District Plan Hauraki Gulf Islands Section - Proposed 2006
(Notified version 2006)
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Annexure 1c - The geology and landforms of the islands
1.0 Introduction
2.0 Basement greywacke rocks, the Waipapa Terrane Group
3.0 Regional subsidence - basal Waitemata Group
4.0 Marine basin sediments - Waitemata flysch basin Group
5.0 Miocene to pleistocene volcanism - includes Coromandel Volcanic Arc Group
6.0 The ice ages and the shape of the islands
7.0 Young volcanoes - Auckland Volcanic Field Group and Auckland lava caves Group
8.0 Geology of individual islands
9.0 Glossary of geological terms
1.0 Introduction
The islands are composed of rocks that were formed during
four different time periods. The islands' shapes and most of their landforms
were produced in the last few hundred thousand years.
2.0 Basement greywacke
rocks, the Waipapa Terrane Group
The greywackes (Waipapa Terrane Group) are the oldest
rocks in the Auckland region. In Auckland City their occurrence at the
surface is limited to the islands. The largest area of greywacke in
the region forms the high-standing Hunua Ranges, but there the visible
exposures of these rocks are small (in streambeds), or weathered in
coastal cliffs. The largest and freshest exposures of these greywacke
rocks and their features are on the more exposed coasts of the islands.
These hard grey rocks accumulated on the sea floor as
sand and mud off the coast of Gondwanaland during the dinosaur age (Triassic
and Jurassic periods, 250-145 million years ago). At this time, the
coast of Gondwanaland lay along a boundary between crustal plates. Here
the leading edge of the ancient oceanic Pacific Plate was sliding under
(subducting) the edge of the Gondwanaland Plate. As the oceanic plate
descended it dragged down the sea floor, creating a deep, elongate ocean
trench parallel to the coast.
Pacific Plate oceanic crust - chert and pillow
lava
The crust of the ancient Pacific Plate consisted of
basalt pillow lava flows that had been extruded onto the ocean floor
at the spreading ridge far out to the east. As the plate moved slowly
westwards towards Gondwanaland, these flows became mantled by thin deposits
of silica-rich mud (called ooze). The ooze was made up of the shells
of dead microplankton that continually fell on the Pacific Ocean floor.
The ooze eventually hardened into splintery red and green chert beds.
Trench sediment
As the ancient Pacific Plate moved down into the deep
trench, the pillow lavas and chert became buried by sand and mud derived
by erosion of the adjacent Gondwanaland. As the sediment layers built
up in the trench, they were deformed by continual movement of the subducting
Pacific Plate. Layers became stacked and progressively tilted towards
Gondwanaland. Younger sediment, and sometimes much older layers of chert
and pillow lava, were scraped off the top of the Pacific Plate (as if
by a giant bulldozer blade) and incorporated into the growing pile of
sedimentary rocks. In this way a wedge of rocks formed and continued
to thicken and extend seawards for as long as subduction and the supply
of sediment lasted.
As the sedimentary pile thickened, the older layers
were compressed and hardened into greywacke. The deeper the rocks were
buried, the more they were subjected to high temperatures and pressures,
which started to metamorphose them. Mineral-rich waters passing through
the rocks at depth deposited quartz and zeolite minerals in fractures
to form the characteristic white veinlets of our greywacke rocks. The
greywackes are Auckland's oldest rocks - we cannot trace the region
and the city's origins back any further in time.
Deposition in the trench finished at the end of the
Jurassic about 145 million years ago, as the large thickness of sediment
that had accumulated was crunched up by collision forces between the
two plates. These forces intensely folded, fractured and faulted the
rocks and started to push them up out of the seas to form a coastal
chain of mountains.
Two slightly different associations of Waipapa Terrane
greywacke occur in the region - a western association (Hunua facies)
of volcanic-derived sandstone with thrust slices of chert and less common
basalt pillow lava; and an eastern association (Morrinsville facies)
of well-bedded, coarser-grained, massive sandstone and occasional granite-bearing
conglomerate. The eastern association, which occurs on Great Barrier,
is usually more coherent, less disrupted, better bedded and less metamorphosed
than the western association, which forms many of the inner islands
of the gulf.
Examples
Localities identified for their significant exposures
of different aspects of these greywacke rocks on the islands are:
- Cherts and their deformation - Island
Bay, Waiheke; Pohutukawa Point, Waiheke; Horuhoru (Gannet Rock); Administration
Bay, Motutapu
- Pillow lavas - Island Bay, Waiheke; Blackpool,
Waiheke
- Trench sediments and their deformation
(western association) - Island Bay, Waiheke
- Trench sediments and conglomerate (eastern
association) - Harataonga, Great Barrier.
