Question

If the sea level dropped 1,000 feet and this sedimentary environment stopped being built and began to break down, what type of weathering would be most likely to occur on these rocks? a. Fracturing from the addition or subtraction of pressure b. Dissolution c. Frost Wedging d. Hydrolysis

Short answer

The most likely weathering type is fracturing from the addition or subtraction of pressure (Option A).

Step-by-step solution

Understanding the Scenario

The sea level has dropped by 1,000 feet, which means that the rocks previously underwater are now exposed to surface conditions. This exposure to air and surface weather conditions changes the processes that can occur on these rocks.

Analyze Each Option

Consider how each type of weathering would be affected by the change in sea level: - **Fracturing due to pressure changes**: When rocks are relieved from the pressure of overlying water, stress release can cause fracturing. - **Dissolution**: Typically occurs when water erodes soluble minerals, not directly linked to sea level changes. - **Frost Wedging**: Requires freeze-thaw cycles, which are more likely at the surface. - **Hydrolysis**: Involves the breakdown of minerals through reactions with water, applicable in a surface environment.

Identify Suitable Weathering Process

With the sea level drop, rocks might expand or contract due to pressure release. **Fracturing from the addition or subtraction of pressure (Option A)** fits well, as the removal of large amounts of water (i.e., sea level decrease) causes pressure changes that can lead to fracturing.

Choose the Most Likely Weathering Type

Considering the analysis: - **Fracturing due to pressure change** makes the most sense for rocks that were once submerged and now exposed. Pressure release from lifting the overlay of water can lead to fracturing. - While frost wedging and hydrolysis are possibilities, they depend more on temperature and water chemistry, rather than the direct impact of pressure release.

Key concepts

Sea Level Change

A significant sea level change, like the drop of 1,000 feet described in the exercise, can have profound effects on geological and environmental processes. When sea levels fall, rocks that were once submerged become exposed to the air and new weather conditions. This exposure can alter the chemical and physical interactions occurring on these rocks.
Effects of sea level change include:

• Exposure of marine environments to terrestrial conditions, impacting organisms that once thrived there.
• Alterations in pressure on rock formations, leading to physical and chemical weathering.
• Changes in erosion and sediment transportation, contributing to the breakdown or reshaping of geological features.

When rocks are no longer submerged, they are subjected to different types of weathering, including fracturing due to pressure release, which is a direct consequence of the immense weight of water being lifted off the rocks.

Sedimentary Rocks

Sedimentary rocks are formed through the accumulation and compaction of mineral and organic particles. They often originate in aquatic settings and are commonly influenced by the surrounding environmental conditions.
Sedimentary rocks include:

• Sandstone: formed from compacted sand particles.
• Limestone: primarily composed of minerals like calcite, often from skeletal fragments of marine organisms.
• Shale: created from compacted clay particles.

These rocks are typically fragile and react significantly to changes in their environment. When sedimentary rocks are uplifted and exposed due to a drop in sea level, they undergo new weathering processes. This exposure brings them into contact with atmospheric conditions, leading to various forms of weathering such as pressure release fracturing.

Pressure Release

Pressure release, also known as unloading, occurs when overlying pressure is suddenly reduced. This happens when sea levels fall or when weight is removed from a rock's surface. Before this release, the rocks are subjected to great pressure, which keeps them stable.
With the release of pressure:

• Rocks may expand slightly, causing stress fractures and cracks.
• Sheets of rock may peel away from the surface, a process known as exfoliation.
• It enhances other weathering processes as new surfaces are exposed.

This process is particularly common in sedimentary rocks that were once underwater. As they are now above the sea level, the pressure exerted by the water disappears, causing expansion and potential fracturing.

Exposure to Surface Conditions

When rocks are exposed from underwater conditions to the surface, they encounter a completely different set of environmental factors. These factors include direct sunlight, air, plant life, and variations in temperature and moisture.
The impact includes:

• Temperature fluctuations that lead to contraction or expansion, enhancing physical weathering methods such as frost wedging.
• Increased interaction with oxygen and water, facilitating chemical weathering processes like oxidation and hydrolysis.
• The growth of plant roots and microbial action, which can further break down rock surfaces over time.

These conditions make rocks more vulnerable to weathering as they transition from stable underwater environments to the dynamic surface. With this exposure, the types of weathering that occur can change significantly, depending on the local climate and environmental conditions.