New Zealand History I – Geology and Environments

Learning Outcomes for New Zealand History I – Geology and Environments

• LO1: Describe the relationships between Gondwana, Zealandia, and Aotearoa (New Zealand).

• LO2: Recall the role that plate shifting has in creating landscapes in Aotearoa.

• LO3: Explain how mountains in Aotearoa are formed.

• LO4: Explain how sea level changes have affected the exposed area of Aotearoa.

• LO5: Outline important post-Pleistocene changes in the Aotearoa environment.

• LO6: Recall species for which a Gondwanan origin is possible.

Key Geological Concepts and the Formation of Zealandia

• Speciation and Extinction Rates (Concept 25.4): The rise and fall of groups of organisms reflect differences in speciation and extinction rates. Continental drift and isolation influenced the speciation events and extinctions that gave rise to the unique biota of Aotearoa.

• Geological Components: Aotearoa is part of the continent of Zealandia. Its landscape has been shaped by sea level changes, uplift, volcanism, and glaciation. Pre-human habitats were dynamic over time, and the distribution of plants and animals reflects both historic and recent environmental changes.

• Plate Tectonics Overview: The earth's surface resides on moving plates. There is a distinction between thick, old continental crust and thin, young oceanic crust.

• Plate Boundaries:

  • Divergent Boundary: Plates moving apart.

  • Convergent Boundary: Plates colliding.

  • Transform Boundary: Plates sliding past each other.

• Supercontinents History: The Earth has formed supercontinents three times:

  • $1$ billion years ago.

  • $600$ million years ago.

  • $250$ million years ago (Pangaea).

• The Breakup of Pangaea: Pangaea split into Laurasia and Gondwana. Zealandia was a part of Gondwana.

• Zealandia Separation: Around $83\,Ma$, Zealandia began to break away from Gondwana. Over a period of $30$ million years, oceanic crust formed in what is now the Tasman Sea, separating Zealandia from Australia and Antarctica.

• Continental Status: Aotearoa is currently considered the exposed portions of an otherwise submarine continent named Zealandia. The continental crust of Zealandia is mostly too thin to project above sea level.

Tectonic Mechanics Specific to Aotearoa

• The Junction: Aotearoa formed at the junction of the Australian and Pacific Plates.

• Plate Interactions:

  • North: The Pacific Plate subducts under the Australian Plate.

  • South: The Australian Plate subducts under the Pacific Plate (Macquarie Fault Zone).

  • The Alpine Fault: This is a transform boundary where plates slide. The northern part of Te Waipounamu (South Island) is on the same plate as most of Te Ika-a-Māui (North Island) and Rangitāhua (Kermadecs).

Origins of the Aotearoa Landscape

• Oligocene Drowning: During the Oligocene epoch ($33.9$ to $23.03\,Ma$), much of current Aotearoa, possibly almost all, was submerged. Evidence for this comes from limestone deposits formed by the deposition of marine organisms.

• Mountain Uplift and Erosion: The landscape results from the balance between uplift from plate collisions and erosion/deposition.

  • Alpine Fault: Responsible for the formation of the Southern Alps, mostly within the past $5$ million years.

  • Relative Motion: Movements along the fault range from $37\,mm/yr$ to $47\,mm/yr$ depending on the specific fault system (Alpine Fault vs. North Island fault system).

• Volcanism: Volcanoes have been active for more than $25$ million years, though many prominent ones are recent.

  • Auckland Volcanic Zone: Active during the last $200,000$ years.

  • Taupō Volcanic Zone: Active over the previous $1.6$ million years, including Ruapehu, Ngāuruhoe, and Tongariro.

  • Lake Taupō: A giant collapsed caldera resulting from repeated, massive eruptions.

Case Study: Taupō Volcanism

• AD 232 Eruption: A powerful eruption where the towering column collapsed, generating pyroclastic flows (clouds of hot gas, ash, pumice, and rock) racing radially outward at high speed. It covered $20,000\,km^2$ in a layer of loose ignimbrite.