3.0 Regional subsidence
- basal Waitemata Group
Following deposition, induration and deformation of
the greywacke rocks, a long period (100-30 million years ago) followed
of which we have no record in the Auckland region as no rocks of this
age are preserved. During this time (80-55 million years ago) the Tasman
Sea opened up and New Zealand split off from Gondwanaland. It would
appear that for most of this long period the Auckland-Northland region
was land which gradually eroded down to a subdued flat-lying landscape
by the Oligocene (30 million years ago).
Towards the end of this period, a new plate boundary
began forming through New Zealand and the new collision forces resulted
in a phase of rapid subsidence in the Auckland region, between 22 and
20 million years ago (early Miocene). A detailed record of this period
of subsidence has been captured and preserved in the fossil-bearing
sedimentary rocks of the basal Waitemata Group, which is best exposed
in the cliffs of the islands.
As the low-lying Auckland region subsided, the sea flooded
the land forming islands out of the low greywacke hills and ridges.
Gravels and shelly sands were deposited on the beaches and in shallow
water around the rocky shore of these islands. These deposits contain
the fossilised remains of numerous shellfish, lampshells, sea eggs and
corals. Intertidal and shallow subtidal rocky shore fossils are not
often preserved and several sections at the west end of Waiheke contain
examples of several hundred different species - a number of which are
known only from this area. As the region subsided the sediments that
were deposited (and the fossils they contain) record a progressive deepening.
First the islands were submerged and eventually depths of 1000-2000m
were reached and the Waitemata sedimentary basin was fully formed.
Examples
Localities identified for their significant exposures
of different aspects of these greywacke rocks on the islands are:
- Most complete sequences: Fossil Bay, Waiheke;
Ocean Beach, Motuihe
- Unique and rich fossils: Double U Bay,
Waiheke; Oneroa, Waiheke
- Limestone and coastal karst: Limestone
Point, Motuihe
- Deeper water barnacles and sea stacks:
West coast of Motutapu.
4.0 Marine basin
sediments - Waitemata flysch basin Group
The Waitemata sedimentary basin (which is unrelated
to the modern Waitemata Harbour) was fully formed by 20 million years
ago (early Miocene). It shallowed up to the north with the land beyond
occupying most of the present-day Northland region. Erosion of this
northern land produced sediment that was carried down rivers and streams
to the coast, which lay in the vicinity of where Wellsford is today.
Large quantities of sand and mud that accumulated along
the coastal shelf periodically became unstable and flowed in a slurry
into the basin. This sediment was funneled down submarine canyons and
on reaching the gentler slopes of the basin floor, spread out to form
undersea fans of sediment, rather like a delta at the mouth of a river.
As the sediment flowed down into the basin, the larger clasts dropped
out first, followed by progressively finer and finer grains. These turbulent
slurries, called turbidity currents, deposited the 10cm-3m thick layers
of sandstone called the Waitemata Sandstones. These layers grade upwards
from coarse or medium sand at their base to fine sand or mud at the
top. Between the sandstone layers there are usually 5-20cm thick layers
of softer, grey mudstone. These mudstone layers accumulated very slowly
on the sea floor as mud settled out of suspension from the seawater
overhead. Each sandstone bed was deposited in only a matter of hours,
whereas the thinner mudstone layers accumulated during the periods of
hundreds of years between each successive sediment flow.
The western boundary of the Waitemata Basin was formed
by the large and actively growing submarine Waitakere volcano. Occasional
volcanic quakes loosened the sea floor high on the volcano's slopes,
causing volcanic gravel and sand to slide eastwards down into the Waitemata
Basin as undersea lahars. These deposited thick beds of darker-coloured
volcanic sediment (called Parnell Grit) within the sequence of more
normal Waitemata Sandstones. Thick Parnell Grit beds can be seen in
the cliffs and shore platforms of Motutapu and Motuihe.
In many places the layering we see in the Waitemata
Sandstone cliffs is flat lying or only gently tilted, but elsewhere
the layers are broken, folded or crumpled. Much of the tight folding
of layers seen within otherwise unfolded sequences was probably produced
by sliding or slumping of the near-surface layers within a few thousand
years of their deposition.
During the 3-5 million years of the Waitemata Basin's
existence, up to 1km thickness of sand and mud accumulated on its floor.
