Question

The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. What changes do rocks undergo during formation of a mountain belt? (A)

Short answer

During the formation of a mountain belt, rocks undergo various changes depending on the specific type of mountain building involved. In Fold Mountains, rocks experience deformation, leading to the formation of new metamorphic rocks such as slate, schist, and gneiss. In Fault-block Mountains, rocks can become fractured, faulted, or incorporated into new rocks like breccia or mylonite. In Volcanic Mountains, volcanic rocks form and existing rocks can be altered or metamorphosed due to heat and fluids from magma.

Step-by-step solution

Understand Mountain Formation Types

There are three primary types of mountain building: (1) Fold Mountains, which result from the folding of rock layers due to compression; (2) Fault-block Mountains, which occur when large blocks of rock are uplifted and tilted due to tension; and (3) Volcanic Mountains, which form when magma rises to the surface and solidifies. Each of these types of mountains results in different changes to rocks during their formation.

Fold Mountains

Fold Mountains form when the Earth's tectonic plates collide, causing rock layers to be compressed and folded. As a result, rocks in Fold Mountains undergo significant deformation. For instance, sedimentary rock layers may become bent, folded or broken. This can lead to the formation of new metamorphic rocks, such as slate, schist, and gneiss, due to increased temperature and pressure during the folding process.

Fault-block Mountains

Fault-block Mountains form when tension in the Earth's crust causes large blocks of rock to break apart and move relative to each other along faults. The rocks in these mountains undergo changes as a result of the faulting process. For example, the rocks can become fractured or faulted, with minerals filling in the spaces created by the fractures. In some cases, minerals and rock fragments can become incorporated into the surrounding rock, forming new rocks such as breccia or mylonite.

Volcanic Mountains

Volcanic Mountains form when magma rises from the Earth's mantle to the surface and solidifies, creating new igneous rock. The rocks found in volcanic mountains undergo changes due to the extreme heat and pressure involved in the process. For example, previously existing rock layers can be melted and incorporated into the newly formed volcanic rocks. Additionally, magma can cause the formation of new minerals and the alteration or metamorphism of surrounding rocks, as heat and fluids from the magma interact with the existing rocks. In conclusion, rocks undergo various changes during the formation of a mountain belt, depending on the specific type of mountain building involved. These changes can include deformation, metamorphism, fracturing, and the formation of new rocks and minerals.

Key concepts

Tectonic Plates

The Earth's crust is divided into large pieces called tectonic plates. These plates are constantly moving. They do so slowly, but with enormous force. Without tectonic plates, we wouldn't have mountains at all! This movement is what causes the formation of mountains. Here's how it works:

• When two tectonic plates collide, they can create fold mountains. This is because the rock layers on the plates are squeezed and folded.
• In cases where the plates pull apart, tension leads to the formation of fault-block mountains. Here, blocks of crust may fall or rise, resulting in a variety of landscapes.

In essence, tectonic plates are the primary drivers of the mountain-building process. Their interactions can lead to different types of stresses on the Earth's crust, ultimately producing stunning and varied mountain formations.

Metamorphic Rocks

Metamorphic rocks are rocks that have changed over time due to heat and pressure. This usually occurs deep within the Earth's crust where temperatures and pressures are very high. When a mountain forms, existing rocks can transform into metamorphic rocks through a process called metamorphism.
When tectonic plates collide, the resulting pressure and heat can turn sedimentary rocks into metamorphic rocks. For example, limestone can become marble, while shale may turn into slate.

• The process involves the recrystallization of minerals, which means the rock's structure changes without the material melting fully.
• Common metamorphic rocks in mountain belts include gneiss, marble, and schist.

These transformations can give mountains their distinct visual and structural characteristics, often making them much more resistant to erosion.

Igneous Rock

Igneous rocks are formed from solidified magma or lava. In the context of mountain formation, these rocks are born from volcanic activity. When magma from below Earth's surface rises and cools, it creates igneous rocks.

• If the magma cools beneath the surface, it forms intrusive igneous rocks like granite. These rocks are often visible in mountain ranges.
• When the magma erupts as lava and cools on the surface, it forms extrusive igneous rocks such as basalt.

Volcanic mountains, like those found on Hawaii or in the Cascades, are prime examples where igneous rocks play a starring role. These rocks are not only critical to mountain formation but also provide rich minerals that can support diverse ecosystems.

Faulting Process

The faulting process is a geological event where rocks break and slip along a fault line. These faults are essentially cracks in the Earth's crust. Faulting occurs due to the stresses exerted by tectonic movements on the crust and is crucial to the formation of fault-block mountains.

• When tension in the Earth's crust is too high, the rock breaks, causing blocks of rock to shift along these faults.
• This movement can create dramatic landscapes, including steep mountain ranges and deep valleys.

In areas where faults are active, the stress can also cause the minerals within the rocks to reorient. This can lead to the formation of new rock types like breccia. The faulting process not only shapes the surface of the planet but also gives rise to unique geological features.