Clastic rock

Some rocks are made of small bits.

These bits break off from old rocks. The bits can be big or tiny.

Large bits look like pebbles. Small bits look like sand. Very tiny bits look like mud.

These bits get buried deep in the ground. The weight of the ground squeezes them together. This makes them turn into hard rock. It is like how sand turns into sandstone.

Clastic rocks are made of broken pieces. These pieces are called clasts. They come from older rocks. Physical weathering breaks the old rocks into bits. These bits can be big or tiny.

Scientists group these rocks by grain size. Large bits like pebbles make conglomerates. In conglomerates, the pieces are round. In breccias, the pieces are sharp and angular.

Medium bits are called sand. When sand turns into rock, we call it sandstone.

Sandstone often has quartz. Quartz is a very strong mineral.

Tiny bits make mudrocks. Silt is a medium tiny bit. Clay is the smallest bit. If a rock has mostly clay, it is a claystone.

If it has silt, it is a siltstone.

How do these bits become rock? It is a way called lithification. First, new layers of sediment pile on top. This weight causes compaction. Compaction squeezes the bits together. This squeezes out water. Then, cementation happens. Minerals fill the tiny spaces between grains. This acts like glue to hold the rock together.

Clastic rocks are fascinating pieces of Earth's history. They are made of fragments called clasts. These clasts are bits of older rocks or minerals. Physical weathering breaks these older rocks into smaller chunks. These pieces can be huge or very tiny.

Geologists use the word clastic to describe these rocks. They also use it to talk about particles moving in water. This includes things floating in suspension or moving as bed load. Understanding these rocks helps us learn about the past.

There is a special way these rocks form. It is a process called lithification. First, loose sediment like sand or mud settles in layers. More sediment piles on top of the old layers. This creates a lot of weight. The weight causes compaction, which is a squeezing action. This squeezing pushes the grains tightly together. It also pushes water out of the tiny spaces.

Next, cementation happens to hold everything together. Minerals settle into the remaining spaces between grains. These minerals act like a natural glue.

Scientists classify these rocks by their grain size. Large particles are called gravel. This includes pebbles, cobbles, and even boulders. If the gravel is rounded, the rock is a conglomerate.

If the pieces are sharp and angular, it is called a breccia. Medium-sized grains are called sand. When sand becomes hard rock, we call it sandstone.

Sandstones often contain quartz, which is a very stable mineral. Other sandstones might be rich in feldspar or rock fragments.

Very small particles create mudrocks. Silt is a tiny particle between 0.062 and 0.0039 millimeters. Clay is even smaller than that.

If a rock is mostly silt, it is a siltstone. If it is mostly clay, it is a claystone. A mix of both is called mudstone. Some clay rocks are called shale because they have thin layers. These layers are called laminae. These layers form because clay particles are shaped like flat sheets. They stack up neatly on top of one another.

Many different minerals make up these rocks. Quartz is the most common stable mineral in sandstones. It makes up about 65 percent of the grains in those rocks. Feldspars are less stable and make up about 15 percent of sandstone grains.

In shales, quartz makes up about 30 percent of the minerals. Other minerals like mica or heavy minerals might be present in small amounts. The way these minerals settle tells us where the rock formed. For example, pyrite often forms in marine environments. This helps scientists map out where ancient oceans once were.

Clastic rocks are essential components of the Earth's sedimentary layers. They are composed of clasts, which are fragments of pre-existing minerals and rocks. These fragments, or geological detritus, are created when physical weathering breaks older rocks into smaller chunks. Geologists use the term clastic to describe these sedimentary rocks and the particles during transport. This transport can happen in suspension or as bed load within a system. By studying these rocks, scientists can reconstruct ancient environments, such as old river systems.

Siliciclastic sedimentary rocks are a specific type of clastic rock. These rocks are noncarbonate and consist almost entirely of silicon. This silicon appears as quartz or other silicates. The composition includes the framework grains and the cementing material, also called the matrix. Geologists like Sam Boggs, Jr. categorize these into major minerals, accessory minerals, rock fragments, and chemical sediments. Major minerals are classified by their resistance to chemical decomposition. Stable minerals, like quartz, resist breaking down easily. Less stable minerals, such as feldspars, are more prone to decomposition.

Quartz is the most common stable mineral found in these rocks. In sandstones, quartz makes up approximately 65 percent of the framework grains. In an average shale, quartz accounts for about 30 percent of the minerals. Feldspars are less stable and appear in smaller amounts. They make up about 15 percent of framework grains in sandstones and only 5 percent in shales. Clay mineral groups are very common in mudrocks, making up more than 60 percent of the minerals. Accessory minerals, such as heavy minerals or micas, occur in much smaller amounts. Rock fragments can account for 10 to 15 percent of sandstone composition.

Classification is often based on grain size using the Krumbein phi scale. This logarithmic scale orders terms from large to small. Gravel includes particles larger than 2 millimeters, such as pebbles, cobbles, and boulders. When gravel is lithified, it becomes a conglomerate or a breccia. The difference between them is the shape of the clasts. Conglomerates contain well-rounded particles. Breccias contain angular fragments.

Sand grains range from 0.062 to 2 millimeters in diameter. When sand is cemented and lithified, it forms sandstone.

Fine-grained sediments create mudrocks, which consist of at least 50 percent silt and clay. Silt particles range from 0.062 to 0.0039 millimeters. Clay particles are even smaller, measuring less than 0.0039 millimeters. If a mudrock is mostly silt, it is a siltstone. If it is mostly clay, it is a claystone. A mixture of both is called mudstone. Some geologists use the term shale for laminated mudrocks. Clay particles are shaped like tiny sheets. This allows them to stack into layers called laminae. The more clay present, the more laminated the rock becomes.

Loose sediments become hard rock through a process called lithification. This process involves several physical and chemical changes. The first step is compaction, which is a physical squeezing. As new sediment piles on top, the weight increases. This weight increases both temperature and pressure. These forces squeeze the grains tightly together and reduce porosity. Porosity is the amount of empty space between grains. This pressure also helps squeeze water out of the sediment.

Cementation is a key part of the diagenesis process. Diagenesis refers to the chemical and mineralogical changes in sediment. Cementation occurs when minerals precipitate into the remaining pore spaces. These minerals act as a glue to hold the clasts together. This can happen during deposition or later. Early diagenesis, called eogenesis, happens at shallow depths of a few meters to tens of meters. During eogenesis, organisms may rework the sediment by burrowing. This activity, known as bioturbation, can destroy original sedimentary structures like lamination.

Mineralogical changes during eogenesis depend on the depositional environment. In marine environments with reducing conditions, pyrite can form. Pyrite might act as cement or replace organic materials like wood. Other minerals like chlorite, glauconite, and iron oxide can also form. In marine settings, quartz overgrowths and carbonate cements are common. In non-marine environments, oxidizing conditions are usually present. These conditions often lead to the formation of iron oxides. These chemical signatures help geologists understand the history of the Earth's surface.