Cleavage (geology)

Rocks can change deep in the ground.

Heavy pressure pushes on them. This makes the rock form flat layers.

Some rocks have layers that go all the way through. These are called continuous layers. Other rocks have layers with gaps between them. These are called spaced layers.

These layers can even follow the shape of folds in the rock. It is amazing how pressure shapes the Earth!

Rocks can change when they are deep underground.

Heat and heavy pressure push on them. This creates a feature called cleavage. Cleavage is a type of secondary foliation. Foliation is a way that flat layers form in a rock.

There are two main kinds of cleavage. The first kind is continuous cleavage. This happens in fine grained rocks. The minerals in these rocks are spread out evenly. One type is slaty cleavage. This forms when the layers are very thin. The space between layers is 0.01 mm or less.

The second kind is spaced cleavage. This happens when minerals are not spread out evenly. The rock forms layers with gaps between them. These gaps are called microlithons. One type is crenulation cleavage. This happens when a rock is pushed more than once. The new pressure makes tiny folds in the old layers. Cleavage can also follow the shape of rock folds. This is called axial planar foliation.

These layers help engineers understand how strong a rock is.

Geologists study how rocks change deep underground. One important way they change is through cleavage. Cleavage is a type of secondary foliation. Foliation is a term for flat layers or patterns in a rock.

This happens to fine grained rocks when they undergo metamorphism. Metamorphism is when rocks change due to heat and pressure. The way a rock changes depends on its minerals and the stress it feels. This can change how strong a rock mass is for building tunnels or slopes.

There are two main ways cleavage can look. The first is continuous cleavage. In these rocks, minerals are spread out evenly.

One type is slaty cleavage. This forms when the layers are very thin. The space between layers is 0.01 mm or less. The second way is spaced cleavage. This happens when minerals are not spread out evenly. These rocks have layers called cleavage domains and gaps called microlithons.

Cleavage forms through several different ways it works. One way is through the rotation of mineral grains. Grains that are long or flat will turn to face a certain direction. Another way is called solution transfer. This happens when pressure causes minerals to move and redistribute.

Minerals like mica can also grow in new ways. This is called recrystallization. Dynamic recrystallization happens while the rock is being changed. Static recrystallization happens after the change or without much movement.

Scientists use specific names for different patterns. Crenulation cleavage is a type of spaced cleavage. It occurs when a rock is pushed more than once. The new pressure creates tiny microfolds in the old layers.

Another type is transposition cleavage. This happens when a new layer completely replaces an old one. This shows the rock was pushed many times. There is also disjunctive cleavage. In this type, the gaps do not have tiny folds. Some people used to call this fracture cleavage, but that is an old term.

Cleavage often follows the shape of large rock folds. This is called axial planar foliation.

The layers form parallel to the center of the fold. In rocks made of sandstone and mudstone, the pattern can look like a fan. This is called foliation fanning. The mudstone layers bend to fill gaps between the stronger sandstone. This helps engineers know how rocks will behave under pressure. Understanding these patterns is a key part of structural geology.

In the study of structural geology and petrology, cleavage is a vital concept. It describes a specific type of planar rock feature. These features develop as a result of deformation and metamorphism. Metamorphism is the process where rocks change due to heat and pressure. Cleavage is actually a type of secondary foliation. Foliation is a term for a fabric element that describes how planar features develop in a rock. While primary foliation occurs in igneous and sedimentary rocks, secondary foliation occurs in rocks that undergo metamorphism. Cleavage specifically refers to secondary foliation in fine-grained rocks.

Cleavage can be categorized into two main types: continuous and spaced. Continuous cleavage, also called penetrative cleavage, occurs in fine-grained rocks where platy minerals are evenly distributed. In these rocks, minerals like mica or amphibole align in a preferred orientation. Even minerals like quartz or calcite can deform into a specific shape. Slaty cleavage is a specific form of continuous cleavage. It is often the first cleavage feature to form after deformation begins. It is defined by having very thin layers, with only 0.01 mm or less of space between them.

Spaced cleavage is different because the minerals are not evenly distributed. This results in discontinuous layers or lenses of different mineral types. Spaced cleavage consists of two distinct parts: cleavage domains and microlithons. The cleavage domains are the planar boundaries. The microlithons are the areas bounded by those domains.

One specific type is crenulation cleavage. This happens when a rock undergoes multiple phases of deformation. The later deformation creates symmetric or asymmetric microfolds within the previous foliation. Another type is disjunctive cleavage. In this version, the microlithons are not deformed into microfolds. An older, outdated term for this was fracture cleavage.

Several mechanical processes work together to create these patterns. One process is the mechanical rotation of mineral grains. During ductile deformation, grains with a high aspect ratio rotate. They move so their orientation matches the plane of the tectonic strain. Another process is solution transfer. This involves the redistribution of minerals through pressure solution and recrystallization. This process helps elongate and rotate minerals like mica into a preferred orientation.

Recrystallization also plays a major role in how cleavage develops. Dynamic recrystallization occurs during metamorphic conditions. It happens when a mineral's chemical composition reaches a new equilibrium. This occurs because deformed micas can store strain energy. This energy allows for the oriented regrowth of minerals into the damaged crystal lattice. Static recrystallization can happen after deformation or without active movement. If the heat is intense, the foliation might actually weaken. This can turn the rock into a hornfels with randomly oriented crystals. However, minimal heat can strengthen the cleavage by growing micas parallel to the foliation.

Cleavage often shows a geometric relationship with large rock folds. This is known as axial planar foliation. These foliations are arranged symmetrically around the axial plane of the fold. In sequences containing both sandstone and mudstone, a unique pattern can emerge. This is called foliation fanning. In these cases, the cleavage is divergent in the mudstone layers. It becomes convergent in the sandstone layers. This happens because the stronger sandstone beds buckle, while the weaker mudstones deform to fill the gaps.

Understanding these features is essential for many scientific and practical reasons. In geology, scientists look for transposition cleavage. This occurs when a younger foliation erases an older one. This provides evidence that the rock experienced multiple deformation events. For engineers, cleavage is a critical factor in geotechnical work. A cleavage plane creates a discontinuity in the rock. This can significantly influence how a rock mass behaves. Engineers must consider this when constructing tunnels, foundations, or slopes. Knowing the strength and deformation of the rock helps ensure safety and stability.