Dry Valleys Geology

Dry Valleys Geology

Although the geology of Antarctica goes back over three thousand million years, the oldest rocks found in the Dry Valleys are (only!) about 650 million years old. These rocks started out as sediments which were carried by rivers out to the sea at the edge of East Antarctica where they gradually built up and got compressed to form sedimentary rock like limestone, mudstone and marl .

At this time (650 million years ago), East Antarctica was part of a continental rift but 500 million years ago things changed completely. The Earth’s plates started to move towards each other a process known as subduction, so that for about 30 million years, the edge of East Antarctica formed a huge volcanic mountain chain, thousands of kilometres long, which continued north from Antarctica through Australia along the edge of the supercontinent Gondwana. We don’t see any evidence of the volcanoes as they appeared on the Earth’s surface, because once the volcanic activity stopped, the volcanoes and mountains were completely eroded – so much so that this part of East Antarctica was left as a huge flat continent similar to how Australia is today. But we do see evidence of the ‘roots’ of the volcanoes – the huge chambers of semi-molten rock that sat about 4 km deep in the earth and which provided the magma that erupted from the volcanoes as lava flows. Once the volcanic activity stopped, the semi-molten rock in the chambers solidified to form granites and other types of plutonic igneous rock. During the subduction process many of the rocks were metamorphosed as they were squeezed and deformed at high temperatures and pressure. This process turned the limestone into marble, the mudstone and marl into schist and calc-silicate and some of the plutonic rocks forming the volcanic roots are now gneiss. The gradual uplift of the land since the late Cretaceous time period has brought these plutons and metamorphic rocks to the surface where weathering and erosion has exposed them and they are able to be studied by geologists.

We can now fast-forward through the geological time scale to the Cenozoic – the next period for which there is evidence of geological activity in the Miers, Marshall and Garwood valleys. We have now reached the point in time where the Earth’s tectonic plates start moving apart (rifting) to cause the break-up of the Gondwana supercontinent eventually leading to the configuration of continents on Earth as we know them today. The break up produced the present day Transantarctic Mountains and a relatively small amount of volcanic activity.

Mt Erebus is the only active volcano caused by the rifting and is the dominant landmark we see every clear day when working in the Dry Valleys. The volcanic rocks of Mt Erebus are black and very distinctive and are unlike any other rocks of the Transantarctic Mountains. In fact one type of volcanic rock is extremely rare and is only found in two places on Earth – on Mt Erebus and on a volcano in Kenya.

Mount Erebus has been active for about 1 million years and exists because the Earth’s crust beneath it is gradually getting thinner and thinner as the continent slowly splits apart. The thinner the crust, the easier it is more molten rock to reach the Earth’s surface and build up a volcano. The summit crater contains a lake of molten lava that constantly produces small eruptions (every day) and a persistent plume of water vapor and other gasses from its summit. Mt Erebus is part of a long-term volcano monitoring project and is very close to the New Zealand and USA research bases.

The massive, permanent ice sheet that covers East Antarctica first started to form 35 million years ago when Antarctica completely split away from the Gondwana supercontinent and became surrounded and isolated by the Southern Ocean and the Antarctic Circumpolar Current. Since then the landscape of East Antarctica has been shaped by the advance and retreat of glaciers. Many features typical of glaciated landscape are seen in the Dry Valleys and two that are the most significant for our work include moraine and patterned ground.Moraine is made up of the rock fragments that have been carved out of the mountain ranges by glaciers as they flow downhill towards the ocean. When the ice melts, the rock debris that was transported within the glaciers is left behind as rubblely loose piles of rock called moraine and this moraine acts as a foot print of where the glacier has been. In the Dry Valleys there are two types of moraine which look very different and have very different origins. The first moraine is made of fragments of schist, granite, marble and gneiss. These are the rocks which form the Transantarctic Mountains and which were incorporated into the glaciers which moving from East Antarctic across the Transantarctic Mountains to the Ross Sea. The other type of moraine is a distinctive black colour and is made of fragments of Mt Erebus volcanic rocks. But how did this end up in the Dry Valleys given that Mt Erebus is not between the source of ice from East Antarctica and the coast? Watch the animation and find out.

Ross Sea Glacial Cycles – NASA Video

We are looking towards the Ross Sea and Ross Ice Shelf. East Antarctica is on the far right covered by the ice cap. The Transantarctic Mountains run along the edge of East Antarctica, West Antarctica is on the left. Mount Erebus is the isolated high point surrounded by the Ross Sea and Ice Shelf. During the last ice age 20,000 years ago as ice accumulated, the ice of the Ross Ice Shelf gradually thickened so much that the ice was high enough to get pushed from the Ross Sea up into the Dry Valleys. On its way, this ice eroded Mt Erebus so that numerous Mt Erebus volcanic rock fragments were contained in the ice. When eventually the ice melted 12000 years ago, the Mt Erebus volcanic rocks were left behind as moraine in the Dry Valleys and show the maximum distance the ice was able to move up the valley.

Patterned ground occurs in most places there is loose moraine on valleys floors and walls because the active layer of the permafrost thaws slightly in the short summer months. The cracks which form often trap wind-driven sediments and form sheltered or wetter microhabitats.

So several different geological processes have resulted in a variety of rock types being present in the Dry Valleys. A few factors we might need to consider when determining if geology plays a part in the presence of life in the Dry Valleys include the chemistry of the rocks, the size of the crystals in the rock and perhaps their colour (see table). Darker rocks will tend to absorb more heat from the sun than light coloured rocks and so may provide temperatures more favourable to life, or the hotter rocks may melt more snow providing water for life. Perhaps rocks with a wide variety of chemical elements provide a better source of nutrients for life? Perhaps rocks with small crystals break-down easily and provide a better soil habitat for life to prosper?