Here we are at the north end of Takapuna beach on Auckland's North Shore.
We're standing here on the edge of a car park that was built over a reef here in
the 1970s. Since then we've been able to look at the rest of the reef and suddenly realize
that the reef is in fact a national treasure. It's the only example of a fossil forest that's been
fossilized and in part engulfed by a lava flow that came from Pupuki volcano which erupted
about 200 000 years ago and the flows came down through here. So how did we have a forest here?
It's because 200 000 years ago the sea level was a lot lower during the colder periods of the ice
ages and so there were forests growing here and on the slopes of the Waitamata valley which flowed
out underneath Rangitoto Island there during every time of cold ice age period when sea level was
lower. So let's go and have a look at this fossil forest. So here we are coming down onto the reef
and we've got some of the remains of the lava that came down from Pupuki and flowed around the base
of these standing trees that were here. And as it went around the tree the tree wood was cold
and cooled the lava and the lava congealed and solidified around it. You can actually see the
layers of gas bubbles captured within that lava. But before the whole lava flow here had cooled
and solidified in place the rest of the molten lava flowed off and on down the slope. And so it
left the standing mold around this tree that had been set on fire and the tree would be burning
and eventually it all incinerated and it's left this empty hole that we see inside the tree mold.
Now if we look over here at the rest of the reef all of a sudden you can see
dozens of tree molds like this. In fact that over 500 of them have been mapped just on this piece of
reef here. So while that hot lava was cooling around the base of all these standing trees
it also cooled on top in contact with the air. So this is the crust of solid lava
that cooled on top of the molten lava which was still underneath
before that rest of the molten lava flowed away. So we've got portions of the crusted over roof
still left here at the reef as well. So here we've got two tree molds: one there
one there and then the molten lava was still under here has flowed away and we've been left with this
wonderful natural arch - miniature natural arch - here with the roof held up by the two tree molds.
So one of the reasons why this fossil forest is so nationally important is that it's so well
preserved. In fact if you look at it it's so fresh and well preserved it almost looks as if it was
formed yesterday, but in fact we know it erupted 200 000 years ago. So how come it looks so fresh?
If it's been sitting here on the surface for that long It would have weathered away. What's happened
actually is it's been preserved underneath layer upon layer of volcanic ash that was later erupted
from Pupuki. Possibly five to ten metres of ash was above us here and that for 190 000
years was sitting here protecting this lava flow and fossil forest with all its little features.
If we look around we can see some of that ash here but most of it's been eroded off by the sea
just in the last six to seven thousand years. And because that ash is a lot softer than the
basalt lava the sea's come along and it's eroded out the ash and so it's exhumed the fossil forest
and the lava flow surface that we see here it's so fresh today. Oh look what we've got here!
Looks like a big canoe actually with lava cooled on either side of it and clearly it's in fact the
mold of the large straight trunk of a tree. It's straight it's not branching so we think that this
was probably the trunk of a kauri tree. And look it's a a good meter, 1.2 meters across diameter
which is the sort of diameter we might expect with a middle-aged kauri tree and it goes off
here in both directions over 10 meters long. so imagine that lovely tree standing up here before
that lava flow came down knocked it over and then has captured its shape in that molten lava.
Let's have a look down here. We've got these curved ropey structures.
These are typical pahoehoe that formed on the skin of a lava flow that would have been moving in this
direction away from the volcanic center over here. So sometimes we have molds of the trunks in situ
or as fallen into the lava but here we've actually got the mold of a branch and the lava has captured
its shape as it's cooling before the branch has incinerated away. Here we can see the branch is
curved and then it branches out to number little side branches here in fact the one comes out
through here. If I put my hand in - there we are - that's the side branch there that's been captured
by that molten lava and we can see the shape of the branching upper part of this particular tree.
So now we've just moved a little bit further along up beside us here we can see there's quite a small
cave and it doesn't go on any further so it looks like it's just a a distinct and discrete cave,
and these are usually formed by gas. So that we said there's a lot of gas coming out of the lava
as it's moving along and then sitting here and that gas bubbles all congeal together to create
a big gas bubble and it pushes up the skin or upper crust of the lava flow to create a blister.
So this is probably a gas blister underneath the cooled crust of the lava flow that was formed as
the lava flow was cooling. So we're getting a mix of the molds and the blisters in this instance.
Is that a blister? It's the shape of a blister. Or is it in fact
the mold of a log that was still plastic when some more lava came over it and maybe has squashed
it down? so originally it was cylindrical and it's been squashed down and created this shape.
And it's the mold of another of these giant kauri tree trunks
that were standing up here when the lava flow came up around it and captured it.
I hope you've enjoyed what we've been looking at here in the fossil forest and the lava flows
along this section of coast. There's still a lot more to see here, so come along and and explore
it yourself see what you can find and interpret and have a great day next time you'll come here.