Question

While on a field 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, and the sandstone is called arkose.

Step-by-step solution

Identify the Characteristics of the Sandstone

The sandstone observed is described as poorly sorted, rich in feldspar and quartz. Poorly sorted means it has a range of grain sizes, and the presence of feldspar alongside quartz is notable.

Analyze Site #1 - Basaltic Lava Flows

Basaltic lava is typically formed from mafic material, which is composed mainly of minerals like pyroxene and olivine. While basalt weathers to form clay minerals rather than feldspar and quartz, it is less likely to produce sandstone rich in quartz and feldspar.

Analyze Site #2 - Granite

Granite is a felsic rock composed primarily of quartz, feldspar, and mica. Weathering of granite would produce sediments high in quartz and feldspar content, which matches well with the composition of the observed sandstone.

Determine the Likely Source Site

Comparing the compositions, granite is more consistent with the poorly sorted sandstone rich in quartz and feldspar. Thus, Site #2 is the most likely source of the sediment.

Identify the Sandstone Type

Sandstones that are rich in feldspar and quartz and suggestive of a granitic source are commonly called arkosic or arkose sandstones.

Key concepts

Sandstone

Imagine you are holding a rock that feels gritty in your hand, with tiny grains just large enough to see without a magnifying glass. This is sandstone, a sedimentary rock that mainly consists of sand-sized mineral particles or rock fragments. It can come in various colors, including tan, brown, yellow, red, gray, and even white.

Sandstone forms from layers of sand deposited by wind, water, or ice, which become compacted and cemented over time. This process, called lithification, transforms these loose sand grains into a solid rock. One special property of sandstone is its porous nature, meaning it can hold water, making it significant for aquifers.

• Color: Varies widely depending on mineral composition
• Formation: Formed by compaction and cementation of sand-like particles
• Porosity: Can hold water, important for groundwater storage

When we come across a sandstone rich in feldspar and quartz, like in the geology exercise, it suggests a specific origin, often from a felsic rock such as granite.

Granite

Granite is one of the most familiar and abundant types of rocks found across continents. It's an igneous rock, which means it forms from the cooling of magma beneath the Earth's surface. The slow cooling process results in large, visible crystals of its minerals.

This rock is predominantly composed of quartz, feldspar, and mica. Its texture is usually coarse-grained, and it can range in color from pink to gray, based on the mineral composition. Granite is very hard and durable, making it ideal for construction and architectural use such as in countertops and building facades.

• Composition: Mainly feldspar, quartz, and mica
• Formation: Igneous, formed from cooled magma
• Texture: Coarse-grained due to slow cooling

Given its mineral content, granite weathers to produce sediments rich in quartz and feldspar, making it an ideal source for certain types of sandstone.

Feldspar

Feldspar is a group of minerals that are incredibly common in the Earth's crust. In fact, they constitute about 60% of the rocks found on Earth's surface, making them the most abundant mineral group. Feldspar minerals are essentially aluminosilicates containing potassium, sodium, and calcium.

Feldspar can be identified by their blocky crystal shapes, and they come in an array of colors including pink, white, gray, and brown. They are important in the production of ceramics and glass, and they weather over time to form clay minerals.

• Abundance: Makes up about 60% of the Earth's crust
• Uses: Ceramics and glass production
• Weathering: Transforms into clay minerals

In sandstone, the presence of feldspar is significant as it indicates little transportation from the source rock, typically pointing to a nearby origin, often a felsic rock like granite.

Quartz

Quartz is one of the most recognizable minerals due to its abundance and hardness. It is a component of many rock formations and is often the last mineral to melt during the rock-forming process due to its high melting point. Being a stable mineral, quartz is resistant to weathering and stands the test of time.

Microscopically, quartz crystals are hexagonal and can be found in various colors, including clear, white, pink, and purple. This mineral has many uses, from glassmaking to electronics, due to its physical and chemical properties.

• Hardness: Very hard and resistant to weathering
• Structure: Hexagonal crystals
• Uses: Glassmaking, electronics

In sandstones, quartz is usually the most durable common component, which is why it remains prevalent even as other minerals break down. It typically indicates an origin from granite or other rocks rich in quartz.