tectonic plates can carry continents or oceans, and their movement can be constructive, destructive, or conservative, depending on the relative motion.

• Divergent boundaries occur when plates move apart, creating new lithosphere.
• Transform boundaries have plates sliding past one another, leading to earthquakes but not typically creating or destroying landmass.
• Finally, the convergent boundaries we discussed earlier are destructive, as they often result in one plate disappearing beneath another, recycling the Earth's crust back into its mantle.

Understanding the movements and interactions of tectonic plates is crucial for grasping how mountains form and predict the natural events like earthquakes and volcanic eruptions.