Question

Why is it not necessary to indicate the texture of detrital rocks on the identification chart for sedimentary rocks (Figure 7.21\() ?\)

Short answer

Texture is not crucial for detrital rock identification as they are classified by grain size.

Step-by-step solution

Understand Detrital Rocks

Detrital rocks, also known as clastic sedimentary rocks, are primarily composed of fragments or grains of pre-existing rocks that are transported and deposited by natural processes such as water, wind, or ice. Their classification is often based on grain size rather than texture.

Identify Key Properties

In sedimentary rocks, texture refers to the size, shape, and arrangement of the particles that make up the rock. In detrital rocks, however, the most significant property for identification is the grain size, which is usually categorized into clay, silt, sand, and gravel.

Examine Importance of Texture

For detrital rocks, the texture is less important than for other rocks because their classification and identification are primarily dependent on grain size. The texture does not typically provide additional distinguishing information for classifying detrital rocks.

Reason Behind Omission in Charts

In identification charts, such as Figure 7.21, focus is often placed on the properties most relevant for classification. Since grain size is the key classification criterion for detrital rocks, it renders the texture less critical, which is why texture might not be included in the chart.

Key concepts

Grain Size

Grain size is a vital characteristic when it comes to defining detrital rocks. It refers to the diameter of the individual particles or grains within a rock. This characteristic is crucial because detrital rocks are classified based on these sizes rather than their overall texture. This means that the size of the fragments plays a more significant role than other properties such as shape or arrangement.

Grain sizes can be broadly divided into categories such as:

• Clay: Fine particles that are less than 0.004 mm in diameter
• Silt: Particles that range from 0.004 mm to 0.063 mm
• Sand: Grains that range from 0.063 mm to 2 mm
• Gravel: Larger grains over 2 mm

These size distinctions are essential because they reflect the process by which the particles were transported and deposited. Larger grains like gravel signify more energetic environments, such as fast-flowing rivers, while finer particles like clay indicate calm settings, like those found in deep water or sheltered areas.

By focusing on grain size, geologists can infer the environment of deposition and clearly distinguish between different detrital rocks.

Clastic Sedimentary Rocks

Clastic sedimentary rocks, also known as detrital rocks, are formed through the accumulation and lithification of mechanical weathering debris. These types of rocks are one of the three primary categories of sedimentary rocks, the others being chemical and organic sedimentary rocks.

Formation of clastic sedimentary rocks involves several stages:

• Weathering: Breakdown of rocks into smaller particles by physical or chemical processes
• Transportation: Movement of these particles through agents like water, wind, or glaciers
• Deposition: Settling of particles in a new location
• Compaction and Cementation (Lithification): Pressure and mineral growth bind the particles into solid rock

These rocks are characterized predominantly by the size and type of fragments within them. Because the classifying feature is the grain size, clastic rocks showcase a wide array of environments, from riverbeds to deserts. Unlike chemical sedimentary rocks which precipitate from solution, clastic sedimentary rocks highlight the physical processes of disintegration and transport.

Rock Texture

Texture in geology encapsulates the size, shape, and relationship between the grains or crystals in a rock. While critical for some rock types, the texture in detrital rocks is often overshadowed by grain size for classification purposes.

In a broader geological sense, rock texture encompasses:

• Size: The dimension of individual grains or crystals
• Shape: Whether grains are uniform or varied; rounded or angular
• Arrangement: The spatial relationship between particles

For detrital rocks, texture can offer additional insights into the rock's history, such as transport processes and deposition environments. However, it does not significantly alter their classification. This is because, in the context of detrital rocks, detailed texture information does not change the primary classification based on grain size. While texture might assist in further understanding the geological past, it generally holds a secondary role in the initial identification and categorization of clastic sedimentary rocks.