Question

While on a field Michael Collier trip with your geology class, you stop at an outcrop of sandstone. An examination with a hand lens shows that the sandstone is poorly sorted and rich in feldspar and quartz. Your instructor tells you that the sediment was derived from one of two sites in the area: Site 1: A nearby exposure of weathered basaltic lava flows. Site 2: An outcrop of granite at the previous field trip stop up the road. Select the most likely site and explain your choice. What name is given to this type of sandstone?

Short answer

Site 2 is the most likely source. The sandstone is called "arkose."

Step-by-step solution

Analyzing Sediment Composition

First, identify the mineral composition of the sandstone. The sandstone at the outcrop is rich in feldspar and quartz. These minerals are significant as they can indicate the rock's source.

Evaluating Site 1 (Basaltic Lava)

Assessing Site 1, we consider that basaltic lava typically consists of minerals such as pyroxene, plagioclase, and olivine. These are not rich sources of quartz or significant sources of feldspar in the form found in the sandstone. Hence, Site 1 is less likely to be the source of the sandstone.

Evaluating Site 2 (Granite)

Evaluating Site 2, granite is known for containing high amounts of quartz and feldspar. This aligns with the mineral composition of the sandstone. Therefore, the granite outcrop is a more likely source for the sandstone's mineral content.

Determining Sandstone Type

Based on the mineral composition of feldspar and quartz, the sandstone can be classified as "arkose," a type of sandstone that is typically derived from granite material.

Key concepts

Mineral Composition

The mineral composition of a rock refers to the specific minerals that are present within it. In the context of geology, understanding the mineral composition is crucial for identifying the original source of sedimentary rocks, such as sandstone.

In the examination of sandstone, it is important to note the presence of minerals like feldspar and quartz. Feldspar is a group of rock-forming minerals that make up about 41% of the Earth's continental crust. It is commonly found in igneous rocks such as granite. Quartz, another prevalent mineral, is hard and resistant to weathering, making it a common constituent in sedimentary rocks.

When identifying the origin of a sandstone formation, one can analyze its mineral content to trace back to its likely parent rock. For example, a sandstone rich in quartz and feldspar is more likely derived from granite rather than basaltic lava, as basalt contains minerals like pyroxene and olivine instead.

Sedimentary Rocks

Sedimentary rocks are formed by the accumulation and cementation of mineral and organic particles over time, often in distinct layers. They serve as a historical record of the Earth's surface processes, including the deposition environments and the nature of the resources that weathered to form them.

Sedimentary rocks can be classified based on the size and composition of their particles. Common types include shales, sandstones, and conglomerates. Sandstone, in particular, is a sedimentary rock composed mainly of sand-sized mineral particles, and it often contains a significant amount of quartz and feldspar.

• Clastic Sedimentary Rocks: Composed of fragments of pre-existing rocks.
• Chemical Sedimentary Rocks: Form from the precipitation of minerals from water.
• Organic Sedimentary Rocks: Form from accumulation of plant or animal debris.

Understanding the context of sedimentary rocks, such as their formation and classification, is essential for identifying their potential sources and the processes that formed them.

Sandstone Identification

Identifying sandstone involves examining its texture, mineral content, and grain sorting. Sandstone is part of the larger category of clastic sedimentary rocks, which are formed from fragments of other rocks.

One can often identify sandstone by its prominent grainy texture, resulting from sand-sized mineral particles, mostly quartz and feldspar. The sorting of grains – whether they are consistently uniform in size or vary greatly – can provide clues to the rock’s history. Poorly sorted sandstone, with a mix of grain sizes, indicates rapid sedimentation likely from a high-energy environment, such as a river or avalanche, where sediment has not had much time to settle and separate by size.

The identification process also requires checking for the presence of matrix or cement that binds the grains together, which can also relate back to the sandstone's geologic history and the environment of its formation.

Arkose

Arkose is a specific type of sandstone characterized by its high feldspar content, typically making up at least 25% of its overall mineral composition. This type of sandstone is usually indicative of significant erosion from a source rock rich in feldspar, most commonly granite.

Arkose is typically reddish to pinkish due to the presence of potassium feldspar and is often found in deposits close to its source rock due to feldspar's susceptibility to rapid weathering. Its mineral composition not only points to its source but often suggests a relatively short transport distance from that source, with less weathering than other types of sandstone.

Geologists use arkose to infer the history of the region where it is found, including past climatic conditions and the tectonic activity that might have brought granite to the Earth’s surface where it could be weathered and eroded into arkose sandstone.

Granite

Granite is a common and widely occurring type of intrusive, igneous rock. Its composition primarily consists of quartz, feldspar, and mica, giving it a distinct grainy texture. As an intrusive rock, granite forms from the slow cooling of magma deep within the Earth, allowing large crystals to develop.

Granite serves as a significant source rock in the formation of arkose sandstone due to its high feldspar and quartz content. This makes it an excellent reference point for identifying the parent rock of sedimentary deposits.

The weathering and erosion of granite break down its constituent minerals, leaving behind fragments that can be transported and deposited to form arkose. Granite landscapes are often rugged, with distinct topography indicative of its durable composition and resistance to erosion compared to softer rocks. Understanding granite’s role helps in piecing together the geological processes that shape many sedimentary formations.