As the layers built up, they were compressed and hardened into the sandstone
and mudstone we see today. The best examples of Waitemata Sandstone
sequences are exposed in the sea cliffs along the east coast of the
Auckland region, with several examples on the islands.
Examples
Localities identified for their significant exposures
of different aspects of these Waitemata Group sedimentary rocks on the
islands are:
- West coast of Motutapu
- Ocean Beach section, Motuihe.
5.0 Miocene to
pleistocene volcanism - includes Coromandel Volcanic Arc Group
The Auckland region was subjected to further tectonic
compression and the Waitemata Basin and sediments that were deposited
in it were pushed up out of the sea to form land from about 18 million
years ago. As the rock was pushed up above the sea to form land, water
erosion and chemical weathering began eating away at and slowly eroding
it. Over the last 18 million years the eastern part of the region has
been uplifted further than the west, allowing deeper erosion in the
east in the vicinity of the islands which has exposed the older greywacke
rocks that further west underlie the Auckland isthmus at depth.
From about 18-4 million years ago, dry land extended
right across from the Auckland isthmus to the Coromandel Peninsula and
Great Barrier. During most of this time a volcanic arc of large andesitic
stratovolcanoes and rhyolitic caldera volcanoes erupted periodically
along the line of present day Great Barrier and the Coromandel Peninsula.
Andesitic stratovolcanoes have a central steep-sided cone composed of
lava flows and breccias which is surrounded by a gently sloping ring
plain composed of laharic volcanic breccia deposits, like Mount Taranaki
or Mount Ruapehu today. The ring plain of some of these stratovolcanoes
that form Coromandel Peninsula extended a long way west, with small
eroded remnants of a 16-14 million year old volcano still preserved
in two small outcrops on the eastern end of Waiheke.
On northern Great Barrier the deeply eroded plumbing
of one of the oldest (18-16 million years old) Coromandel Volcanic Arc
stratovolcanoes can be seen as dikes of andesite and dacite intruding
greywacke. Most of the southern two-thirds of Great Barrier are also
underlain by the eroded flows, breccias, lahar deposits and shallow
plumbing of further, slightly younger (14-10 million years old) andesite
stratovolcanoes. Rare freshwater lake sediment occurs within the stratovolcano
sequence in the south.
Later in the Miocene, 10-8 million years ago, rhyolitic
volcanism broke out in a number of places at the northern end of the
Coromandel Volcanic Arc. Caldera-forming (collapse crater) volcanoes
erupted ignimbrite and rhyolitic domes in the Hirakimata (Mount Hobson)
area of central Great Barrier and at Rakitu. Te Ahumata plateau on southern
Great Barrier is formed of a thick ignimbrite deposit, probably the
remnant of a formerly much more extensive sheet erupted from Hirakimata
caldera. Large rhyolitic eruptions also occurred to the north of Great
Barrier during this same period. Eroded rhyolite domes and ignimbrite
form the Mokohinau islands and seafloor outcrops over a wide area. A
small quantity of basalt occurs with the rhyolites at Rakitu and the
Mokohinau islands, and its ascent from the mantle may have been the
catalyst that generated the rhyolitic eruptions.
Slightly later in the Miocene, 8-7 million years ago,
a basaltic andesite volcano with extensive lava flows erupted near the
east end of Waiheke. Boulder fields of this rock at Stony Batter are
the eroded remnants that provide the only evidence for this volcano's
former existence. Other volcanoes of similar age also erupted near Cape
Rodney to the north.
Around 5-3 million years ago, the Hauraki Gulf and Firth
of Thames areas were forced upwards, tilting the Coromandel Peninsula
to the east and much of the Auckland Region to the west. Following this
up-doming, the elongate central strip subsided dramatically about 3-2
million years ago to form the gulf, the firth and the Hauraki Plains
to the south. About this time (3-1.5 million years ago), another stratovolcano
was produced by eruptions of dacite in the centre of the gulf, forming
Little Barrier volcano. Despite erosion, the volcano still retains some
of its original shape with a steep-sided central cone surrounded by
gently sloping ring plain remnants.
Examples
Localities identified for their significant exposures
of different aspects of these Miocene to Pleistocene volcanoes on the
islands are:
- Fort Hill, Waiheke
- Stony Batter, Waiheke.
6.0 The ice ages
and the shape of the islands
The world has experienced alternating periods of cold
and warm climate during the ice ages of the last few million years.
