Block Mountains

Introduction

Block mountains are formed by the Earth’s crust stretching and pulling apart. They are also characterized by uplifted blocks of crust separated by faults. Unlike fold mountains, which result from compression, block mountains form due to the extension of the crust. This creates distinctive landscapes with steep cliffs and valleys.

Step By Step Formation of Block Mountains

1. Stretching of Earth’s Crust:
   • Block mountains form when the Earth’s crust is stretched due to tectonic forces.
2. Faulting Occurs:
   • As the crust stretches, faults (cracks in the Earth) develop.
3. Uplifted and Down-Dropped Blocks:
   • Then crustal blocks on either side of the faults move vertically. Thus causing one block to go up (horst), and the other to go down (graben).
4. Distinctive Landscape Emerges:
   • This movement creates a distinctive landscape with steep slopes on the uplifted side and gentler slopes on the down-dropped side.
5. Valleys and Mountains:
   • Due to this, valleys may form in the down-dropped areas (grabens), creating a varied landscape of mountains and valleys.
6. Erosion Shapes the Landscape:
   • Over time, erosion and weathering further shape the block mountain landscape.

Occurrence

Block mountains commonly occur at divergent or extensional plate boundaries where tectonic plates are moving away from each other. Furthermore, as the crust stretches, blocks of rock are lifted along fault lines, creating elevated areas. Additionally, block mountains can form in continental interiors where the crust is under tension, leading to the creation of fault systems and the uplift of large blocks of rock.

Example

An example of a block mountain is the Sierra Nevada in the western United States. The Sierra Nevada range was formed by the extensional forces associated with the stretching of the Earth’s crust. This is because the crust was pulled apart, and large blocks of crust were uplifted along faults. The eastern side of the Sierra Nevada features steep escarpments, including the iconic and dramatic landscape of the Sierra Nevada’s eastern front.