New Zealand's Volcanic North Island

For about a week, I toured the North Island of New Zealand with ten travelers on a Smithsonian Journeys adventure called Journey Through New Zealand. The North Island is where volcanism, geysers, mud pots, and hot springs play an oversized role in shaping this very active landscape. We visited and witnessed many of the features left behind by these dynamic forces.

This is a true color satellite image of New Zealand on a relatively rare cloud-free day. Blueish Lake Taupo is clearly visible in the center of the North Island and the white spine of the snow-clad Southern Alps is also obvious on the South Island.

Zooming in on the same image to the North Island with our stops listed. I flew internationally into Auckland and traveled overland to Rotorua, Lake Taupo, Napier, and the capital at Wellington.

Auckland

A view of the Central Business District (CBD) of Auckland from across Waitemata Bay at Devonport. The view is to the southwest. Auckland was built within a basaltic volcanic field that is still active! This photo is from my trip in 2017 to New Zealand.

A slide from my lecture to the group. Basaltic volcanism began here about 250,000 years ago. This image, from Geoffrey Cox's book "Fountains of Fire" shows the landscape around Auckland before volcanism began and during the Ice Age, when sea level was about 300 feet lower than present. (The modern shoreline in Auckland can be seen as the faint dashed line). I have annotated the future CBD (shown in the photo above this) and the International Airport where I arrived. Note that the Waitemata River in the top of the diagram flows east into the Pacific Ocean (left to right), but the Manukau River flows opposite this towards the west and into the Tasman Sea.

Each yellow circle represents one of the approximately 50 scoria cones located in central Auckland (not all 50 are within this image). The city center and international airport are also shown for reference to the previous diagram. The red circle in the upper right is Rangitoto Island and it is the most recent of the volcanoes to erupt - only about 500 years ago.

This is a view of Rangitoto Island from the top of Mt. Victoria in Devonport (one of the oldest volcanoes in this field). This photo is from my trip in 2017 to New Zealand.

The formation of Rangitoto Island with Maori people watching from the future site of Auckland city. Image used from "Fountains of Fire".

Drive South

Heading south, we passed Mt. Pirongia, a late Pliocene-Pleistocene basaltic center. The various peaks show basaltic vents on the volcano, now inactive. A native forest can still be found here. New Zealand was a land of unique wildlife and flora inherited from its separation with the Gondwana supercontinent. However, the introduction of exotic mammals and other pests, begin Ning with the Maori about 800 years ago, decimated native bird populations, with about 40% of endemic species now extinct. Therefore, the New Zealand government has strict requirements for entry into the country. I once traveled on a trip here where two of my fellow companions were fined $400 US each for having mud within their lug-soled boots.

Ruakuri Cave

No, this is not the extreme vertical entrance to the cave we visited. But it is a natural opening in the limestone near the town of Waitomo, where a series of solution caves have been carved into the Te Kuiti Group. This marine limestone is dated at between 27 and 23 Ma, at a time when most of New Zealand was submerged beneath sea level.

This is the man-made entrance to the cave, which is nearly identical to the natural outcrops of the limestone.

The cavern system is extensive and has numerous speleothems (cave formations) such as stalactites, stalagmites, and curtains such this one.

The caves in New Zealand are famous for their glow worms (one is curled here in the yellow oval). These are the larval stage of a dipteran fly. Their scientific name is Arachnocampa luminosa.

The natural "glow" of the glow worms. The ISO on my camera is set at 6400 and this is a hand-held image.

This is the big room in Ruakuri Cave showing the many formation to be seen here. A river runs beneath this section of the cave and "blackwater rafting" is offered inside the cave.

Rotorua

The Pohutu geyser. (This photo is from my trip to New Zealand in 2017).

Tree ferns and the Pohutu geyser on March 25, 2019. This is a localized venting area within the much larger Rotorua caldera. The rim of the caldera can be seen in the distance through the steam.

We arrived just as the geyser began its 20-minute display of steam and fountaining.

A nearby mud pot erupts with its boiling mud. The chemistry of the thermal water is such that it dissolves some of the enclosing volcanic rock, which is rich in silica. The tiny particles of silica mix with the boiling water to create the mud. I just love boiling mud pots!

At the Te Puia park, the local Maori people share their culture and crafts with the public. We enjoyed a traditional hangi, where the food is cooked in the ground traditional style.

On the road south from Rotorua toward Lake Taupo.