There have been at least 30 of these cold-warm cycles in the last 2
million years. Each cycle lasted 40,000-100,000 years and included a
warm period similar to today and a cold or glacial period when large
ice caps formed on southern and northern hemisphere continents. These
ice caps froze large amounts of the earth's water on land and resulted
in major worldwide drops in sea level of 130-50m during each ice age
period. Sea level has only been up around its present level during the
peaks of the warmer periods, for about 10 per cent of the time in the
last 2 million years. It has probably never risen more than about 6-10m
higher than what it is today.
During the coldest part of the last ice age, just 20,000
years ago, sea level fell to 130m lower than present. Although other
parts of New Zealand were glacier and ice cap covered, the Auckland
Region was still covered in forest. Today's harbours and the Hauraki
Gulf were forested valleys, with streams flowing seawards across broad
coastal plains. In Auckland a small river flowed down the forested Waitemata
valley and straight out past Motutapu hills beneath what is now Rangitoto.
From there it still had 120km to flow to reach the coast out beyond
Great Barrier and the Mokohinau islands. All of the islands were hills
and ridges joined together by lower lying valleys and plains. Waiheke
and Ponui were separated from the Coromandel Ranges by an extension
of the flat land of the Hauraki Plains. Great Barrier and the Mokohinau
islands were joined together and linked to mainland North Island.
Following the peak of the last ice age, the world's
climate began to warm, the ice caps slowly melted, and the world's sea
level rose correspondingly. Sea level reached its present level about
7000 years ago, although a slightly warmer period 6000-3000 years ago,
resulted in a temporary rise of 1-2m above what it is now. Some low-lying
coastal terraces were formed intertidally at this time (for example,
the flats of Motukorea, carpark flats at Matiatia, Waiheke).
The cycles of wildly fluctuating sea levels had a major
impact on the shape of the islands and their coasts. During each ice
age, erosion on land increased because of colder weather and lowered
base level for the streams. Sediment poured down the rivers (especially
the ancestral Waikato River flowing down through the Hauraki Plains)
and was spread along the coasts by longshore drift. The valleys on and
between the islands' hills were progressively eroded down to levels
well below present sea level. At the end of the last ice age the rising
sea encroached on the land and the sand that had built up along the
coast was swept shoreward. For several thousand years after the sea
reached its present level, vast quantities of sand were thrown up against
the land to form beaches, terraces and dune barriers.
Inside the Hauraki Gulf, where sand supply was not so
great, the river and stream valleys were drowned to become the modern
embayed coastlines and islands. Outside the Hauraki Gulf, on the east
side of Great Barrier, there was a greater supply of white quartz sand
which was thrown up on the beaches and formed sand dune barriers across
the mouth of several valleys forming Whangapoua Harbour and Kaitoke
Swamp. Cobbles eroding from the laharic breccias forming the cliffs
of Little Barrier have been transported by storm waves down both sides
of the island and formed the extensive cuspate boulder flat of Te Titoki
Point on the leeward southwest corner of the island.
Most of the cliffs around the islands are very young
and have eroded out of the sloping hillsides in just the last 7000 years.
The intertidal reefs in front of the cliffs are an indication of the
amount of cliff retreat since sea level rose. Some of the higher cliffs
in harder greywacke or andesite rocks, such as those on the northern
coasts of Waiheke and Little Barrier and eastern coast of Great Barrier,
would have been carved back during each successive period of higher
sea level. These would have become weathering inland bluffs and forested
scree slopes during the intervening intervals between the ice ages.
Today the youthful coasts of the islands are still changing.
They erode in some places and grow in others as nature continues to
respond to the post-ice age rise in sea level, the present pulse of
sea level rise, and to the variable patterns of winds, waves and currents.
Examples
Localities identified for their significant examples
of young coastal and terrestrial landforms on the islands are:
- Motukaha tombolo, Waiheke; Te Matuku Spit,
Waiheke; Stony Batter, Waiheke
- Te Titoki Point cuspate foreland, and
Pohutukawa Flat rock fall, both Little Barrier
- Man o' War Passage, Kaitoke Swamp, and
Whangapoua Harbour, all Great Barrier.
7.0 Young volcanoes
- Auckland Volcanic Field Group and Auckland lava caves Group
Much of Auckland city is built over the products and
landforms of the young Auckland Volcanic Field in which basalt was erupted
from about 50 volcanoes over the last 300,000 years. Three styles of
eruption produced the small basalt volcanoes of the Auckland Volcanic
Field. While some volcanoes were formed by only one style of eruption,
many were formed by a combination of all three. The style of eruption
at any particular time depended upon how much gas was dissolved in the
magma, the rate of magma upwelling, and whether it came in contact with
water.
