Geology of the Canterbury Region facts for kids

The Canterbury region

Canterbury is a special part of New Zealand's South Island. It lies to the east of the mighty Southern Alps. This area stretches from the Waiau Uwha River in the north to the Waitaki River in the south.

The Southern Alps were formed by a huge crack in the Earth's crust called the Alpine Fault. This fault runs right through the South Island. On one side of the Alpine Fault is the Pacific Plate, and on the other side is the Australian Plate. These giant plates of rock are always moving! The Pacific Plate is slowly sliding southwest by about 3.5 centimeters each year. It's also pushing upwards by about 1 centimeter a year. This slow but powerful movement is what created the towering Southern Alps.

The Southern Alps didn't always look like they do today. They started forming around 23 million years ago. About 10 million years ago, they were just small hills. It was only about 5 million years ago that they grew into the tall mountains we see now. East of the Southern Alps are the Canterbury Plains. These flat lands were made from all the rocks and dirt that eroded off the mountains over millions of years. Near the coast, just southeast of Christchurch, is Banks Peninsula. This area was formed by two very old volcanoes.

What Are the Oldest Rocks in Canterbury?

The oldest rocks in Canterbury are called the Torlesse Composite Terrane. They are mostly made of a hard rock called greywacke. Greywacke is a type of sandstone and mudstone that formed deep in the ocean. These rocks became part of the ancient supercontinent Gondwana a very long time ago, before the Tasman Sea even existed (about 80 million years ago).

You can see these old rocks in the Southern Alps and their foothills. South of Rangiora, you'll find Rakaia Terrane rocks, which are between 300 and 200 million years old. North of Rangiora are the Pahau Terrane rocks, which are a bit younger, from 160 to 100 million years ago. Where these two rock types meet, there's a special zone called the Esk Head Belt. It's 11 kilometers wide and filled with broken and twisted rocks.

Some of the rocks near the Alpine Fault, south of Timaru, have been changed by heat and pressure into a different type of rock called schist.

How the Southern Alps Were Formed

The Southern Alps began as layers of sand and mud deposited between 230 and 170 million years ago. These layers, mostly greywacke, were first pushed up into mountains about 140 to 120 million years ago.

Then, about 26 million years ago, a second major period of uplift started. This is what created the Southern Alps we see today. It happened because the Pacific tectonic plate is constantly crashing into the Australian Plate. The main boundary where these plates meet is the Alpine Fault. It runs slightly west of the highest peaks of the Southern Alps.

Ancient Volcanoes in Canterbury

Long ago, between 100 and 66 million years ago, there was volcanic activity around the Mount Somers / Te Kiekie area. You can still see remnants of these ancient volcanoes in the foothills, like in the Malvern Hills and near the Rangitata River.

Limestone Deposits

Over millions of years, from the late Cretaceous to Pliocene periods, layers of sandstone, mudstone, and some limestone were laid down across many parts of Canterbury. You can find limestone rocks that are 34 to 24 million years old in South Canterbury near the Opihi River. In North Canterbury, you can see them around Omihi and further north near Waiau. There are also limestone outcrops south of Castle Hill.

Banks Peninsula: Volcanoes by the Sea

Banks Peninsula was formed by two very large volcanoes that overlapped each other. These volcanoes were active between 11 and 6 million years ago. Today, the areas where these volcanoes once stood are now the beautiful Lyttelton / Whakaraupō and Akaroa Harbours.

Around 2.5 million years ago, there were also basalt eruptions near Timaru and Geraldine. Basalt is a type of dark, fine-grained volcanic rock.

Lakes Formed by Glaciers

If you visit the MacKenzie Country, you'll see stunning lakes like Lake Pukaki and Lake Tekapo. These lakes are actually the beds of ancient glaciers that once covered the area. As the glaciers moved, they carved out these deep valleys, which later filled with water when the ice melted.

The Canterbury Plains: River Deposits

Most of the Canterbury Plains are covered by layers of sand, gravel, and silt. These materials were carried down from the Southern Alps by the many large rivers flowing out of the mountains. Over time, these deposits built up the flat plains we see today.

Earthquakes in Canterbury

While Canterbury doesn't have earthquakes all the time, it's important to know about them. The Alpine Fault, which is a major fault line, last had a big earthquake in 1717 CE. Scientists expect a large earthquake on the Alpine Fault to happen every 200 to 300 years.

When a big earthquake occurs, the ground can sometimes turn into a liquid-like state, a process called liquefaction. This can happen up to 150 kilometers away from where the earthquake starts.

Some important earthquakes in Canterbury's history include:

• The 1888 North Canterbury earthquake
• The 2010 Canterbury earthquake, which had a magnitude of 7.1 and caused a lot of damage.
• The 2011 Canterbury earthquake, which had a magnitude of 6.3 and caused widespread damage and sadly, deaths.

Cool Geological Places to Visit

Canterbury has many amazing places where you can see geology in action:

• Explore Banks Peninsula. Look for interesting rock formations like Castle Rock above Heathcote Valley, and The Remarkable Dikes near Kaituna Pass.
• Drive from Christchurch to Arthurs Pass and over the Southern Alps to Greymouth. As you get closer to the Alpine Fault, you'll notice the rocks change from greywacke to schist.
• Visit Aoraki / Mount Cook and see the impressive Hooker and Haupapa / Tasman Glaciers.
• Take a trip from Christchurch to Hanmer Springs for a hot swim. On the way, you can see the limestone formations at Weka Pass.

Learning More with Geological Maps

If you want to learn even more about New Zealand's geology, you can get special maps from GNS Science. This is a New Zealand Government Research Institute that studies the Earth. They publish a map called "New Zealand's Geological Foundations."

They also have a series of detailed maps called the 1 : 250 000 QMap series. Low-resolution versions of these maps can often be downloaded for free from the GNS Science website. For example, the map for the Christchurch Area was published in 2008, and the map for the Aoraki Area came out in 2007.