Geology of the Raukumara Region facts for kids

The Raukumara Region is a special part of New Zealand. It's located on the East Cape of the North Island and includes its mountain ranges. This area has a fascinating geological story, shaped by huge forces deep within the Earth.

The Raukumara Region used to be right next to another area called Northland. Over millions of years, it slowly moved to where it is today. Because of this, many of the rocks found in Raukumara are very similar to those in Northland.

To the east of the North Island is a place called the Hikurangi Trough. This is where two giant pieces of the Earth's crust, the Pacific Plate and the Australian Plate, are crashing into each other. The Pacific Plate is slowly sliding underneath the Australian Plate. This movement pushes and squeezes the eastern part of the North Island. This squeezing creates the North Island Fault System and a series of long, narrow valleys and mountain ranges, including the Raukumara Range. Over time, new layers of rock have been added to the East Coast.

To the north of the Raukumara Range is the Bay of Plenty. Here, you can find very old rocks called Torlesse (Waioeka) Greywacke. The central mountains are covered by rocks from the Cretaceous to Oligocene periods. Some of these rocks are in their original place, while others have been moved a long distance. Further south, you'll find younger rocks, mostly from the Miocene and Pliocene times.

What are the Oldest Rocks in Raukumara?

The very oldest rocks beneath the Raukumara Region are part of something called the Torlesse Composite (Waioeka) Terrane. These rocks formed between 150 and 100 million years ago, during the late Jurassic and early Cretaceous periods.

They are mostly made of Greywacke. Greywacke is a type of hard sandstone and mudstone. These rocks formed deep under the ocean. You can see these ancient rocks exposed in the northern part of the Raukumara Range, stretching from Whakatane to Papatea Bay.

How Did Rocks Get Moved in the East Cape?

Between 100 and 24 million years ago, during the Cretaceous to Oligocene times, layers of sandstone and mudstone were laid down across the Raukumara region. These rocks still cover the Raukumara Range today.

However, something dramatic happened about 24 to 21 million years ago, in the Early Miocene period. At this time, the Raukumara region was still next to Northland. Huge sheets of rock were pushed and slid over much of the East Cape area. This event is called the East Cape Allochthon.

Large sections of the older Cretaceous to Oligocene rocks were moved many tens or even hundreds of kilometres! These rocks came from the Northeast. Interestingly, they were placed in a "reverse order" on top of other rocks, but they were still the right way up. These moved rocks include mudstones, limestone, and even basalt lava.

Most of the rocks east of the line from Papatea Bay to Waipiro Bay, and rocks just north of Whatatutu, are these "allochthonous" (moved) rocks. Scientists believe these moved rocks also lie underneath the younger rocks further south.

In the north, from Cape Runaway to Tokata, you can find allochthonous basaltic rocks. These are thought to be pieces of the ancient seafloor that were pushed up onto the land. These rocks are similar to the Tangihua rocks found in Northland.

What are the Younger Rocks in the Region?

During the Miocene and Pliocene times, which were more recent geological periods, new layers of sandstone, mudstone, and some limestone were deposited. These younger rocks cover much of the southern part of the Raukumara region.

What are Marine Terraces?

You can often see flat, step-like areas along the coast called Marine terraces. These are old shorelines that have been lifted up by Earth's movements. They are common between Ōpōtiki and East Cape, and also at Māhia Peninsula.

Are There Hot Springs in Raukumara?

Yes! You can find sinter deposits around the Te Puia Springs. Sinter is a type of mineral deposit formed by hot spring water.

What Natural Resources are Found Here?

The area south of the Raukumara region has natural oil seeps. This means oil naturally leaks out of the ground, like in the Waimata Valley and at Waitangi Station. Scientists believe there might be large amounts of gas and oil hidden underground here. Even though the region has been explored, and some small amounts of oil and gas have been found, no big commercial reserves have been discovered yet.

What are the Geological Risks in Raukumara?

The Raukumara Region and the Hikurangi Trough are areas where earthquakes happen often. Because of earthquakes, there's also a risk of tsunamis. For example, a strong 6.8 magnitude earthquake hit the Gisborne area on December 20, 2007. There were also two big tsunamis on March 26, 1947, and May 17, 1947.

Much of the land is made of soft mudstones. These rocks can easily be washed away by rain and rivers, especially in areas where there aren't many trees or plants to hold the soil.

The region can also experience floods when tropical cyclones travel far enough south. Sometimes, the area also gets small amounts of volcanic ash falling from the Taupo Volcanic Zone.

Where Can You Explore Geology in Raukumara?

The Raukumara Region offers many cool places to see its geology up close:

• You can drive a loop from Gisborne, through the Waioeka Gorge. Here, you'll see the greywacke rocks. Continue to Opotiki, then around East Cape. You can see the Matakaoa volcanic rocks between Cape Runaway and Tokata (like at Haupara Point, near Hicks Bay). Don't forget to visit East Cape itself! Then, drive back to Gisborne, passing through areas with Miocene sandstones and mudstones.
• At Oponae, in the Waioeka Gorge (about 19 km from Opotiki), you can see huge blocks of red, green, and white chert. These rocks contain tiny fossils called radiolarians, which are from the Jurassic age. These blocks are stuck inside the younger Cretaceous greywacke. You can see them sticking out of road cuttings and forming the narrow Hell's Gate Rapids on the river.
• You could try white water rafting down the Motu River, or drive to see the beautiful Motu Falls.
• Visit Waimata Valley and Waitangi Station, north of Gisborne, to see the interesting mud volcanoes and oil seeps. (These might be a bit tricky to find on your own.)
• Have a relaxing hot swim at Te Puia Springs and observe the unique sinter deposits formed by the hot water.

Where Can You Find Geological Maps?

You can get geological maps of New Zealand from the New Zealand Institute of Geological and Nuclear Science, also known as GNS Science. This is a New Zealand Government Research Institute.

The main maps are part of the 1 : 250 000 QMap series. This series was completed in 2010 and has 21 maps. You can download lower-resolution versions of these maps (without the detailed booklet) from the GNS Science website. The map specifically for the Raukumara Area was published in 2001.

You can also find digital map layers using the free GNS map viewer online.