Most Auckland volcanoes started life with a series of
explosive eruptions. These occurred when rising magma encountered ground
or surface water, which produced superheated steam. Gas dissolved in
the magma was released explosively with the steam and a mushroom-shaped
cloud of ash and shattered rock from the volcano's throat was thrown
hundreds of metres into the air. A shallow explosion crater up to 2km
across and 100m deep was formed and debris from the collapsing cloud
built up a low, circular rim of bedded ash and debris, known as a tuff
ring.
Lava-fountaining eruptions occurred when gas-rich magma
reached the surface without coming into contact with water. The gas
was released quickly, creating frothy lava that was sprayed from the
vent as a near-continuous stream of brightly glowing fragments. As they
flew through the air, the fragments cooled to form red-brown or black
scoria, which accumulated around the vent and built up a steep-sided
scoria cone, often with a deep central crater.
Lava flows developed when degassed magma rose in the
vent and burst out from the base of the cone or breached the explosion
crater rim. Rivers of lava initially flowed off down existing valleys,
but if the outpouring continued a sequence of overlapping flows was
sometimes erupted. This built up a cone called a shield volcano that
gently sloped away in all directions. When the lava flows cooled they
solidified into a hard, dark, fine-grained rock, called basalt. This
has been used extensively in Auckland for kerbstones and many older
buildings, including a number made of basalt from Rangitoto (for example,
the Melanesian Mission House, Mission Bay; and Kinder House, Parnell).
Two of the youngest volcanoes in the Auckland field
formed Rangitoto and Motukorea (Browns Island) in the Hauraki Gulf.
Motukorea is thought to have erupted between 15,000 and 10,000 years
ago, when sea level was considerably lower than now and the Waitemata
Harbour was still a forested valley system. Motukorea initially erupted
explosively forming a shallow explosion crater and surrounding tuff
ring. Parts of the tuff ring form the ridge, cliffs and reefs on the
north-east side of the island. Lava-fountaining built a scoria cone
in the middle of the explosion crater. Portions of some of the early-formed
scoria cone were rafted off by lava flow and form small hills around
the main cone. Degassed lava poured out from around the base of the
western and southern sides of the scoria cone, breached and overtopped
the tuff ring and flowed up to 2km west and south to form an extensive
lava flow field. Most of this field is now drowned beneath the harbour,
but the flat southern and western parts of Motukorea and the surrounding
reefs are the upper parts of the lava flow field.
Rangitoto is the youngest and by far the largest volcano
in the Auckland volcanic field. It erupted just 600 years ago and the
finding of footprints in wet ash on nearby Motutapu indicates that its
eruption was witnessed by local Maori. Rangitoto erupted in the middle
of the main channel into the Waitemata Harbour and its initial eruptions
appear to have been highly explosive with large volumes of ash being
deposited over the northern half of Motutapu. Once sufficient of the
volcano had built up above sea level the dominant styles of eruption
switched to fire-fountaining over the central vent area and the outpouring
of enormous volumes of relatively hot, fluid basalt lava flows. The
fire-fountaining built up a series of scoria cones in the centre of
the growing island with remnants of two earlier cones forming distinctive
bumps on either side of the last formed steep-sided cone with its deep
crater. The lava flows poured out in all directions from around the
flanks of the scoria cones and built up a gently sloping, circular shield
volcano of many overlapping flows. Soon after the last flows had been
erupted some of the still fluid magma withdrew down the vent causing
the scoria cones to subside and creating a shallow moat around them.
Some of the thicker feeder flows high on Rangitoto formed thick crusts
around them while the lava flowing inside was still molten. Later the
fluid lava flowed out of the internal tubes leaving behind lava caves,
which are mostly accessed through sections of collapsed roof.
Examples
Localities identified for their significant examples
of the volcanic landforms and structures of the Auckland Volcanic Field
on the islands are:
- Motukorea
- Rangitoto; lava caves, hornito, lava flow
crust, flow levees and flow lobes, all on Rangitoto.
8.0 Geology of
individual islands
8.1 Pakatoa, Ponui
and Rotoroa
These three islands are composed entirely of Mesozoic
Waipapa Terrane greywacke rocks. Because their east coasts are exposed
to rough seas from the Hauraki Gulf, they are more eroded on this side
and have some of the freshest exposures of these rocks in the gulf.