Lake Taupo

The Taupo Volcanic Zone is one of the most active, land-based volcanic areas on planet Earth. Its volcanic history is revealed through the many deposits scattered across New Zealand and out to sea. New Zealand is located in a very complex tectonic setting.

First off, New Zealand is the emergent part of an 8th continent - Zealandia. The continent is outlined in the black dotted line above. Part of the continent rests upon the Pacific tectonic plate and the other is part of the Australian plate. Continental crust, rich in silica and alumina makes up the crust of all continents including Zealandia, even though 85% of its rock remains submerged beneath sea level. Zealandia was once part of the supercontinent Gondwana but broke away from Antarctica and Australia beginning about 85 Ma.

Today New Zealand sits astride the boundary between the Pacific and Australian plates. Note however, that the two islands reside are on different plates. This explains the degree of complexity in New Zealand's geology.

A north-projecting oblique view of Zealandia with cross-sections cut across both the southern and northern ends of the continent. Note how the Australian plate is being subducted beneath the Pacific plate in the south, while the opposite occurs in the north with the Pacific plate subducting beneath the Australian plate. In between these two opposing subduction zones runs the Alpine fault, a right lateral offset much like our own San Andreas fault. With two opposing subduction zones and a San Andreas-type fault, it is an understatement to say that New Zealand is geologically active.

Close-up of the Taupo Volcanic Zone showing the four large caldera complexes of Rotorua, Okataina, Maria, and Taupo, and the three stratovolcanoes of Tongariro, Ngauruhoe, and Ruapehu (New Zealand will challenge those who suffer from vowel-phobia). 26,500 years ago a tremendously large eruption from Taupo created the caldera there.

These slides are from my lecture to the group and highlight some of the enormity of this eruption, the largest on planet Earth since the Toba (Indonesia) eruption about 71,000 years ago.

The 26,500 year Taupo eruption is known as the Oruanui eruption and this illustration shows the relative amounts of material erupted in other, more well-known eruptions. About 100 cubic miles of rock was erupted. For comparison, about 1/4 of a cubic mile was erupted at Mt. St. Helens in 1980.

Lake Taupo today is a quiet, peaceful place with a small town of retirees living along it quiet shores. I would imagine that most residents know something about the lake's violent past but maybe don't want to dig too deeply into the details. For myself, I find it interesting how things change with time.

Napier

These changes can be readily understood by those who live in and visit Napier, a unique city on the North Island's Pacific shore. On February 3, 1931 the entire Central Business District (CBD) was destroyed in a 7.8 magnitude earthquake. Within two years, the CBD had rebuilt in a modern style for the day - what we call Art Deco style today. Only in the 1990s, when some of the buildings from this era were being razed for newer development did the residents realize that they had a treasure - a city entirely built when the rest of the world's economy was stagnant during the Great Depression. Now, Napier's CBD is a tourist draw. The building above highlights the specific time when the city was rebuilt. To learn more about this devastating New Zealand earthquake, see here.

This could easily be a building from the 1930s movie, Superman. The Daily Telegraph was Napier's newspaper back in the day.

Another Art Deco building in there CBD of Napier. Of course, I have many other photographs but free time is short on this trip and I cannot include as much as I would like.

Wellington

We spent the day driving south toward Wellington, New Zealand's seat of Parliament and its capital. The city is located on a beautiful bay with many long-running ridges trending southwest-northeast. This fabric is due to a splay of faults that also have historic movements resulting in tragic human casualties. The January 23, 1855 earthquake with a magnitude of 8.2-8.3 is the strongest jolt in New Zealand in historic time. A whopping 59 feet of lateral offset was documented from this one quake. It seems as if we have been traveling in New Zealand with me talking to our small group only about one devastating event after the other. Don't think for a single moment that I am not keenly aware of how this must appear to my fellow companions, who may not have the time-honed ability to separate the stupendous natural history story of these events from the resulting loss of life and property. The scale of the events here is impressive and New Zealand is a MUST visit for anyone with an interest in the natural world and especially geology.

The red cable cars climb the hills to the west.

A view to the west from Mt. Victoria toward the CBD and the harbor area. What a lovely setting and we all loved this gentile city of about 400,000 people. Note the faceted spurs on the hill in the upper right of the photo. These triangular-shaped spurs highlighted here in sunlight denote where the Wellington fault cuts across the view. This fault is one of the many strands of a fault system that runs through this area. The Wellington fault runs along the base of the hill.

Flying out of Wellington toward the west, I took this last shot of the southern end of the North Island. I will be posing about my South Island journey next.

