Question

This detrital rock consists of angular grains and is rich in potassium feldspar and quartz. What do the angular grains indicate about the distance the sediment was transported? The source of the sediment in this rock was an igneous mass. Name the likely rock type. Did the sediment in this sample undergo a great deal of chemical weathering? Explain.

Short answer

The rock is breccia. Angular grains imply short transport. Minimal chemical weathering occurred.

Step-by-step solution

Identify the Key Characteristics

The rock consists of angular grains, indicates rich content of potassium feldspar and quartz, and has its origin as an igneous mass. These characteristics are essential in determining the identity and transport history of the sedimentary rock.

Analyze Angular Grains

Angular grains suggest that the sediments have not been transported over great distances. Transportation processes like water, wind, or ice usually round the grains. Sharp edges that remain imply a short transport distance.

Link Grain Composition to Rock Type

Potassium feldspar and quartz are primary minerals from felsic igneous rocks, like granite. The angular nature and composition of the grains indicate that this sedimentary rock is likely breccia, particularly if it is sourced from nearby outcrops of felsic igneous rocks.

Assess Chemical Weathering

The presence of potassium feldspar and quartz suggests limited chemical weathering. These minerals are quite resistant to chemical breakdown compared to more unstable minerals. Since the grains are angular and not rounded, it further indicates minimal chemical alteration.

Final Step: Conclusion

Based on angular grains, mineral composition, and source material, the rock is a breccia formed from feldspar-rich igneous rocks like granite. The sediment underwent little chemical weathering due to the preserved angularity and mineral stability.

Key concepts

Angular Grains

When we describe grains in sedimentary rocks as angular, we're referring to their jagged, sharp-edged appearance. These grains haven't been smoothed out by transportation. Imagine a rock fragment that breaks off from a larger block. If it doesn't travel far, it retains its sharp edges because it hasn't been tumbled enough to get rounded.
This is because transportation processes like water, wind, or glacial movement generally cause grains to bump into each other and other surfaces, rounding them over time.
An angular grain in a rock like breccia indicates that the sediment was deposited relatively close to its source area.

Chemical Weathering

Chemical weathering refers to the breakdown of rocks through chemical reactions, often involving water and natural acids. This process alters the mineral composition of rocks, transforming original minerals into more stable forms.
In our example, the minimal chemical weathering is evident because of the presence of potassium feldspar and quartz. These minerals are relatively durable, resistant to alteration, and they stay intact or change very slowly in natural environments.
The limited weathering implies that the sedimentary rock hasn't been exposed to conditions like high moisture and acidic environments that speed up chemical weathering.

Igneous Mass

An igneous mass is a body of solidified molten rock (magma). It serves as a key source for many minerals found in sedimentary rocks. Igneous rocks are formed through cooling and solidification of magma or lava.
These rocks are the origin of many minerals such as quartz and feldspar, which are commonly found in igneous rocks like granite.
In the example, the original sediment for the described detrital rock comes from an igneous mass, which is typical with the presence of minerals like potassium feldspar and quartz.

Breccia

Breccia is a type of sedimentary rock characterized by angular, broken fragments. These fragments are held together by a matrix that can be mineral cement or fine material.
The key feature that sets breccia apart from other sedimentary rocks is its sharp, angular clasts. Unlike conglomerates, whose rounded clasts suggest longer transportation, breccia's angular clasts imply short transport from the source to the point of deposition.
In our given example, the angular grains combined with the mineral composition indicate the rock type as breccia, originating likely from nearby felsic igneous outcrops.

Potassium Feldspar

Potassium feldspar is a group of minerals that are important components in many igneous rocks, such as granite. They are characterized by their pink to reddish colors, although they can also appear white or green.
This mineral is relatively resistant to chemical weathering, which is why it is often found intact in sedimentary deposits.
In the example rock, the presence of potassium feldspar helps identify both the rock’s origin and its geological history, confirming minimal chemical changes during its transport and deposition.

Quartz

Quartz is one of the most abundant and hard minerals found in the Earth's crust. It is a key mineral in many sedimentary, metamorphic, and igneous rocks.
In the context of sedimentary rocks, quartz's durability and resistance to weathering make it a common grain even after long transportation. It survives when other minerals break down.
In the example rock, quartz is one of the main minerals alongside potassium feldspar, further indicating limited chemical weathering and suggesting the sediment derived from a felsic igneous source.