Lesson video

Hello, my name's Mrs. Niven, and today we're going to be talking about fossils and the rock cycle as part of our unit on the carbon cycle and climate change.

Now, you may have some experience of what we talk about in today's lesson from your previous learning, but what we do in today's lesson will help us to better answer that question of, how can we explain changes that we see in the air, the oceans, and the land? By the end of today's lesson, you should hopefully feel more comfortable being able to describe the formation of fossils and how the rock cycle affects our fossil record.

Now, throughout the lesson, I will be referring to some keywords, and these include fossil, mineral, sedimentary rock, sediment, and the rock cycle.

Now, the definitions for these keywords are given in sentence form on the next slide, and you may wish to pause the video here to read through them or perhaps make a note of them so you can refer back to the definitions later on in this lesson or later on in your learning.

Today's lesson is broken into two parts.

Firstly, we'll look at how a fossil forms in the first place, and then we'll move on to look at the rock cycle and how it affects the fossil record.

So let's get started by looking at forming a fossil.

Some rocks actually contain physical evidence of ancient life, and these are known as fossils.

The word fossil comes from the Latin fossilis which means unearthed.

It gives us a clue of where we can find fossils.

Now, a fossil is something that is typically more than 10,000 years old, and they can be either the remains or impressions of an organism itself, or it could be traces of that organism's behaviour like its footprints or even its waste.

Now, there are three main types of fossils that could be unearthed.

The first occurs when an entire organism is preserved, including its soft parts.

Now, that is the rarest type of fossil, and it tends to be something like an insect that's been trapped in amber.

And amber is simply fossilised tree resin.

The second type of fossil is when an organism's hard parts are preserved, but in their original chemical form.

Now, those fossils are normally found in young sediments, things like a tar pit, and would include some things like bones, some shells, and maybe even exoskeletons.

Now, the third type of fossil is made when the hard parts of an organism are preserved but in an altered or changed chemical form.

Now, those are the most common fossils that we can find, and they're the ones you're most likely to find, say, in a natural history museum.

And what's happened here is that the remains have been essentially mineralized.

Let's stop here for a quick check.

Which of the following would be considered a fossil? Well done if you said all of them.

Absolutely every single one of these would be considered a fossil.

So very well done if you managed to get that correct.

Great start, guys.

Now, you may recall that sedimentary rocks form over millions of years, and it follows a very distinct process.

The first thing that happens is that sediments are deposited, then there's compaction that occurs, so extra layers of sediments fall on top and then compress those lower sediments together.

And then finally, sedimentation occurs, so minerals in the water would glue those sediment grains together.

Most fossils, then, are formed in a very similar way to sedimentary rocks.

The first thing that happens is that an organism dies and then is quickly buried in some sediment.

Decomposers then break down those soft parts, leaving the hard parts behind of that organism.

Now, more layers of sediment then accumulate or build up, and that's gonna increase the pressure and cause the strata, or those layers of sediments, to compact together.

And it's that compaction that creates the conditions necessary for mineral crystals to form.

So the conditions now exist for the remaining hard parts of the organisms to become mineralized, altered, or changed.

And this can happen in one of two ways.

The first way is that the pores within the organism's hard parts become filled with mineral-rich water.

Mineral crystals then can grow in these pores.

The other way is that the hard parts could dissolve in that mineral-rich water and be replaced with mineral crystals from the water.

What this does then is essentially create a stone photocopy of the organism.

Now, that mineralization process takes an incredibly long time to occur, and so after many thousands to millions of years, that is when a fossil has formed.

Let's stop here for another quick check.

Which of the following statements does not occur to an organism's remains when a fossil forms? Well done if you said B.

They are not heated to a high temperature underground.

So well done if you got that correct.

Let's try another one.

True or false, fossils are the mineralized soft parts of an organism.

Well done if you said false.

But which of these statements best justifies that answer? Well done if you said B.

The hard parts of an organism are mineralized.

Fantastic work, guys.

Great job.

Now, a collection of fossils documenting the history of life on Earth is known as the fossil record.

The youngest fossils are about 10,000 years old, but the oldest fossils are about 3.

4 billion years old.

And by studying this fossil record, scientists can learn about past environments and how life has evolved over time.

The problem is less than 1% of organisms become a fossil.

Besides time, specific conditions are needed for a fossil to actually form.

For instance, they need to have the right body type.

Organisms that contain hard parts are more likely to become fossils than those with soft parts.

It's one of the reasons why a natural history museum has more fossils of dinosaurs than they do of jellyfish.

The next thing to consider is exposure.

The organism needs to be buried pretty quickly after death, and that's more likely to occur if an organism has died near flowing water or wind because they can readily erode and deposit sediments, burying those organisms very quickly.

The other thing to consider is that smaller organisms would actually require less sediment in order to become fully buried.

Another thing to consider is the type of sediment that that organism has been buried in.

Bones, shells, exoskeletons of organisms contain a chemical called calcium phosphate, and it's this chemical that makes an organism's hard parts hard.

