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The Difference Between Fossilization and Petrification

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Unless you are a scientist, it is unlikely that you know the actual difference between fossilization and petrification. Even though many untrained people use these terms interchangeably, there is a distinct difference between the two processes.

Fossilization refers to any process which produces fossils. One of these fossilization processes is called petrification. What separates petrification from other types of fossilization is that during petrification, minerals replace organic matter.

If you’re still a bit confused, that’s ok. The differences between fossilization and petrification can be challenging to grasp. Let’s continue by discussing more details about each of these processes.

The Difference Between Fossilization and Petrification
The Difference Between Fossilized and Petrified

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Understanding the Fossilization Process

The clearest way to describe the fossilization process is to say that it produces fossils. But what exactly are fossils?

To most, the term fossil conjures up images of dinosaur bones at a museum. But fossils are more than the bones of once-living animals. Fossils are the remains of an organism or evidence of that organism’s presence. Here are some examples:

  • Footprints
  • Shells
  • Bones
  • DNA Remains

It is important to know that when paleontologists find fossils, they are never finding the actual organism. Instead, they are finding evidence of that organism’s existence.

For example, when scientists discover a dinosaur fossil, they do not dig that animal’s body out of the ground. What they are uncovering might be footprint or bone from that animal.

This is because an organism’s remains tend to break down very quickly. This may be due to another animal eating the remains or simply through the natural decaying process. Either way, the flesh of an animal is gone long before scientists arrive with their shovels.

Building the Fossil Record

Countless researchers invest their time trying to establish a better understanding of the past. These professionals are concerned with understanding forms of life that existed before the rise of human beings.

This information helps us to develop a better grasp of evolutionary processes. Through this inquiry, we learn more about how the world today came to be.

Because organic matter decays relatively quickly, paleontologists rely on indirect proof of an organism’s existence. That indirect proof comes in the form of fossils.

There are a few different types of fossils. Below are some of the more common classifications:

  1. Petrified Fossils
  2. Mold Fossils
  3. Cast Fossils
  4. Trace Fossils
  5. Body Fossils
  6. Carbon Films
  7. True-Form Fossils

By studying these fossil types, scientists work to create what is known as the fossil record. The fossil record refers to a group of fossils that scientists have studied and arranged in chronological order.

By organizing this data, we get a glimpse into the past. Each year we learn more about species that died off before our time.

So, what role does petrification play in this quest for knowledge? Read on to find out.

Petrification Process

Let’s again start with a straightforward definition.

Much like how the fossilization process produces fossils, the petrification process produces petrified fossils. But what makes a fossil petrified?

Petrified Fossil – A fossil formed when minerals replace parts of an organism.

That is the technical definition. But a more apt way of describing petrification would be to say that it is a process by which dead plants or animals turn to stone.

When petrification is complete, the organism is no longer there. But in its place, is a mineral version of the organism. The mineral version shares the same size and shape as the original organism it replaced.

That is why petrified fossils are so useful to paleontologists. While the original organism is long gone, an accurate replica remains.

For petrification to occur, a specific sequence of events is necessary:

  1. The organism becomes buried shortly after dying
  2. Ground seeps into the organisms’ remains
  3. Minerals in the water slowly replace the organism’s body

The three steps above describe a natural process called permineralization.

Permineralization is the precursor to a petrified fossil. The next section will break down the details of how this process takes place.


The creation of petrified fossils requires permineralization. This process only occurs when a specific set of circumstances are in place after the death of the organism.

Organism Burial

The organism must become buried after dying. If the organism dies and remains in the open air, it will decompose before permineralization can begin.

You can see how this first step prevents most organisms from turning into petrified fossils.

Funeral burials are a cultural creation by humans. For the vast majority of earth’s history, species died off with little fanfare.

When an animal died, most often, another animal would come along and eat it. But even if no other animal ate it, the dead organism would still decompose fairly quickly.

Burial after death would be a rare occurrence in pre-human times. This would have to result from some natural event. For example, a landslide might kill an animal and then quickly bury it.

Considering that type of event is rare, you can see why petrified fossils don’t arise in large quantities. But even if the dead organism is buried, there are other factors needed for permineralization to happen.

Groundwater Seeps In

The next needed step is for groundwater to seep into the area where the organism is buried. But it can’t be just any water. Water for permineralization must fall into one of two categories of water:

  1. Hard Water – Water with high concentrations of minerals
  2. Soft Water – Water with low concentrations of minerals

Water can have a lot of variation in its mineral content. Since soft water has limited mineral density, it is not suitable for permineralization.

