Conglomerate and breccia - Amazing Geology

Introduction

On site you will find, among other things, two rocks: one with conglomerates and another with breccia. This EarthCache invites you to discover the differences between these two types of rock, understand how they were formed, explore their properties and which geological processes shape them.

Tasks on site

To log this EarthCache, go to the given coordinates, examine the two rocks and answer the following questions and send them to me by email or via the message center:

1. Describe the visible differences: Examine the two rocks. How do the shape and size of the fragments (grains) differ in both rocks? Pay particular attention to the roundness or angularity of the fragments.
    
2. Determine the classification: Which of the two rocks is a conglomerate and which is a breccia (Name the number regarding to the picture above)? Justify your decision based on the observed characteristics
    
3. Is the breccia polymict or monomict? Justify your answer.
    
4. MANDATORY: Take a photo!!! Since June 10, 2019, a log photo may be required.
   Please attach a photo with the James Hutton memorial stone that documents your visit there, but does not reveal answers to the questions asked - your GPS device, talisman or other personal item in the log photo is welcome!

What are conglomerates and breccias?

Conglomerates and breccias are clastic sedimentary rocks that consist of fragments of other rocks (clasts). Between these larger fragments there is a fine-grained matrix that cements the larger fragments together. They differ mainly in the shape of the clasts:

• Conglomerate: The clasts are rounded. They have been rounded by long transport routes, for example in rivers.
• Breccia: The clasts are angular and sharp-edged. These indicate a short transport route or a rapid break-off, e.g. due to a landslide or a tectonic fault.
• There are also rocks with round and angular parts; such a mixture is referred to as a fanglomerate. The rock contains a lot of coarse material and only a small amount of fine-grained matrix as a binding agent.

Conglomerate	Breccia	Fanglomerate

Formation, classification and properties
 

Conglomerates:

• Formation: Conglomerates often form in river systems, on erosion coasts or in glacier areas. Transport by water or ice ensures that the clasts are rounded off. (Wind is not enough for these larger chunks, the water must also have a high flow speed and transport capacity). After the diagenetic solidification of these pebbles, conglomerates are formed. The individual grains of the conglomerates can consist of all possible types of rock that are present in the area of ​​origin.
• Classification: Clasts must be larger than 2 mm and held together by a fine-grained matrix.
• Properties: The round shape of the clasts indicates that they have been transported for a long time. Conglomerates often have a well-sorted composition and appear stable.
   

Breccias:

• Formation: Breccias typically form in landslide areas, on tectonic faults or through volcanic activity, where material is quickly broken and deposited and which are cemented by a fine-grained matrix.
• Classification: As with conglomerate, the clasts are larger than 2 mm, but here they are angular. The grain size of breccias exceeds two millimeters; rocks with angular grains of smaller grain size are called microbreccias, and grain sizes over 50 centimeters are called megabreccias.
• Properties: The sharp edges and angular shapes show that the transport route was short. Breccias often have a less well-sorted composition and can appear fragile. If the fragments consist of only one type of rock, the breccia is monomictic; if there are different types, it is polymictic.

Breccias are differentiated according to their origin, such as sedimentary, volcanic, diagenetic, tectonic and impact breccias:

1. Sedimentary breccias:

These breccias are formed by the rapid disintegration of rock material in landslide areas or on steep slopes. The material is only transported and deposited to a small extent, which preserves the angular shape of the clasts.
 

2. Volcanic breccias:

Volcanic breccias are formed during explosive volcanic eruptions when lava fragments and rock are torn apart and deposited by the explosion. They are often found near vents or lava flows.
 

3. Tectonic breccias:

These are formed along fault zones when rocks are crushed and broken by tectonic movements, such as faults or shearing. The angular clasts are often accompanied by fine-grained rock dust (cataclasite).
 

4. Diagenetic breccias:

Diagenetic breccias are formed by chemical processes during the solidification of sediments. One example is the dissolution of rocks by groundwater, creating cavities that collapse and form breccias.
 

5. Impact breccias:

These breccias are formed by meteorite impacts, where the rock is torn apart by the enormous pressure and shock wave. They are often found in craters and can contain melted and shattered rock fragments.
 

Have fun discovering these fascinating rocks!

Sources
Wikipedia
Geologyscience
Press  Siever Allgemeine Geologie