The problem is, acidic sediments can react with that calcium phosphate, but alkaline sediments don't.

So if a hard part is buried in acidic sediments, that reaction that would occur would destroy the organism's hard parts, and that would leave little if anything left to mineralize and eventually become a fossil.

Another quick check.

In which of the following conditions would a fossil most likely form? Well done if you chose B and C.

It would need to be buried in alkaline sediments and the organism would need mostly hard parts.

So well done if you managed to get one of those correct and incredibly well done if you managed to choose both of them.

Great job, guys.

Let's move now to the first task of today's lesson.

What I'd like you to do is to match each keyword to the most appropriate description.

So pause the video and come back when you're ready to check your answers.

Okay, let's see how you got on.

So a fossil is physical evidence of ancient life, whereas mineralize means crystals of minerals have grown in the pores of an organism's hard parts or replaced those hard parts completely.

A sediment is material that's been deposited by water, wind, or ice.

A trace fossil shows an organism's behaviour, so that's things like footprints or its waste.

And compaction occurs due to the accumulation of many sediment layers.

So very, very well done if you managed to get those correct.

Great job, guys.

The next thing I'd like you to do is to match each of these pictures to the most appropriate description of what's going on.

Pause the video and come back when you're ready to check your work.

Okay, let's see how you got on.

Now, you had to read these descriptions very carefully to make sure you matched them up correctly, and this is the way I would have done it.

The first picture seems to indicate that an organism has died and been very quickly buried.

The second image looks like it contains water, and so I would say that it's showing those remaining parts being mineralized.

The third picture shows the most number of layers, so I would say that that is the layers of sediment that's accumulating and causing compaction.

The fourth picture is a tricky one, but looking at the number of sediment layers, I would have said that this is where the decomposers have broken down an organism's soft parts and leaving its hard parts.

And that would mean that the final picture shows that final fossil that has formed.

So well done if you got those correct.

So now that we've matched up the pictures with the correct descriptions, I'd like you to put them in the correct order to describe how a fossil is formed.

So pause the video and come back when you're ready to check your work.

Okay, so if you've done this correctly, you should have the paired picture and description in the following order.

A, D, C, B, E.

Very well done if you got that order correct, guys.

Fantastic work.

Now that we're feeling more comfortable in how fossils are formed, let's move on to look at the rock cycle and how it might affect our fossil record.

Firstly, we need to recall that Earth is dynamic, which means it's constantly changing and that these changes are due to interactions between Earth's crust, its mantle, and its core.

Now, changes in one part of Earth can result in changes in another.

Now, the crust is broken into pieces called tectonic plates.

And tectonic plates are moved in a variety of ways, and each method depends on how those tectonic plates are interacting with each other or interacting with the mantle.

And we also need to remember that energy transferred from Earth's core drives some of these movements.

Now, as tectonic plates move, conditions in the crust and the upper mantle can change, and these changing conditions then affect the type of rock that might form.

As a reminder, sedimentary rocks form when sediments are deposited, compacted, and cemented together.

Igneous rocks form when magma or melted rock cools and crystallises, and metamorphic rocks form when other rocks are put under intense heat and pressure.

And all of these types of rocks undergo weathering and erosion.

Now, scientists use what's called a rock cycle to keep track of the conditions that form each type of rock and any changes between them.

Now, diagrams as well as its name suggest that the rock cycle, cycle, means that it's circular.

Now, the problem with a circular style rock cycle is that it incorrectly implies that one type of rock will always change into another specific type of rock.

A more accurate rock cycle might look a little bit like this, where you have your three types of rock, and we know that all of them can undergo weathering and erosion to form a sediment.

Now, a sediment can undergo compaction and sedimentation to form a sedimentary rock.

Now, both sedimentary rocks and igneous rocks could be put under some intense heat and pressure to form a metamorphic rock.

And metamorphic rocks and igneous rocks can be melted to form magma.

Magma, then, could cool and crystallise through the process of crystallisation to then form an igneous rock.

Let's stop here for a quick check.

Which of the following processes do all types of rock undergo? Well done if you said C, weathering and erosion.

All rocks can be broken apart and all rock fragments, then, can be moved along the process of erosion.

So, very well done if you got that correct.

Great job, guys.

Let's try another one.

True or false, all sedimentary rocks form metamorphic rocks.

Well done if you said false.

But which of these statements best supports that answer? Well done if you said A, some sedimentary rocks undergo weathering and erosion to form sediments.

Metamorphic rocks are not formed when magma cools and crystallises.

That's an igneous rock.

So well done if you got those correct.

Now, if you recall, a fossil takes many thousands of years as a minimum, to form, and that's really similar to the formation of a sedimentary rock.

In fact, fossils are usually found in sedimentary rocks, and that makes sense because they're formed by a similar process.

The thing is, fossils can be affected by geological processes.

For instance, any process that involves intense heat and pressure could distort fossils, and melting would destroy fossils entirely.

Now, because of that, fossils are not found in metamorphic or igneous rocks.

Another quick check, then.