Hard water, by contrast, is the perfect ingredient to continue the permineralization process. Luckily for fossil enthusiasts, most groundwater is hard water.

This means that on the off chance that an ancient organism was buried after death, there is a reasonable likelihood that permineralization will occur.

Once the animal is in the ground, and the mineral-rich groundwater has arrived, there is one last step to the permineralization process.

Cell Replacement

Provided that the first two prerequisites are in place, permineralization can proceed. This third step involves the replacement of the organism’s cells.

After an organism is dead, many of its cells take a little longer to die off. As each individual cell dies, a void is left where it once was.

When that void appears, the groundwater will quickly fill it. The solid minerals in the water will take the place of the dead cells’ former position.

Over a long time, each cell will die off, leaving its unique void. So long as there is plenty of hard groundwater, the minerals will continue to fill in those gaps.

Eventually, the minerals will replace every cell that was a part of the organism’s body. The result is a stone in the exact shape of the organism’s remains. The organism is completely gone, and the process of permineralization has successfully produced a petrified fossil.

TIP: Many kinds of fossils come from the ocean. Do you know which other gemstones come from the ocean? I recommend reading this article:
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Different Minerals, Different Fossils

There is more to permineralization than simply replacing cells with minerals. The types of minerals also matter. Depending on the minerals present in the groundwater, petrified fossils will have different levels of detail.

Of course, many minerals could be present in groundwater. But the three types of minerals which contribute most to the creation of petrified fossils are:

  1. Silicates
  2. Carbonates
  3. Iron

These three mineral types have differing effects on the petrification process. The quality of the resultant petrified fossil depends on which minerals happen to be present at the time.

Mineral TypeLevel of Detail
Effects on the petrification process by different minerals

As the above table indicates, silicates and carbonates are more likely to produce petrified fossils with a high level of detail. This has to do with the nature of the crystalized forms of these minerals.

Silicates and carbonates consist of fine-grained crystals. Because of this, these minerals can replicate minute details of an organism’s structure.

In contrast, iron has a larger crystallized form. This means that iron is only able to reproduce the larger, more generalized forms of an organism.

Environmental Factors

Each of these minerals is more common in specific settings. The presence of silicates, carbonates, or iron depends on a certain area’s particular environmental conditions.

Mineral TypeEnvironmental Precursor
SilicatesIgneous Rock – Granite, Basalt, Volcanic Ash
CarbonatesMarine or Non-Marine Settings
IronHigh Levels of Sulfur – Common in marine settings
Minerals types and environmental precursors

Since these minerals arise in distinct environmental settings, each one is likely to fossilize different organisms.

For example:

  • Since iron occurs most frequently in a marine setting, it is more likely to petrify a marine organism.
  • Silicates are more likely to fossilize land animals since they occur in rocky terrestrial areas.
  • Carbonates often exist in both settings.

However, regardless of the mineral type and environmental setting, some organisms are more likely to become petrified than others.

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Common Petrified Fossils

Petrified Fossils Rockhounding
Petrified Fossils

Because of the unique process involved in petrification, some organisms are more likely to become petrified than others. By far, the most commonly petrified organisms are plants.

Petrified Wood

Although it takes hundreds of millions of years to produce, petrified wood is the most ubiquitous type of petrified fossil. But you wouldn’t know that petrified wood is a fossil just by looking at it.

Petrified wood looks just like regular wood. But if you were to attempt to lift it, you would find it is far heavier than you expected. As described above, even though these fossils look just like wood, they have actually turned into stone.

Petrified wood is so common, as a fossil, that there are entire petrified forests that you can visit. The most famous is Petrified Forest National Park in eastern Arizona.

This popular park receives thousands of visitors each year. These visitors come from near and far to view petrified trees that are purported to be over 200 million years old.

At the park, there is a multitude of logs from conifer trees. These logs became petrified over the eons and now lie scattered throughout the desert in intriguing sculptural forms.

But even though wood is most common, any organism can become a petrified fossil. Because of this, petrified fossils make a significant contribution to the fossil record.

In addition to petrified fossils, other fossil types contribute to our knowledge of the past as well.

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Non-Petrified Fossils

Non-petrified fossils rockhounding
Non-petrified fossils

Petrified fossils are highly informative to researchers, but they are far from the only type of fossil. In fact, there are many types of fossils, all of which form through their unique process.

