So today we are at Piha beach which is a really popular destination for people from
Auckland because it's not too far away on the West Coast. But today we're going to focus
on the geology here and we are going to be guided by Bruce Hayward who knows everything there is to
know about the geology of Piha and the whole area around here. So Bruce, you're going to show us
what these rocks are made of, how they originated
as well as how they have been sculpted by the forces of erosion more recently since they were
created. So I suggest we wander on down there and have a look at Whakaari / Lion Rock first,
and let's discuss what we see and let's see if we can figure out what it's doing there
So we're getting closer to Lion Rock and Bruce what I see is not very clear to me. It looks
very chaotic, I see layers going at different angles, I see big blocks, little blocks... Is
there something I'm missing? No no no, absolutely right! When we look at it from here it looks like
a big jumble of rocks all mixed together. And as you say we can see some layering that's horizontal
in the middle of that orange brown weathered area, but as we go higher up in it you can see
layering that's at a different angle. Let's have a closer look to try and figure out what
we might be seeing and how it got there. Here's a couple of nice big boulders that just come down
from up there and we look at it and we see that all the pieces are quite angular here.
We call this a breccia and when we look at it all the pieces are of the same composition
which is telling us that it was one lava flow or one injection of lava that's been
broken up as it was cooling and solidifying and moving. It's broken up into all these pieces
and so they're more or less in place where they've broken up and be brecciated into these these
fragments. Okay and what's in between? So they've got this bright red matrix in between of much
finer particles and the bright red is oxidized iron or iron oxide, all right? So the oxidation
only occurs in air - it doesn't occur under the water - so that that's telling us that this
eruption here was actually into the air and not under the sea like many of the rocks around us.
If we come over here just a little bit we can see all these little
creamy white fragments. Those are fragments of andesitic pumice. Okay now if we go over here...
That's amazing! This looks like one of those lava bombs that you were showing us the other day on
Mangere mountain. Absolutely! So that's a spindle or fusiform bomb of andesite.
I'm delighted to have made such a discovery! You're magnificent! I first saw this in
1973 - this particular bomb - the same one i've been tracking it ever since and it's
only abraded just a little bit in those 49 years. Okay. So you've made a great discovery in fact
if... So if you ever want to make a discovery of Auckland geology good luck to you! But dream on
because Bruce has probably been there first a long time ago. So a lot of this is a bomb material and
splatter that's been erupted into the air. So as it was being erupted these globs of molten
lava were quite plastic but not completely cool and solid and as they went through the air they
pull out, they might spin and they develop these these spindle shapes. And then they've
cooled a bit more, become solid as they're going through the air and as they've landed
they've retained that shape and been incorporated into this deposit of other
material that's been thrown out from the vent. So for your interest we call this agglomerate.
"Agglomerate" - Yes - That's everything together. That's a breccia that's been thrown out from
the vent. Great yeah okay. Well why don't we just walk around the corner here,
there's something interesting we can have a look at. Okay. There's solid lava here,
and it's jointed with these cooling joints all through it. So here we see the the bottom of this
tongue of lava coming in. You can see its contact there with the broken up breccia here underneath
it. Well as we get further back we can see this is a a rounded tongue intruding into the breccia.
So far we've got breccia, we've got ash, we've got pumice, we've got lava bombs - spindle bombs
or whatever That's right. And then we've got a piece of - a chunk of lava intruding into
that mixture. And we've also got red oxidized matrix which is also telling us about erupting
in the air rather than under the sea. Right okay so that's... Okay so we've got a volcano.
What's the overall story that we're learning about Lion Rock? Let's go across the stream and
I'll draw you a picture. Oh okay let's do that. So Bruce you've done some artwork here on the
beach. I've started it Julian! - You started an art piece. Is this related to the topic?