Which picture below shows a sedimentary rock and why? Well done if you chose A.

A is definitely our sedimentary rock, so well done if you chose the correct answer there.

Why? Because it's the only rock that shows a fossil, and fossils are only found in sedimentary rocks.

So well done if you were able to explain your choice as well.

Great job.

Geological processes don't just affect the type of rocks that we can find fossils in, but they can also impact the quality of the fossils that are unearthed.

For instance, some fossils are found incomplete, and that's usually due to weathering or erosion that's taken place over time.

Other fossils could be broken due to extreme compaction or due to some kind of sudden shift like an earthquake or a landslide.

And in fact, that's what's normally found with a larger fossil, that it's more likely to be found in a broken state.

Now, you may recall that the fossil record is a collection of fossils documenting the history of life on Earth and the constant geological changes, though, that have occurred during Earth's history means that the fossils in this record are usually broken or incomplete.

Nevertheless, these plentiful, even if broken and incomplete, these fossils provide a lot of evidence that supports theories that scientists have about how organisms might have died, but also how they might have lived and also conditions that might have scattered or weathered those organisms' remains.

So even if they're broken or incomplete, they can still be very, very useful.

Let's stop for another quick check.

Which of the following may have happened that means a fossil is not found at all, or if it is, it's found broken or incomplete? Well done if you said all of them.

That's exactly right.

All of these are reasons why a fossil might not be found or if it is found, it's broken or incomplete.

Well done.

Okay, let's move on, then, to the last task in today's lesson.

For this first part, what I'd like you to do is match each statement to the type of rock it describes and then match each type of rock to an example of it.

So you may wish to pause the video here and come back when you're ready to check your work.

Okay, let's see how you got on.

So sediments that are compacted and cemented together and contain fossils are known as sedimentary rocks, and shale is an example.

Magma or lava that cools and crystallises is igneous rock, and granite is an example.

And rocks that change due to intense heat or pressure are metamorphic rock, and marble is an excellent example.

So well done if you managed to match those up correctly.

Great job, guys.

For this second part, I'd like you to add arrows and labels to show the processes that are involved in the rock cycle.

Now, this might take a little bit of time.

You might want to look at some reference material, maybe notes from a previous lesson or other resources, and definitely talk it over with people nearest you.

I personally would recommend creating a key and maybe using some colours to color-code any processes that occur multiple times in this rock cycle.

So definitely pause the video and then come back when you're ready to check your work.

Now, this task allowed for quite a lot of freedom in how you completed it, so as you check your work, please pay close attention to the arrows.

Are they starting and pointing in the correct direction? And is there some way of determining what that arrow represents? Have you created a key or did you perhaps label those arrows in some way? If you've managed to do that, very, very well done.

And I would challenge you, if you have time, to maybe go back and add a few examples of each of the three different types of rocks 'cause that just helps to flesh it out a little bit as well.

But very, very well done on what could be a very tricky task.

Great job, guys.

I'm so impressed.

For this last task, we want to look at a very impressive fossil.

In 1990, the largest, most extensive, and best preserved fossil of a Tyrannosaurus rex was found in South Dakota in the United States of America, and this fossil was nicknamed Sue.

Now, Sue was unique for a variety of reasons, but one was because of how many of its bones were unearthed.

250 bones of a possible 380 were discovered.

What I'd like you to do is to describe another reason why the discovery of Sue might be considered unique.

So pause the video and come back to check your work in a moment.

Now, we already know that the discovery of Sue was impressive because of the number of bones that were unearthed.

And you were asked to describe another reason why Sue's discovery might be unique, and your answer could have included any of the following.

For instance, Sue is a Tyrannosaurus rex, which we know is a very large dinosaur.

And it being quite large, larger organisms were less likely to become fossils because they were more difficult to bury fully and quickly.

So Sue must have died somewhere where it could be buried quickly and fully.

Moreover, that burial took place in good chemical conditions, meaning Sue was not buried in acidic sediments that might have reacted with some of the chemicals in its hard parts, its bones, meaning those bones were left and they were allowed to mineralize into a fossil.

And finally, Sue was lucky enough not to be affected by geological processes that might have broken or melted those fossilised parts before it was finally discovered.

So lots of different things that needed to fall into place in order for the fossil to be formed in the first place, for the fossil not to be broken or melted before discovery, and then finally, all those bones that were discovered as well.

An incredibly impressive find and very useful for our fossil record.

So incredibly well done if you managed to come up with even one of those reasons why Sue was unique, and very well done if you managed to get more than one answer.

Great job.

Wow, we've gone through a lot of information in today's lesson, so let's take a moment to summarise what we've done.

Well, we learned that fossils are often the remains or the imprints of living plants or animals and that bones buried in these sediments can be very slowly dissolved by water and replaced by minerals in it to become mineralized to form that fossil.

But that geological activities can break, destroy, or distort fossils along the way.

And because of that, fossils can only be found in sedimentary rocks.

I've had a fantastic time learning with you today.

I hope you've had a good time learning with me, and I hope to see you again soon.

Bye for now.