Mold Fossils

Mold fossils are a perfect example of the fact that many fossils represent evidence of an organism’s existence.

In this scenario, no part of the organism’s remains has become fossilized. Instead, a mold fossil is a hollow space where the organism once existed.

Again, mold fossils require burial. When an organism dies and remains in the open air, animals, insects, and microorganisms feed on the dead organism’s flesh. This leaves little possibility for fossilization.

A mold fossils form when a dead organism is buried and leaves an imprint on the earth. This happens when the deceased organism is lying in sand or mud.

As future layers of mud form over time, the mud surrounding the organism will turn into stone. But the shape of the animal will remain.

This type of fossilization most often serves as evidence of past species with exoskeletons or shells. Because these animals have hard exteriors, they are more likely to leave a discernible imprint in the sedimentary rock which surrounds them.

There are two types of mold fossils:

  1. External Mold Fossils – Mold of the outside of an organism’s anatomy
  2. Internal Mold Fossils – Mold of the inside of an organism’s anatomy

To understand the difference between an external and an internal mold, imagine a snail shell. A snail shell has a definite exterior shape. But it also has a hollow internal shape. This internal shape is where the snail lives.

If a snail were to become a mold fossil, it could either produce an external or an internal mold.

The external pattern would represent the outside of the snail shell pressing into sedimentary rock. The inner cavity would represent sediment filling the inside of the shell.

These mold types are opposites of each other, but either way, both these mold types are evidence of the snail’s existence. Beyond that, they also give rise to another kind of fossil.

Cast Fossils

Cast fossils rely on mold fossils to form first. Cast fossils are an opposite version of mold fossils, but they also share a relationship with petrified fossils.

After a mold fossil forms, groundwater may fill the mold later on. This presents the opportunity for minerals to settle in the void space the organism left behind.

However, in contrast to the permineralization process, the minerals present in the formation of cast fossils do not replace organic matter. Instead, they are merely filling the space. The result is another stone similar to a petrified fossil.

The critical difference is that cast fossils don’t directly replace the tissues of organisms. Instead, they fill the space left by the organism after it has decayed.

Trace Fossils

Trace fossils are indirect evidence of an organism’s existence. The fossils are also formed in soon-to-been sedimentary rock. However, unlike previously mentioned fossils, trace fossils are not representations of an organism’s body.

On the contrary, trace fossils represent an organism’s activity. Trace fossils can tell scientists a lot about an animal’s behavior, but they don’t necessarily help us clearly identify the animal.

Here are some examples of trace fossils:

  • Footprints
  • Burrows
  • Scratch Marks
  • Feces
  • Tooth Marks

A paleontologist studying a tooth mark might be able to determine many facts about the animal’s life. It may be clear that the animal was a carnivore or an herbivore. But the tooth mark alone won’t always tell them to which carnivore or herbivore species the tooth belonged.

TIP: How is it that fossils are often found in sedimentary rock but not igneous or metamorphic rock? Find out the answer in the article below:
Fossils Can Be Often Found in Sedimentary Rocks & Here’s Why

Body Fossils

Body fossils are very similar to trace fossils. Neither one represents an animal’s entire body. While trace fossils show an animal’s activity, body fossils represent one part of the animal’s body.

Here are some examples of body fossils:

  • Teeth
  • Claws
  • Bones
  • Eggs

As with fossils in general, body fossils most often form from the harder parts of an organism’s anatomy. This is because materials like bone take longer to break down than flesh and are more likely to impact the surrounding earth.

Carbon Fossils

Up to this point, the fossils we have discussed have typically represented animals with hard anatomies. But what about the softer organism, like insects or plant leaves? This is where carbon fossils come in.

Carbon fossils form when a soft-bodied organism dies and is pressed between layers of sedimentary rock. This pressure produces heat, and eventually, the organism dissolves.

Other gases like oxygen and nitrogen tend to dissipate, but carbon often remains. This carbon forms a thin layer of film which remains preserved.

The film appears in the form of a dark residue. This residue is painted onto the rock in the shape of the organism it represents. Carbon fossils are a great resource when studying prehistoric plant life.

True-Form Fossils

As we have covered so far, fossils are usually indirect evidence of an organism. When scientists discover fossils, the actual body is not present.

But as with most topics, there are always exceptions. True-form fossils are rare cases where much of an organism’s body remains intact and preserved.

