Dinosaur layer cake

Some of the boys in Primaria are really into dinosaurs and have been
asking for a dinosaur-related experiment.  By talking to them on
previous visits, I got a sense of what would be useful.  They knew
that dinosaurs did not live at the same time as cavemen, but they
didn't know how we know that.  Understanding this brings together a
lot of key ideas in geology and in scientific reasoning, so I thought
it would make a great activity.  But it turned out to be more of a
demo than a small-group activity, so it fit the schedule well on a day
when there was less time for science due to the Easter egg hunt.

I brought a large, clear plastic box and set it on a table in the
outdoor area.  As part of the setup I also filled some buckets with
different materials in the yard: sand, wood chips, and black dirt from
the planter boxes.  I started, as usual, by asking them what they know
about the topic, and I tried to steer the resulting conversation
toward how they know what they know. (Aside: this is one of the few
times I had a conversation with the entire class of 20+ kids at once,
and it was surprisingly not chaotic.  It really helped to have them
seated before the start, with everyone able to see because I was on a
platform.)  One boy was able to give an answer like "men hadn't
evolved yet" but no one know how we know that.  So that provided the
motivation for the following demo.

As part of the preparation, I had also printed out skeletons of
different dinosaurs as well as Lucy and a modern human, and glued
these to pieces of cardboard.  I pulled out the stegosaurus and asked,
"Who knows what this is?"  Then we imagined stegosaurus caught in a
mudslide.  I had a volunteer help me pour the bucket of sand over the
stegosaurus (in the large clear plastic box).  Then, some time later,
here comes a...does anyone know what this is? Triceratops.
Triceratops dies and gets buried in a layer of wood chips, symbolizing
a different type of soil in that area at that time, which ultimately
forms a different layer of rock.  We repeated with a T. Rex and
another layer of sand.

Then we imagined that the area was underwater for a time.  We talked
about how an area could be underwater at times and above water at
other times.  We reviewed what they had learned about rivers and the
water cycle, and decided that layers of sediment can build up on the
lake's bottom or the sea floor.  We also related it to what they had
learned about the deep ocean, that things (like whale bones and
smaller bits of nutrients) rain down from above.  We simulated this by
having a few volunteers rain down black dirt, while I dropped an
elasmosaurus skeleton in.

Next, I did a special, thin, brightly colored layer using a bottle of
paprika.  They guessed it represented lava but I said we would come
back to discuss it later.

Then I brought out Lucy and discussed her, buried her in another layer
of wood chips and then brought out the modern human skeleton and
buried him in a final layer of sand.  The final product was
impressive, clearly showing seven different layers of "rock" through
the clear plastic.  (The box was about 2.5 feet long by 1.5 wide by
1.5 feet deep, and was about 2/3 filled by the end.)  We discussed how
the oldest rock layers are on the bottom and the newest are on the
top, so that the fossils we find on the bottom layers are of creatures
who lived long ago, and the fossils we find on the top layers are of
creatures who lived recently.  (This is true even if an earthquake
comes later and tilts the layers.  I tilted the box and asked who had
been to the Grand Canyon and seen the tilted layers there; a
substantial minority had seen it.)  Do we ever find cavemen (Lucy) on
the bottom layers? No.  Do we ever find dinosaurs on the top layers?
No.  We can even tell which dinosaurs lived earlier, and which lived
later.

Next, I had them exercise their hypothetico-deductive reasoning
skills.  If Lucy had lived as early as the dinosaurs, what would we
find?  If the dinosaurs had lived as late as Lucy, what would we find?

Finally, I returned to the thin paprika band. All over the world, we
find an easily identifiable band called the K-T boundary, and we find
dinosaur fossils only below that band, indicating that dinosaurs died
out around the time the band was formed.  And the band has been found
to contain an element, iridium, in much higher concentrations than
normally found on Earth, but consistent with a certain type of
asteroid.  The conclusion is that an asteroid impact and its aftermath
killed the dinosaurs.

I'm aware that this model is not universally accepted; some scientists
think volcanism played a role in the demise of the dinosaurs, and some
think the dinosaurs were dying out before the asteroid impact, which
perhaps only delivered the coup de grace. But there's only so much
detail you can go into with five-year-olds.  The best thing I can do
to help them deal with nuance as they grow more sophisticated is to
give them practice reasoning with evidence, just as I did.

I left the whole layer cake for the kids to excavate in their free time after lunch.
I had originally envisioned doing something which would make the layers set more
like stone so they would really have to chip away at it, but after finding out that
plaster of paris is toxic, decided not to go there.  I suppose a weak concrete might work,
and I may return to this idea in future years.  If I had done plaster or concrete, I would
have found something to color the layers slightly so they would show a bit of contrast.
As it happened, the sand/woodchips/black dirt made a beautiful set of layers.

I highly recommend reading this story of how Walter Alvarez and collaborators figured out the K-T boundary.  It really shows how
science works; it involves far more creativity and discovery than most
students are led to believe by being forced to do contrived lab
exercises in school.  Unfortunately, many K12 teachers have
experienced science only in that contrived, uninteresting context, and
themselves do not believe science requires creativity, and therefore
create a vicious cycle when they pass that attitude on to their
students.  I'll sign off with this link to a list of misconceptions about science.