What is the Difference Between Mudstone and Shale

The main difference between mudstone and shale is that mudstone is a type of sedimentary rock with parallel layers of minerals, whereas shale is one type of these layers in the mudstone.

Mudstone and shale are two common terms in geology that describe rocks. Mudstone is a type of sedimentary rock that forms from the deposition of clay and mud over a long time period. These mudstones are layered structures where one layer is named as a shale.

Key Areas Covered

1. What is Mudstone 
     – Definition, Features
2. What is Shale
     – Definition, Features
3. What is the Difference Between Mudstone and Shale
    – Comparison of Key Differences

Key Terms

Mudstone, Shale

What is Mudstone

Mudstone is a type of sedimentary rock which is finely grained, and original constituents are clay and mud. The grain size of this material is less than 0.063 mm. It is difficult to distinguish the individual grains because these grains are too small. Therefore, grains are observed under the aid of a microscope. When high pressure is applied on the material over time, the platy clay minerals become aligned, giving the mudstone an appearance of fissility or parallel layering.

Mudstone appears as a finely bedded material that can split readily into thin layers. These thin layers are called shale. Sometimes, mudstones may lack fissility or parallel layering due to disruption. About 65% of sedimentary rocks are mudstones and shale. We can often observe mudstone as hardened clay. Under some circumstances, these mudstones show some cracks or fissures.

Figure 1: Mudstone in a Beach Area

There are a few categories of mudstone. These categories include siltstone, claystone, mudstone (mudstone can be further categorized into shale and argillite).

What is Shale

Shale is a type of clastic sedimentary rock that contains mud, flakes of clay minerals and a trace amount of other minerals. More precisely, shale is a thin layer of mudstone. Mudstones have several parallel layers of material. Shale primarily contains mud and clay. But there can be trace amounts of quartz, and calcite. We can observe shale as breaks along thin laminae or parallel layering. This is the most common type of sedimentary rock. It has bedding less than 1 cm thickness.

Figure 2: Shale

Typically, shale has varying degrees of fissility. This makes the material break into thin layers, usually in the parallel direction. Shale appears in grey colour. However, the colour can change depending on the other minerals present in the shale. For example, there is a black shale that occurs due to the high amount of carbonaceous matter. In addition, there can be red, green, and brown colours due to the presence of compounds such as ferric hydroxide, iron hydroxide, micaceous minerals, etc.

Difference Between Mudstone and Shale

Definition

Mudstone is a type of sedimentary rock which is finely grained, and original constituents are clay and mud. In contrast, shale is a type of clastic sedimentary rock which contains mud, flakes of clay minerals and a trace amount of other minerals.

Structure

Mudstone has a layered structure while shale has a single layer

Composition

Furthermore, mudstone mainly contains mainly clay and mud while shale contains clay, mud, and some minerals. 

Conclusion

Mudstone is a type of sedimentary rock that is abundant on Earth. It has a layered structure. The main difference between mudstone and shale is that mudstone is a type of sedimentary rock with parallel layers of minerals, whereas shale is one of these layers in the mudstone.

Reference:

1. “Mudstone Rock Definition, Uses, Formation and Properties.” Geology Science, 9 Apr. 2019, Available here.
2. “Mudstone.” Wikipedia, Wikimedia Foundation, 17 Apr. 2020, Available here.
3. “Shale.” Wikipedia, Wikimedia Foundation, 1 Dec. 2019, Available here.

Image Courtesy:

1. “East Beach 1 2006” By Ballista at the English language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “ShaleUSGOV” (Public Domain) via Commons Wikimedia