The primary way that this happens is when an animal becomes encased. The following substances can allow this to happen:

  • Ice
  • Resin
  • Tar

If you have seen the movie Jurassic Park, you may remember a scene featuring one of the most common types of true-form fossils – amber.

Amber fossils are made of tree resin that typically encapsulates an insect. The resin then hardens, and the insect’s pure form remains throughout time.

Clearly, true-form fossils are of great use to scientists. These fossils reveal what a past organism looked like in real life. This provides a much more direct study of a species rather than relying on indirect evidence.

TIP: Amber fakes are very diverse. Real amber is an organic gemstone, so there is another way to spot amber fakes. Find out how to spot fake amber in the article below:
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Making Sense of Fossils

The fact that an organism can be preserved at all is an intriguing concept. But studying fossils is about more than marveling at past species and the passage of time.

The goal of collecting and analyzing fossils is to learn about the past. By doing so, we can have a better understanding of the present. As we further understand past species, we are better able to determine how contemporary species came to be.

Questions about the past are so enthralling that there are multiple professions dedicated to establishing and drawing conclusions based on the fossil record. Each of these professions requires years of specialized academic study.

Without these specialized individuals, we would have no way of making sense of fossils. The professionals most responsible for furthering our understanding of the history of life on earth are listed below.

You will likely be familiar with the first listed profession, as we have mentioned it many times in this article alone. The others are not as clearly defined in the mind of the average person:

  • Paleontologists
  • Anthropologists
  • Archeologists

Each of these professions uses a specific lens to study the past. Each one also makes a unique contribution to our collective scientific knowledge.


Paleontologists study the history of life. They use fossils to learn about specific organisms and how they lived throughout time. They use the many types of fossils listed above as clues. By combining these clues, they can classify different species of animals.

These scientists pay close attention to change over time as well as relationships between species. All of this information allows for a more in-depth study of evolution.

Unlike the next two professions, paleontology is a much broader field. The members of this field study all forms of life throughout all periods of history. This is the purest study of fossils in current existence.

As you will see below, anthropology and archeology are very different fields of study. These professions use a more specified focus as well as different methods of inquiry.


Anthropology is the study of human beings. This field focuses on human culture throughout time rather than studying all forms of life.

Anthropologists use fossils in their pursuits, but that is only one tool at their disposal. The approach is more holistic, taking into account all relevant factors to human activity. The goal of an anthropologist is to understand, in a broad sense, why humans act the way they do.

This is not to be confused with psychology, which is more focused on individual behaviors. Instead, anthropologists look at larger treads about groups of people around the world. They study how these groups have lived and developed cultures over the centuries.

Another interesting distinction about anthropology is that it is not solely focused on the past. Unlike paleontologists, anthropologists often study people living today.

As a large field of study, anthropology is divided into four subcategories. These subfields differ in their specific topic but share the same methodology:

  1. Biological Anthropology – The study of human adaptation and the evolutionary process by which humans came into being.
  2. Cultural Anthropology – The study of how humans derive meaning, including through interactions with each other and the establishment of societal rules and expectations.
  3. Linguistic Anthropology – The study of how humans communicate and how language is structured as a means to share information.


All of these branches of anthropology are relevant to humanity. But biological anthropology and archeology may be the most relevant to the search for fossils.

Archaeology is the study of human artifacts. It is the branch of anthropology that focuses on collecting and interpreting human artifacts.

These artifacts come in a variety of forms. Some of the most useful and common artifacts are below.

  • Pottery
  • Stone Tools
  • Teeth 
  • Bones

In the same way, that a paleontologist uses the fossil record to determine facts about past species, archeologists use artifacts to learn about past human activity. This activity can be related to the day-to-day lifestyle of past humans.

Archeology also teaches us how humans have used elements of the natural world to their benefit.

Understanding the Past

The obvious challenge of learning what happened before our time is that we weren’t there to see it. Because of this, there are countless questions to resolve and mysteries to solve.

Paleontologists, anthropologists, and archeologists all play a vital role in this effort. By combining the knowledge from these three fields, we can learn about the forces that formed the present day.


Learning what happened in the past is a challenging task. It seems that there is a never-ending amount of information relevant to the story of life on earth. After all, the past constitutes everything that happened from the beginning of time until now.

But this learning is much easier when you have a good understanding of relevant terms. Hopefully, I have helped define the differences between fossilization and petrification.

Next time these topics come up, you will be ready to actively share your knowledge of fossils and how the past has led to life as we know it today.

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