Question

Which type of crust is formed when the older rocks are pushed above the younger rocks in earth's crust?

• A. Normal fault
• B. Reverse fault
• C. Strike-slip fault
• D. Thrust fault

Hint:

• Faults are fractures where rock blocks move relative to each other.
• Reverse faults and thrust faults are caused by compressional forces.
• In both, the hanging wall moves up relative to the footwall, which can place older rocks on top of younger rocks.
• A thrust fault is a low-angle reverse fault (dip<45°, often<30°). They are responsible for large-scale overthrusting of rock sheets.
• Normal faults (tensional) result in the hanging wall moving down.
• Strike-slip faults involve horizontal movement.

Answer 1

The Correct Option is D

When older rocks are pushed above younger rocks in Earth's crust, this geological structure is formed by a Thrust fault.

Step-by-Step Explanation:

Step 1: Understanding Faults and Rock Movement

A fault is a fracture or zone of fractures between two blocks of rock. It allows the blocks to move relative to each other. Geologists classify faults based on the direction of movement along the fault plane and the forces that cause them.

To understand the movement, consider the concepts of 'hanging wall' and 'footwall':

• Hanging Wall: The block of rock that lies above an inclined fault plane. (Imagine hanging a lantern on it).
• Footwall: The block of rock that lies below the inclined fault plane. (Imagine standing on it).

Step 2: Analyzing Different Fault Types

• Normal Fault:
  • Movement: The hanging wall moves down relative to the footwall.
  • Forces: Caused by tensional (pull-apart) forces.
  • Rock Age Relationship: Typically results in younger rocks being juxtaposed against, or lying above, older rocks. This is characteristic of crustal extension.
• Reverse Fault:
  • Movement: The hanging wall moves up relative to the footwall.
  • Forces: Caused by compressional (squeezing) forces.
  • Rock Age Relationship: Results in older rocks being pushed above younger rocks. The fault plane has a relatively steep dip angle (greater than 45 degrees).
• Strike-slip Fault:
  • Movement: Blocks of rock slide past each other horizontally along the fault plane, with little to no vertical motion.
  • Forces: Caused by shearing forces.
  • Rock Age Relationship: Does not typically result in older rocks being pushed vertically above younger rocks.
• Thrust Fault:
  • Movement: This is a specific type of reverse fault where the hanging wall moves up and over the footwall, but the fault plane has a very low dip angle (typically less than 45 degrees, often much shallower).
  • Forces: Caused by strong compressional forces.
  • Rock Age Relationship: Due to the low angle, the overlying block (hanging wall) can be transported a significant horizontal distance, resulting in large masses of older rock being effectively "thrust" or pushed horizontally over younger rocks. This is a hallmark feature of mountain-building (orogenic) processes.

Step 3: Identifying the Best Fit for the Description

The description "older rocks are pushed above the younger rocks" is characteristic of both reverse faults and thrust faults. However, a thrust fault specifically emphasizes the low-angle nature and the often extensive horizontal transport that results in older rock units overriding younger ones, making it the most precise answer for this phenomenon, especially when large-scale crustal shortening is involved.