Several of the eastern sections on these islands provide excellent exposures
of the complex deformed structure and sedimentary features of the western
association.
8.2 Waiheke
Waiheke is mainly composed of greywacke rocks of the
western association of the Waipapa Terrane. Hard beds of red chert occur
in a number of places and because of their resistance to erosion they
often form the more prominent coastal points and high-standing ridges
on the island. Basal Waitemata Group sedimentary sequences outcrop along
the coast in several places at the west end of the island and provide
an excellent record of the initiation of the region's subsidence about
22 million years ago. High on the ridges of the east end of Waiheke
are the greatly eroded remnants of two periods of Miocene volcanism
- 16-14 million year old andesitic breccia transported here as lahars
from the Coromandel volcanoes, and 8-7 million year old andesitic basalt
lava flows from a small shield volcano that now form the distinctive
boulders of Stony Batter.
8.3 Motutapu,
Rakino and the Noises
These islands are predominantly made of greywacke rocks
of the Waipapa Terrane's western association, with a mix of hardened
sandstone, chert and pillow lava basalt. Small sections of basal Waitemata
Group conglomerate and siltstone outcrop in the cliffs on the west side
of Motutapu and east side of Rakino. On Motutapu the basal sequence
passes up into the deeper water Waitemata Sandstones that accumulated
in the early Miocene Waitemata sedimentary basin. Several beds of submarine
lahar deposits (Parnell Grit) are present within the Waitemata Sandstone
sequence. Volcanic ash from Rangitoto thickly drapes the surface of
Motutapu, especially over its northern half.
8.4 Motuihe
The tops of several ancient early Miocene sea stacks
and a somewhat larger island made of Waipapa Terrane greywacke occur
in the southern half of Motuihe. These are overlain by thin sequences
of basal Waitemata Group conglomerate and sandy limestone that accumulated
in shallow water as the region was subsiding around 22 million years
ago, but most of the island is composed of sediment that accumulated
in the Waitemata sedimentary basin following subsidence. This is interbedded
sandstone and mudstone (Waitemata Sandstones) and one 20m thick submarine
volcanic lahar deposit (Parnell Grit) which outcrops in the cliffs around
much of the island.
8.5 Motukorea
Motukorea is one of the youngest and least modified
of Auckland's young basalt volcanoes. The northwest cliffs are eroded
into the remnant northwest arc of its tuff ring, which had been produced
by early explosive eruptions of ash. The high central hill is a scoria
cone with a deep crater. Surrounding low knolls of scoria are portions
of cone that had been rafted away by lava flows. The southern and western
sides of the island are underlain by lava flows, which extend well out
beneath the waves. The extensive 1-2m high flat that forms the southern
and western sides of the island is a high tidal terrace that was built
up in the lee of the island during the Holocene high sea-level stand,
6000-4000 years ago.
8.6 Rangitoto
Rangitoto is the youngest (600 years old) and by far
the largest of the volcanoes in the young Auckland basalt volcanic field.
It erupted in the middle of the Waitemata Harbour's main channel, initially
with voluminous ash eruptions, some of which mantled nearby Motutapu.
These explosive ash eruptions were followed by fire fountaining which
built up a series of scoria cones around the main central vents and
form the steeper knobs of the island's summit. Around the base of these
growing scoria cones, enormous quantities of basalt lava flowed out
in all directions forming a gently dipping, circular shield volcano,
which now forms the bulk of the island. Many of these flows were slow
moving aa types with cooled carapaces of angular blocks, but when they
reached the sea they often developed branching, finger-like tubes when
the hot lava came in contact with cold water. Many of the flows, especially
those higher on the island, developed a thick crust of cooled solid
basalt with molten lava still flowing inside. Sometimes the lava flowed
out from inside the flows, leaving empty elongate lava tubes or caves.
8.7 Little Barrier
(Hauturu)
Little Barrier is the eroded remains of two dacite volcanoes.
Remnants of the older 3 million year old dacite dome only occur near
sea level in the northeast corner, with less eroded remnants of a younger
1.5 million year old dacite stratovolcano forming the bulk of the island.
The distant profile of Little Barrier preserves the original shape of
this stratovolcano with a steep-sided central cone and surrounding gently
dipping laharic ring plain.
8.8 The Mokohinau
islands
The Mokohinau islands have an entirely volcanic origin,
being the eroded remains of late Miocene rhyolitic volcanism at the
northern end of the Coromandel volcanic arc. Also present is evidence
of minor associated andesitic and basaltic volcanism. Burgess and the
associated northern islands in the group are composed predominantly
of ignimbrite and rhyolite lava, whereas the southern island Fanal is
entirely formed by a rhyolite dome.
