Learn about sedimentary, igneous, and metamorphic rocks, and how the rock cycle transforms them.

What Is Sedimentary Rock?

A video by Kurtis Baute. The resources accompanying this video were created by Ethan and are shared with a CC BY-NC-SA license.

Summary

Sedimentary rock is one stage of the rock cycle, where the rocks are formed from compression between layers of sediment. The slow formation of these rocks allows for various layers to form, preserving Earth’s history within them.  One form of this history is in fossils, which undergo fossilization and become part of the rock.

Why Watch This Video?

• Have you ever wondered how fossils become entrapped in rocks and what the necessary conditions are in order for them to form?
• Would you like to know how sedimentary rocks help preserve the relative geologic time scale?
• Have you ever been confused by where and how billions of pounds of loose material come together to form sedimentary rocks?

Key Terms

Lithification is the process in which compressed molecules gradually dispel excess gases and liquid and become bonded as a solid rock.  This process can take millions of years as it relies on the weight of the overlaying sediments to apply enough pressure to expel the gas and liquid trapped in small bubbles within the forming rock.

Petrification is the process by which organic materials become permeated with minerals which are preserved as fossils within the rocks.  Petrification is quite a rare form of fossilization, and it is most often found to affect wood, however all organisms can be fossilized through petrification.

Stratification. In geology, this is the formation of the layers which can be seen in exposed sedimentary rocks, they are usually made of varying materials causing unique colours to emerge throughout the strata.  This process always occurs perpendicular to the surface of the Earth due to its gravitational pull.

Loose Ends

Why do sedimentary rocks tilt?

In the final seconds of the video, the narrator shows an example of sedimentary rocks tilting, however he never truly explains why this is.  It is true that sedimentary rocks always form parallel to the Earth’s surface. The tilting of the rock can occur because of uplift caused by the subduction of another rock underneath the one that is tilted. This subduction causes the overlaying rock to lift up, in the case of a tilted rock it has been pushed up more on one side than the other.

What other ways can fossilization happen?

The video only mentions petrification as a method for bones to become fossils, in reality there are many ways for fossilization to occur.  Including mold and cast, carbonization, preservation, and replication to name a few.  All of these methods have a different process for preservation, but all of them result in the formation of fossils.

How long do all the steps of sedimentary rock formation take?

This video fails to justify just how much time is required in order for each step of sedimentary rock formation to occur.  The erosion step alone may take hundreds of millions of years.  Deposition will not happen all at once, and new deposits on top of old deposits will not be compressed into rock at the same time, the new sediment may take millions of more years to be compressed enough to lithify.

Self-Test

Try these questions to test your understanding.

The Rock Cycle

A video by Mike Sammartano. The resources accompanying this video were created by Karla Panchuk and are shared with a CC BY-NC-SA license.

Summary

This video uses a diagram to illustrate the pathways that can transform the three types of rocks—igneous, sedimentary, and metamorphic—into one of the other types, or into a different version of the same type of rock. It’s different from other rock cycle videos because it explains the processes in greater depth, and gives specific examples of different scenarios. If you want to follow along with your own version of the rock cycle diagram, the one used in the video in on page 6 of the New York State Earth Science Reference Tables available here.

Why Watch This Video?

• Have you ever wondered what type of rock was the first to form on Earth?
• Would you like to know how to apply the concept of the rock cycle to understanding processes on Earth?
• Have you ever been confused by the fact that most cycles (like life cycles) happen in a particular order and repeat, but the rock cycle doesn’t appear to work that way?

Key Terms

Cemented. This refers to the process of sedimentary rock formation when minerals grow around grains of sediments. The video refers to “squeezing sediments back together again” to convert an igneous rock into a sedimentary one, but the grains have to get locked together somehow, and one way that happens is with cement.

Recrystallize. In the video it says that intense heat and pressure causes one type of rock to recrystallize into a metamorphic one. Recrystallization is the process where existing mineral crystals in a rock re-form into different shapes. For example, when this happens in a sedimentary rock, fossils made of calcite can be transformed into blocks of calcite, completely erasing the fossil shapes. Note that recrystallization is not the only process that counts as metamorphism.

Gneiss. Gneiss (pronounced “nice”) is a metamorphic rock that forms at high pressures and temperatures (high even for a metamorphic rock). It’s defining feature (aside from being fodder for many puns, frequently in the company of another metamorphic rock, schist) is bands of colour from layers with different minerals. In the video, the gneiss sample looks striped. Unlike sedimentary rocks, where layers reflect sequential accumulation of sediments, the layers in gneiss result from “unmixing” of minerals within the rock as metamorphism proceeds.

Loose Ends

How do specific rock types fit in?

The rock cycle is about how any of the three rock types can be changed into any other type, but those types refer to igneous, sedimentary, and metamorphic rocks in general. The video mentions two specific kinds of igneous rocks (granite and obsidian), and a specific metamorphic rock (gneiss). There are many other conditions that determine what specific rocks will form, meaning that specific rock types are not necessarily interchangeable through rock cycle processes into any other specific rock type. Take the sedimentary rock sandstone as an example. It’s rich in the mineral quartz, and when metamorphosed becomes quartzite. It can’t become the metamorphic rock marble, because marble is made of a completely different mineral, called calcite.

Does a rock have to go through the entire rock cycle?

No. The rock cycle is more a description of possibilities than a list of steps written in stone (so to speak). At one point the narrator says that “a handful of things can happen” to granite, and goes on to describe a scenario where granite is weathered and transformed into a sedimentary rock, a second scenario where it is heated and compressed into a metamorphic rock, and a third scenario where it is melted then cooled, forming an igneous rock all over again. Only one of these things will happen next. Some scenarios in the rock cycle may never happen to a particular rock.

Not all sedimentary rocks form from cemented sediments.

The video describes the formation of clastic sedimentary rocks made from particles that get stuck together, but not all sedimentary rocks form that way. Some sedimentary rocks form when ions dissolved in water precipitate as minerals. The rock cycle could be expanded to include those by showing that weathering can also release ions into water, and minerals can precipitate. To be clear, the rock cycle diagram discussed here is necessarily incomplete because there are simply too many processes to describe in a single simple chart. The rock cycle itself includes all of those processes and steps, by definition.

Self-Test

Try these questions to test your understanding.