Unlocking past climates from New Zealand’s glaciers

Not only do glaciers look spectacular, but they hold valuable secrets which help scientists understand climate change.

Last year, I was lucky enough to be part of a geography field-trip class where we travelled to New Zealand and discovered the mysteries of Earth's climate history for ourselves.

On our journey around New Zealand's countryside, we scribbled meticulous notes, attempted sketches of rocks and landforms, and took some stunning photos. I had an absolutely fabulous time, but also learnt loads (about as much as in three years of theory classes!).

What can glaciers tell us about past climates?

You may have heard that ice cores from modern glaciers store a wealth of climate information such as past atmospheric gas concentrations and temperature. Scientists can extract this valuable data and use it to improve our understanding of past and future climate changes.

But even after glaciers have melted and the ice is long gone, clues remain in landforms which reveal the presence of past glaciers and when they occurred. So by looking at signs in modern landscapes, we can learn about Earth's climate history.

What clues do glaciers leave behind?

As a glacier scrapes along the ground, its gigantic mass carves out the land into a U-shaped valley. At the same time, the glacier picks up rocks and debris in its path and carries them along with the ice.

When the climate warms and the glacier ice melts, the U-shaped valley is revealed and the glacier deposits all the debris it has collected. These haphazard piles of rocks and sand - known as "moraines" - are like precious treasure to geologists. Our lecturer delighted in endlessly pointing out moraines to the class so that by the end of the field trip, we were all "experts" too!

The distinctive shapes, sizes and layout of moraine rocks reveal the presence of a glacier to the well-trained eye. Unlike rocks from rivers which are smoothed by the flow of the water and sorted into sizes (by how much force the river has at a given point), glacier rocks are angular and poorly sorted.

There are many other signs of past glaciers: glacial lakes, roche moutonnées, outwash plains ... Geologists could list loads more, but I'll spare you the jargon and just explain one.

My favourite is the roche moutonnées aka "rock sheep", named because they apparently look just like little sheep standing in a field. These rock formations are created when a glacier passes over the bedrock and smoothes one side of the rock whilst picking up rocks/gravel from the other side. This leaves an asymmetric formation which indicates the direction of ice flow through the valley, and the minimum height of the glacier. 

By the end of the field-trip we had documented stacks of field-evidence on glaciers. Next came the monumental task of sifting through the vast jumble of data, and consulting scientific literature, in an attempt to piece together the story of Earth's climate. For example, the roche moutonnées told us where the glaciers had been, but we needed to refer to scientific studies which had dated the rocks to know when the glaciations occurred.

The field-trip class was an amazing holiday and fantastic learning experience. And it displayed how New Zealand's glaciers help us unravel Earth's climate history.

Other sources of interest:

How climate change made the melting of New Zealand's glaciers 10 times more likely, LAUREN VARGO, 2020

10 Interesting Things About Glaciers, 2021

This article was originally published via Scientific Scribbles. 

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