8.9 Great Barrier
(Aotea)
Eastern association greywacke rocks underlie all of
Great Barrier and appear above sea level to form the high northern part
of the island, north of Katherine Bay and also along a small area of
coast in the east around Harataonga. The largest part of the island,
in the centre and south, is composed of the eroded remnants of mid Miocene
andesitic stratovolcanoes and their laharic ring plain deposits. In
the north, the subvolcanic plumbing from beneath a slightly older stratovolcano
has been exposed by erosion to reveal numerous andesite and dacite dikes
cutting through the underlying greywacke. The high central Hirakimata
(Mount Hobson) part of Great Barrier is composed of the erupted products
of a late Miocene rhyolitic caldera volcano. Ignimbrite is widespread.
The high, flat-topped Te Ahumata plateau of southern Great Barrier is
composed of the eroded remnants of a formerly far more extensive sheet
of ignimbrite that may have been erupted from the Hirakimata caldera
volcano. The eroded columns and flow-banded rhyolite of several steep-sided
domes form high pinnacles and ridges around Hirakimata.
The shape of Great Barrier today is a result of a long
period of erosion of the andesite and rhyolite volcanoes by streams
and waves. When sea level rose after the end of the last ice age the
lower reaches of the stream valleys were flooded by the sea and became
bays and inlets. Those on the western side remain today as Port Fitzroy,
Whangaparapara and Blind Bay. On the eastern side an abundant supply
of quartz sand was thrown up as sand dune barriers across the entrance
to the bays creating Whangapoua Harbour and Kaitoke Swamp.
8.10 Rakitu
Rakitu is the eroded remains of a small rhyolitic caldera
volcano that erupted in the late Miocene. Welded and non-welded ignimbrite
is the predominant rock type with older, rather altered rhyolite lava
outcropping on the north coast and fresh intrusive rhyolite forming
the high southwest corner. A small outcrop of dark basalt lava occurs
within the rhyolite sequence of Black and White Rock, off the west side
of the island.
9.0 Glossary of
geological terms
| Term |
Meaning |
| aa flow |
viscous lava flow with sharp rubbly
outer surface |
| andesite |
grey volcanic rock formed by cooling
lava with an intermediate silica content (52-65 per cent) |
| argillite |
hardened mudstone |
| basalt |
dark volcanic rock formed by cooling
lava with a low silica content (45-52 per cent) |
| bathyal |
water depths of 200-2000m |
| beach rock |
beach rocks, shells and sand cemented
together |
| bomb impact sags |
depression made by a volcanic
bomb landing |
| boudin |
layer of sandstone pulled apart
to form 'a string of sausages' |
| boxwork weathering |
rectangular pattern of hard rusty
ribs (of limonite) produced by weathering of jointed greywacke
|
| breccia |
rock composed of angular gravel-sized
fragments of rock |
| broken formation |
beds broken-up by deformation |
| chenier |
long, narrow beach ridge of shell
or sand built out across intertidal flats
|
| chert |
extremely hard siliceous rock |
| clast |
fragment of pre-existing rock |
| conglomerate |
sedimentary rock composed of rounded
pebbles, cobbles or boulders |
| dacite |
light grey volcanic rock formed
by cooling lava with a high silica content (>65 per cent) |
| dike |
a sheet-like body of igneous rock
that cooled and solidified after being intruded in molten state, and
cross-cutting an existing rock |
| eastern association |
the eastern association (or Morrinsville
facies) of Waipapa Terrane greywacke |
| exposure |
place where weathered rock and
soil has been removed to expose rock beneath |
| flaggy |
rock with a natural tendency to
split into flat oblong slabs |
| fluting |
grooves dissolved on surface of
rock by water |
| foraminifera |
microscopic shell-bearing marine
amoeba-like organisms |
| geology |
study of rocks |
| geomorphology |
study of landforms |
| Gondwanaland |
ancient southern supercontinent |
| greywacke |
hard compacted sedimentary rocks
forming the basement rock of the Auckland Region |
| hornito |
small spatter cone formed on top
of lava flow |
| ignimbrite |
volcanic rock formed by deposition
and partial welding of a high temperature, high velocity flow of fragmented
magma |
| intra-plate volcanism |
volcanism erupting through a tectonic
plate |
| joint |
a fracture in rock |
| karst |
distinctive landforms produced
by solution of limestone rock by rainwater |
| keystone fault |
x-shaped double fault |