Absolutely - so as I said at the beginning the hills right around Piha are composed of
volcanic conglomerate. So these are the volcanic conglomerates here that surround this whole area
and they're layered okay? So that's what the most of the country around here is. But we've seen
that Lion Rock is quite different. Lion rock is composed of these brecciated rocks all of
one composition and bombs and things that have been thrown out of a volcanic vent it appears.
So this is my volcanic vent coming up through the surrounding rock. And it's been erupting
big bombs and things have gone up here and then some of them have landed back in here and we've
got this big jumble of material that's landed up here. Some of the ash would have been building up
on the sides up here, and some of these sides are then collapsed back down into here. So we've got
layered blocks different angles in here mixed in with the the bombs, the breccias, etc. And then
we've also seen that lava. So the magma coming up at depth may be intruding into this with tongues
coming up like so, of that lava that's coming up and including the material that's fallen
back into the vent. And then we've had erosion and in this instance it's eroded right off this
and now we've got Whakaari / Lion Rock here. Ah right now um if you can interpret that -
that chaotic mess, it's very similar to the chaotic mess of Lion Rock
is that correct? Yes indeed! Great ,so we're done with Lion Rock. We're going to go to
the other part of the beach to see the cliffs there, and we'll let you tell us more! Okay!
We've come a few hundred meters southwest to the end of the beach here
and like you've said we've got quite a different rocks to look at. Well first
of all let's talk about the introduction to the volcano itself Okay! So these rocks here
are believed to have been deposited under the sea on the submarine slopes
of a giant volcano that was located out here to the west. Okay so out there to the west right now
is just nothing. That's out in the Tasman Sea. Okay. And here's a picture, a map if you like
,of the Waitākere Ranges today. Here's the Manukau Harbour coming around here, the Āwhitu Peninsula,
so that's the coast. Auckland is in here - there's the Auckland Isthmus here, so this is Auckland,
this is the Waitākere Ranges here. Right? So the Waitākere Ranges are the eroded remnants
of the eastern submarine slopes of this giant volcano that we can use remote sensing -
like so. So that was the size of the Waitākere Volcano: 60 kilometers across this way, 40
kilometers across that way. And we know by remote sensing techniques that the stump of it eroded off
is out to sea offshore from where we are here at Piha today. Okay so this is all land,
that's a Waitākere Range and that's all out to sea but disappeared. That's all eroded off. Okay. Now
this was the New Zealand's largest strato volcano and it grew on the floor of the sea. If we come
over here, here's the floor of the deep sea and here's the sea surface. Here it began erupting
here about 22, 23 million years ago. Gradually it would have built up - gradually it built up,
and gradually it built up until it was an island, an erupting volcanic island like this.
And these conglomerates that we've got here, volcanic conglomerates,
are material from the slopes of this volcano, some on land
some under the sea, that have flowed down the slopes and been deposited here as layer upon layer
of course volcanic conglomerate. So unlike Lion Rock which was just single composition we're
looking close at these rocks here we'll see that the cobbles and pebbles are of different
compositions all mixed together. Okay and we've got some right here so can you show what we are
looking at? Absolutely! Now if we look at these clasts or these cobbles we can see some are red
so they have been oxidized and erupted on land so they've come from the volcanic island, others
of these are fresh and gray light coloured, dark coloured, some are full of holes like this one,
some are quite smooth, some are orangey, some are light gray with big crystals in them.
They're all different compositions. So these have eroded from different parts of the volcano
all been mixed together as they've been flowing down those submarine slopes and were deposited
here. Great! And just finally perhaps there's a lot of this stuff all along this coastline is
that correct? Yes most of the Waitākere Ranges is made of this conglomerate which we call the Piha
Conglomerate named for this locality. Where to next? Just around the corner! Okay let's do that.
Okay so we can now start looking at the coastal erosion and how it's working on these rocks here.
So if we look at the base of the cliff we get a platform - that's at high tide level that platform
there. And what's happening here is that the rocks above high tide level are eroding back much faster
than the ones that covered in the water at each high tide. Which which is interesting because
you'd expect the waves at high tide level to be quite strong and be washing out... Absolutely!