• ŀruanui Eruption ($26,500$ years ago): Formed the initial caldera.

  • This eruption was $10 \times$ the volume of Tambora ($1815$).

  • It was $100 \times$ the volume of the Taupō eruption of c. AD $200$.

  • It was $1,000 \times$ the volume of Mt St Helens ($1980$).

  • It was $10,000 \times$ the volume of Ruapehu ($1995$-$96$).

• Global Impact: These eruptions deposited ash across much of Aotearoa and into the Southern Hemisphere atmosphere.

Sea Level and Vegetation Changes

• Climate Mechanisms: Water is locked in glaciers and ice sheets. Global sea levels rise when climates warm and release this water.

• Antarctic Melting: About $3$ million years ago, the melting of the Antarctic ice sheet increased sea levels around New Zealand.

• Pleistocene Ice Ages: Caused repeated changes in sea level over the past million years.

• Last Glacial Maximum ($20,000$ years ago):

  • Sea levels were more than $100\,m$ lower than today, reaching the edge of the continental shelf.

  • Glaciers were extensive in southern New Zealand.

  • Vegetation patterns shifted; broad patterns at the Glacial Maximum have been reconstructed using pollen, macrofossil, beetle, and geomorphic evidence.

Summary Timeline of New Zealand Physical History

• $150\,mya$: Began as part of Gondwana.

• $83\,mya$: Sea floor spreading begins separating Zealandia from Gondwana.

• $25\,mya$: Subsidence and extensive "drowning" during the Oligocene.

• $5\,mya$: Recent mountain formation through uplift at plate boundaries and volcanoes.

• $2.6\,million$ to $12,000$ years ago: Pulsing of warm/cool periods (Pleistocene) affected sea levels and connections between the North and South Islands.

• <25,000 years ago: Recent volcanism obliterated large areas of vegetation.

Recruiting a Biota

Three historically important explanations for the acquisition of species in New Zealand:

1. Continental Drift: Vicariant origin before $83\,Ma$.

2. Land Bridges: Via Zealandian islands.

3. Transoceanic Dispersal: Flying, floating, or wind-blown.

Flora Origins

• Nothofagus (Southern Beech): Molecular data indicates independent divergences between Australian and NZ taxa occurred less than $50\,Myr$ ago (more than $30\,Myr$ after the Tasman Sea opened). Thus, Nothofagus is likely not vicariant; earlier populations likely became extinct, and the current trees represent more recent arrivals.

Fauna Origins

• Peripatus (Velvet Worms): Estimated age of $85$-$94\,mya$. A strong candidate for Gondwanan origin.

• Amphibians: Leiopelmatid frogs; estimated age $67$-$130\,mya$.

• Reptiles:

  • Tuatara: Estimated age $100$-$130\,mya$.

  • Skinks and Geckos: Molecular clock analyses suggest skinks colonized in the early Miocene ($16$-$22.6\,mya$) via long-distance overwater dispersal from New Caledonia. Gecko divergence from Australian groups occurred $40\,mya$. Both are too recent for vicariant Gondwanan origin.

  • Dinosaurs: Evidence of Ankylosaurs (Nodosaur), Compsognathids, Ornithopods, Theropods, and Titanosaurs exist in the fossil record.

• Mammals: Three bats of recent descent. However, Miocene fossils revealed much higher bat diversity and a small land mammal of probable Gondwanan origin.

• Birds:

  • Moa: Previously thought to be Gondwanan, but now known to be related to the flying Tinamous of South America.

  • Kiwi: Related to the Elephant Bird of Madagascar.

  • New Zealand Wrens: The most basal passerines; possibly originated in Australia; origins are likely too recent for vicariance.

  • While the ratite group is Gondwanan, the specific New Zealand representatives have more recent origins.

Summary of Best Extant Candidates for Gondwanan Origin

• Leiopelmatid Frogs: $67$-$130\,mya$.

• Tuatara: $100$-$130\,mya$.

• Peripatus: $85$-$94\,mya$.