| laharic breccia |
deposit of angular boulders and
cobbles left behind by a passing volcanic mudflow (called a lahar) |
| landform |
form of the surface of the land |
| lava cave/tube |
an elongate hollow left behind
inside the solidified outer crust of a lava flow when the molten lava
inside flowed out |
| levee |
elongate ridge of rocks or sediment
deposited on either side of a river or lava flow |
| limestone |
a sedimentary rock comprising
more than 50 per cent calcium carbonate (lime, shell) |
| limonite |
rust-coloured iron oxide mineral
formed during the weathering of iron-rich rocks |
| lozenge |
rhomb-shaped pieces of rock
|
| macrofossil |
fossils that can be easily seen
without magnification |
| Mesozoic |
age of reptiles, between 235 and
65 million years ago |
| microfossil |
fossils that cannot be seen without
magnification |
| Miocene |
period of time between 23 and
5 million years ago |
| mollusc |
shellfish and snails |
| mudstone |
sedimentary rock made of mud |
| normal fault |
fault in which overhanging rocks
moved downwards |
| octocoral |
group of deepwater tree corals |
| ostracods |
tiny crustaceans sometimes known
as water fleas or seed shrimps |
| pahoehoe flow |
fluid lava flow with wrinkled
outer skin |
| paleontology |
the study of fossils |
| Parnell Grit |
thick beds of volcanic pebbles,
grit and sand deposited on the floor of the Waitemata Basin by submarine
lahars |
| pillow lava |
elongate, pillow-like fingers
of lava formed when lava flows under water, often seawater |
| rhyolite |
light grey, pink or white volcanic
rock formed by cooling lava with a high silica content (>65 per cent) |
| rock |
any mass of mineral matter, whether
consolidated or not, which forms part of the Earth's crust |
| sandstone |
sedimentary rock made of sand
grains |
| scoria |
red or black vesicular material
erupted from a volcano |
| scoria cone |
steep-sided volcanic cone composed
of scoria, produced by fire-fountaining eruptions |
| shell spit |
narrow spit of shells built-up
on or near the shoreline |
| shield volcano |
gently-sloping volcanic cone composed
of numerous overlapping, fluid lava flows |
| stack |
small rocky islet |
| stratotype |
a section of sedimentary rock
deposited during a period of geological time and used to define that
period of time |
| stratovolcano |
steep-sided volcanic cone made
of andesite lava flows and breccia |
| subtidal |
shallow marine depths below low
tide |
| surge deposit |
volcanic ash bed left behind by
a sideways explosive surge of hot, often wet, gas-rich ash |
| tension gash |
gash-like split in rock caused
by pull-apart tension |
| thrust |
low angle fault |
| tombolo |
a spit of sand, shell or rocks
linking an island to the mainland |
| top hat island |
a small island with the shape
of a top hat when the tide is out, formed by a central remnant islet
surrounded by a wide shore platform |
| tsunami |
tidal wave generated during an
earthquake |
| tuff |
rock made of hardened ash |
| tuff ring |
a raised circular ring of bedded
ash built up around a volcanic explosion crater |
| type locality |
the rock locality where the type
specimen of a fossil species, mineral or rock was first found and named |
| type section |
the section of rock exposure designated
as the most typical when a formation is named and described |
| vein |
a sheet-like body of minerals
crystallized in a joint or fissure |
| volcanic bomb |
a solid block thrown out by an
erupting volcano |
| Waipapa Terrane |
group name for greywacke rocks
of east Auckland |
| Waitakere Volcano |
giant volcano that erupted off
the west coast of the Waitakere Ranges between 22 and 15 million years
ago forming a volcanic island that has since been eroded away, except
for its eastern slopes which now form the Waitakere Ranges |
| Waitemata Basin |
deep submarine depression that
lay off Auckland 20 million years ago, on the floor of which the Waitemata
Sandstones accumulated |
| Waitemata Sandstone |
interbedded sandstones and mudstones
that accumulated on the floor of the deep Waitemata Basin |
| Waitemata Group |
group name for sedimentary rocks
that accumulated in the Waitemata Basin, as it was forming and after
it became deeply submerged |
| water expulsion structure |
swirly shape of bedding disrupted
by the outflow of water from the sediment beneath |
| weathering |
the processes that break down
rocks to clay and sediment |
| western association |
the western association (or Hunua
facies) of Waipapa Terrane greywacke |