You'd expect the rocks that are being inundated and battered by the waves during the tidal cycle
to be eroded more than the ones above the tide but it's the opposite. And we think the reason
is that these rocks at high tide level and below are almost constantly saturated with sea water.
So that they're covered with water at high tide and then exposed for maybe 12 hours and they're
completely covered again and saturated every 12 hours. So they stay pretty much waterlogged the
whole time and are therefore more resistant to erosion than the rocks above which are getting
wet by spray etc and then drying out. That wetting and drying will get to break apart and erode much
faster. So we can see often right around the coast here if you look at Lion Rock there's
a lovely high tide platform. It's everywhere around the coast. Another example is this rib
of rock that's vertical in the cliff here. Notice how it's wet. That's because of this tiny stream
flowing over the top there. And so that little bit of rock in the cliff is constantly wet,
it's more resistant to erosion and so now it's sticking out and either side is eroding back
faster where it's getting wet and dry. Ah great okay, that's interesting. So that explains to us
this ledge going around the coast. And what's next? If you look higher in the cliff up here
it's not nice and smooth is it? It hasn't been eroding back evenly, it's cavernous. It's got
all these caves scalloped out of the cliff. And we believe that this is a result of what we call salt
crystallization weathering, or cavernous weathering, or honeycomb weathering. What's
happening there is that we're on the West Coast, we get lots of strong winds and salt spray coming
up and it's wetting the cliff and as it dries the salt crystals grow on the rock and between the
grains and the matrix of the rock. And the growing of those salt crystals pushes the grains apart
and starts to break them open. The wind comes along and sweeps those grains away and it's
gradually erodes back into the cliff. The more cavernous they become the more they collect the
salt water and the more exaggerated that cavernous honeycomb weathering becomes.
So here's Taitomo island here and we can see a wonderful example of how some of the
erosion on the coast occurs. We can see a big flat face here of the rock and underneath you can see
it's being cut back with the high tide platform there and the wetting and drying creating a
cut underneath this flat face. The flat face itself was in fact a fracture or joint - a
big plane that's parallel to that face there. And the rocks have slid down that joint to the
bottom when they've been undercut. This looks exciting! Yes very exciting. So this is Taitomo
island and this is what it's named after, this beautiful tomo or tunnel through the island here.
Here we're on a much wilder part of the sea with the huge waves crashing in.
Hopefully we won't get drowned. It's called the Gap - so everybody at Piha knows this is the Gap.
But one interesting aspect of the Gap is that going across the Gap here is a complex of three
dikes. These are sheets of lava that have intruded up between the the conglomerate on either side,
and forced the conglomerate apart. So dikes don't necessarily have to be just a single intrusion.
In this instance there's been three intrusions come up the one fracture over a period of time.
So just south of the Gap we can walk up through the hills and come down through the blow hole here
and come through this tunnel that's gone right through the cliff here.
And if we look up high in the cliff we can see in fact there's a fracture plane. Let's go and have
a look at another cave just a bit further along. Okay great! The sea gets into that soft rock and
that fracture and erodes progressively along it until it's eroded right through the cliff.
When you look at this particular sea cave you can see in fact it's eroded along two joints or two
planes to create this interesting shaped cave and we've got this lovely tunnel we can go on.
So this is a relatively big cave. This is a long cave that's gone in along this
one major fracture plane but in several places it branches out doesn't it? So here we've got
a column in the middle you can go around either side of that in the middle down here.
Exciting! And the sea would be surging in here at high tide. Yep that's all still
quite wet. And right at the head it's above high tide in the sand which we'll see. Looks like...
You can go that way if you want. Oh okay that sounds exciting...
I'm going this way around - oh there's another pillar!
Bruce thanks very much for this adventurous bit of geology today. I've really enjoyed it
Julian thanks for the opportunity to show you around. So if you ever here for a holiday
don't forget to have a closer look at the rocks! Thanks Bruce!