What is Mudstone?

How Does Mudstone Form?

Mudstone is a fine grained sedimentary rock made up of muds and clays.  Grains are so fine they can only be seen under a microscope.

The particles that make up mudstone typically come from the weathering of other rocks. Weathering is when the physical and chemical properties of rocks and other materials is altered by exposure to the elements. 

During physical weathering rock is broken down into smaller pieces.  This may happen because of the freezing and thawing of water in cracks or when rocks smash against each other whilst being carried in fast moving water.

Chemical weathering occurs when the chemical composition of rock is altered by exposure to the elements. An example may be acid from rainwater dissolving certain minerals in a rock which leads to the rock's structure breaking down.

Biological weathering occurs when plants, animals and other organisms play a role in breaking down a rock. An example could be tree roots growing into cracks which eventually cause the rock to break apart.

Weathering is a natural process that plays an important role in shaping Earth's surface.  It's an ongoing process that will continue to shape Earth's surface for millions of years. As a result of weathering new rocks, minerals and soils form and new landscapes take shape.
Once materials are broken down they're transported by erosion.  

Erosion is the process by which natural forces such as water, wind, ice and gravity transport materials from one location to another.

The final stage of the process is deposition. This is when sediments are laid down in a new location.  Deposition occurs in environments such as rivers, oceans and deserts.
 
Mudstone formed when tiny grains of mud and clay were deposited in water and sank to the bottom.  Over time they became buried by sediment and were slowly compacted.  This squeezed out any water which led to the hardening of the particles.

The process of cementation eventually led to the formation of the sedimentary rock known as mudstone.

Mudstone typically forms in environments where sediment is being deposited at a relatively slow rate.  The slow deposition of sediment allows clay sized particles to gradually build up.  Over millions of years it hardens into rock.

Sedimentary rock always forms in layers.  The layers which are known as "beds" or “strata” are a prominent feature of sedimentary rock. 

Mudstone is typically a fine-grained dense rock that breaks up fairly easily.  It can range in colour from black to dark grey to yellowish-brown.

Mudstone often contains fossils which can provide important information about the environment in which the rock formed.

With mudstone being composed mostly of clay minerals it's relatively soft.

NASA's Curiosity rover has drilled into mudstone on Mars.

Article Photo

The mudstone in our photograph is at least ninety million years old.  Photo courtesy of James St. John.  The image is clickable and redirects to the original photo.