New Zealand Journey

I'm traveling in New Zealand with Smithsonian Journeys and a wonderful, small group of ten people. We've toured the North Island and I'll be posting photos in the next few days. Friday, March 29 is a National Day of Mourning for the shooting victims in Christchurch, where I will be on March 30 and 31. As you can imagine, a small, safe country like this is reeling from the trauma To begin my postings from here, I include headlines from a single edition of the Auckland newspaper, the New Zealand Herald. I hope this will give you sense of the gravity of the situation here.

The Alamo (Nevada) Breccia - Evidence for a Late Devonian Impact Event on the Continental Shelf of Ancient North America

Last week I accompanied my colleague Dr. John Warme, retired professor of geology from the Colorado School of Mines, and three other geologists to look at the Alamo breccia, a quite out-of-place layer of conglomerate sandwiched within many thousands of feet of fine-grained, shallow marine limestone. The opportunity was not to be missed and so I was off to southern Nevada.

Physiographic map of Nevada showing the location of the type section of the Alamo breccia, about 100 miles north of Las Vegas. Note the fabric of mountain ranges within Nevada that trend north to south. This pattern is due to extension (stretching) of the Earth's crust that began about 20 to 17 Ma (millions of years ago). Blocks of crust were pulled apart, likely as a result of the subduction of the East Pacific Rise beneath this part of North America. As the crust stretched, once continuous sheets of rock were faulted and rotated along a north/south trend. Nevada has widened twice as much as it was prior to 20 Ma. That's 100% extension!

Dr. John Warme stands in front of the geologic map of Nevada at the beginning of our field trip. The entire state is located within the Basin and Range Province, which also includes eastern California, western Utah and south and central Arizona.

On the ground, this is what the Basin and Range looks like. Rocks in the foreground are also exposed in the far mountain range but they are separated by faults and the extension that occurred here.

To understand this story, it is helpful to see what this part of North America looked like around 380 Ma. The emergent part of North America lay to the east and off the right side of the map. An early incarnation of the Pacific Ocean covered most of Utah and Nevada as was part of the continental shelf. These conditions were present for hundreds of millions of years here! Thick accumulations of quiet-water limestone and shale piled up over this period of time to over 130,000 feet. In spite of their later deformation, the rocks are essentially limestone, after limestone, after limestone. 

Evidence from the quiet-water days of the Late Devonian comes from the Guilmette Formation. Here a stromatoporoid sponge fossil lies in quiet repose on a tilted bed of limestone.

These stromatoporoids sometimes made reef-like colonies. Silica-replacement of the calcium "shell" highlights the outline of the sponges.

This example shows the concentric layers within the sponge. These fossils provide evidence for quiet, shallow marine settings. With apologies to paleontologists, this was quite boring tectonically.

Except for this - an approximately 300-foot section of coarse, angular breccia (foreground) found within the Guilmette Formation. This anomalous deposit is the subject of this posting.

To further complicate the setting, this area was compressed and the crust was shortened extensively by a mountain building event before the crust was stretched and thinned. The once horizontal layers of limestone were squished and interleaved along giant thrust faults stacking rocks over themselves. See the diagram above for how the layers became stacked. When a geologist goes into the field here, they have to mentally "remove" the compressive event and the thinning event to sense the nature of the rocks before they were deformed. This is the how the paleogeography can be envisioned.

One can  rather easily see a pronounced tilt to the rocks in this view. But within the tilted beds are numerous thrust faults that reveal that the layers were also compressed and shorted before they were stretched. Yikes - makes a guy like me run for the Colorado Plateau where the rocks are still essentially horizontal like the day they were born!

From the South Hancock Hills near Hancock Pass on State Highway 375 a view of the section can be seen. The lighter-colored bed of limestone in the center of the photo is a part of the Alamo breccia.

A view of the same photograph with the breccia horizon highlighted in yellow.

So, let's look at the Alamo breccia up close. It contains pieces of rock (called clasts) of the very same limestone rocks and are always quite angular when found. This suggests they were emplaced quickly without prolonged agitation, which would tend to round their edges.

There are many beds of this material within the breccia.

The mechanical pencil (center right) lies on the contact between in-place limestone below and brecciated limestone above. This represents the surface upon which the breccia was emplaced.

Close-up of the same contact (photo center). Note the rather distinct bed of breccia with lighter-colored and angular clasts within a darker matrix.

John Warme next to a dislodged boulder of the Alamo breccia.

What is inherently interesting about the Alamo breccia are seemingly coherent beds of limestone within it. Here the 1/2 meter-long rock hammer lies across a brecciated horizon in the middle part of the photo. Note the small clasts within an apparent coherent bed above that. But a lens-shaped block (with its left margin terminating at the top of the hammer) is exposed on top. This suggests that clasts of ANY size can be found in the breccia.

Like this - note the twin beds of limestone "floating" on top of the clastic breccia. These were likely shoved into the mobile mass of breccia without becoming disrupted.

Or this! Note the overturned bed of limestone, surely pushed around to obtain this shape. Wow!

And this - a series of beds that trail off to the left into equidimensional breccia. How does this happen?

Dr. Warme was made aware of this anomalous occurrence of breccia in January, 1991, when a PhD student told him about breccia he saw in the Guilmette Formation. They eventually ruled out tectonic collapse of the continental shelf, since it was rather featureless and was located in a stable environment. Plus, the scale of the breccia and the texture hinted at something more catastrophic, especially when it was found that some clasts are 500 meters long! A meteoroid impact in  the Late Devonian was the interpretation!

Rocks of this age are full of fossils called conodonts and they provide rather tight ages for when this impact might have occurred. Conodonts were ubiquitous in early Paleozoic oceans worldwide but evolved and changed rapidly. Thus, they make great index fossils, since their evolution can be traced in the rocks. The Alamo breccia was emplaced during the Palmatolepis punctata conodont stage, about 382 million years ago. A paper on Late Devonian conodont stratigraphy in Nevada can be downloaded here.

Although not age-diagnostic, Warme also took us to these rather large stromatolite fossils exposed beneath the breccia (still in place). Note the cauliflower shape to these 3-4 feet high stromatolites.

Another view of two stromatolites side by side.

These petroglyphs were found on a yellow sandstone boulder that had rolled down from above the breccia horizon. Warme thinks the sandstone may have resulted from uplift of the ocean floor concentrically around the impact zone.

Beautiful Nevada!

This feature in the southern Hiko Hills was our destination on another day of the field trip. We were headed to the lens-shaped beds of lighter-colored limestone in the center of the hill. These are part of the Alamo breccia.

No trails here - just walk where you can.

As we went up the hill, we arrived at the top surface of the breccia deposit, seen here where the lower, darker cliff of limestone gives way to an upper slope that angles down to the left. At the top of the breccia are graded beds (where the clasts get smaller towards the top) interpreted by Warme as tsunamites - rocks formed when giant tsunamis raced across the sea after impact.

Note this fining-up sequence.

This is not a great example of how these tsunamites fine upward, but the rock is from the tsunami horizon. Incredible to think that all of this - 300 feet of debris - may have been emplaced within a matter of hours 382 million years ago!

Still gotta go higher to see one more thing. Believe it or not, the lighter gray bed in the center of this photo is likely a very large clast that was transported eastward from the impact. It remained largely intact as it came to rest here.

We finally made it to a cliff where a lighter-colored bed, almost yellowish, was exposed.

Closer examination revealed the presence of spheres within a limey matrix.

Closer still, it was observed that the spheres had concentric interiors. Hmm?

The finger points to one of these features. containing a nucleus, a mantle and a thin crust. It turns out that when the bolide (or comet) impacted the earth, it vaporized the previously consolidated limestone layers and water from the ocean. A gigantic cloud of pulverized limestone and vaporized water was injected into the rising and very hot mushroom cloud. The temperature was so intense that the limestone powder was cooked into quick lime - essentially cement. This mixed with the water vapor in the cloud to create nodules of cement that then adhered to more pulverized quick lime (like the way a snowball accumulates more snow as it rolls downhill). Warme was advised early on that these sphere's are technically lapilli. But not volcanic lapilli, rather meteoroid impact lapilli.

Warme found one example where numerous small lapilli were fused with other lapilli to create a meteorite-impact bomb (cut in half and polished here). Note the similar outline to the shape of a volcanic bomb. As various lapilli were incorporated into the mushroom cloud, they may have risen on upwellings again and again from the rising heat, much as hailstones behave inside a thunderhead.

In this view we can see the orientation of the bomb as it fell to Earth, with its edges trailing upward as it descended to the ground.

State Highway 375 which bisects this area, has been named The Extraterrestrial Highway by the Nevada State Legislature. Rachel, where this photo was taken, lies just outside the boundary of the Nevada Test Site and Area 51. For these reasons, the name of the highway was given.

But how appropriate that although the reason for the name is likely a farce, there is something very real about this area containing material that is extraterrestrial - the Alamo breccia and impact.