Mordor Pound: the geology of Lord Sauron's 'Australian home'

16 December 2002

Mordor, J.R.R. Tolkien'' imaginary land of the dark Lord Sauron, actually exists in Australia.

Landsat 7 satellite image showing Mordor Pound, 70km north-east of Alice Springs in the Northern Territory, Australia

Landsat 7 satellite image
showing Mordor Pound,
70km north-east of Alice Springs
in the Northern Territory, Australia
© Geoscience Australia

The Mordor Pound bears an uncanny resemblance to J.R.R. Tolkein''s land of Mordor from his novel "Lord of the Rings".

Mordor Pound lies 70 km north-east of Alice Springs, in Australia's Northern Territory. Such was the resemblance between the landscapes that geologist Alan Langworthy renamed the land form which was previously called Spring Pound to Mordor Pound in the 1970s.

Since Langworthy and his colleagues initially mapped the area for the Bureau of Mineral Resources (now called Geoscience Australia), Mordor Pound and the surrounding Arunta Region have been providing clues about how the Red Centre formed hundreds of millions of years ago for geological detectives from Geoscience Australia and the Northern Territory Geological Survey.

What is a Pound?

The Mordor Pound is an example of a landscape feature referred to as a 'Pound' in Australia. The term 'Pound' appears to have been derived from the late Old English term for a stone enclosure to shelter or accommodate livestock.

In Australia, Pounds are deep valleys that are enclosed by cliffs. They form when the soft rocks in the valley floor erode more easily than the harder rocks which form the cliffs.

Mordor Pound is a spectacular example of a rectangular pound that is enclosed by high cliffs.

Other examples of Pounds in Australia include: the Wilpena Pound in South Australia and the Ormiston Pound west of Alice Springs in the Northern Territory.

Image of Wilpena Pound from the air courtesy of Wilpena Pound Resort.

Image of Wilpena Pound
from the air.
© Geoscience Australia

What do rocks in the Mordor Pound tell us about the geology of central Australia?

The valley floor of Mordor Pound is made up of igneous rocks that are formed by the cooling of magma, or molten rock, below the surface of the earth. Compared to the surrounding sandstone cliffs, these rocks are soft, which is the reason Mordor Pound formed.

Dark, iron and magnesium-rich rocks in the floor of Mordor Pound.

Dark, iron and magnesium
rich rocks in the floor
of Mordor Pound
© Geoscience Australia

In the image you can see the dark rocks in the floor of Mordor Pound which are composed of iron and magnesium-rich minerals with no quartz. One of these minerals, phlogopite, which is a magnesium-rich mica, commonly has a bronze, shimmering effect. The combination of the dark colour along with the shimmering effect gives the rocks of Mordor Pound the feel of a "dark land where the shadows lie" as the Land of Mordor was described in the Lord of the Rings. One of the highest hills made of these unusual dark rocks was christened 'Mt Doom' by Langworthy.

The discovery that rocks in Mordor Pound are about 1130 million years old provides a clue that the Red Centre was connected to North America a billion years ago. The Mordor Pound rocks are important as they are one of the northernmost examples of rocks of this age. These rocks form a belt of rocks to the south in the Musgrave area of South Australia and the Northern Territory.

Reconstruction of Rodinia showing the inferred relationship between Australia and North America about 600-700 million years ago.

Reconstruction of Rodinia
showing the inferred relationship
between Australia and North
America about 600-700 million
years ago.
© Geoscience Australia

Similar aged rocks form a belt of rocks, known as the 'Grenville Front', that extends from eastern Canada to Arizona in the south-western United States through to the Musgrave region.

Many geologists consider that Australia was connected to North America up until about 600-700 million years ago, when the Australia-North America super-continent, called Rodinia by geologists, was split, or rifted apart by subterranean forces to start forming the Pacific Ocean.

The map is a reconstruction of Rodinia showing the inferred relationship between Australia and North America about 600-700 million years ago. It is based on the reconstruction presented by Karlstrom et al. (2001), Precambrian Research, v. 111, p. 5-30.

How old is old?

An age of 1130 million years is not particularly old by Australian standards. The oldest rocks in Australia are more than 3700 million years and occur in Western Australia. Over half the rocks in Australia are more than 1000 million years old. The map on the right shows the distribution of rock ages in Australia. The earth itself was formed about 4500 million years ago.

To put these vast ages into perspective, think of a calendar year matched to the Earth's history of 4500 million years, as seen in the geologic calendar below. This calendar shows important events in the history of the Red Centre (in red), Australia and the Earth.

A single day on this calendar corresponds to 12.5 million years. One hour corresponds to 520 000 years, and one second to 145 years.

According to this 'calendar clock', the earliest fossil evidence of life (blue-green algae) appeared on Earth in late March (about 3500 million years ago).

Many of the rocks covering the vast areas of central Australia were formed in mid-August (1850-1750 million years ago).

The unusual igneous rocks of the Mordor Pound formed on about 3 October (1130 million years ago).

A wide variety of life forms, including jellyfish, worms, trilobites and animals with backbones, burst onto the scene around 19 November (545 million years ago).

Dinosaurs first roamed the Earth on 14 December (225 million years ago) and became extinct on 27 December (65 million years ago).

Early 'Man' (Homo sapiens) did not appear on Earth until about 11.25 p.m. on 31 December (300 000 years ago).

Aboriginal people were painting in rock shelters at 11.58 p.m. on 31 December (20 000 years ago), and explorer John McDougall Stuart traversed the Red Centre (in 1860) one second before the end of the calendar year.

How do scientists measure the ages of rocks?

How can scientists be so confident about measuring the ages of rocks?

Some philosophies regard all time and the Earth as eternal, with no beginning and no ending. Others consider that there was a definite starting point: for example, the 17th century Archbishop Ussher in Ireland counted back through the Biblical and Hebrew records to suggest that the Earth was created in 4004 BC. Since Ussher's calculations, scientists and philosophers have estimated the age of rocks and the Earth using a number of methods - some accurate, some not.

The scientific breakthrough came in 1896 when the French physicist Becquerel discovered that minerals containing the element uranium emit radiation. The radioactive breakdown of uranium yields a stable end product-lead, and the steady rate of lead accumulation from uranium decay enables the ages of uranium-bearing minerals and rocks to be measured using the lead accumulation clock.

Photographs of a zircon taken under a microscope.

Photographs of a zircon
taken under a microscope.
© Geoscience Australia

This photograph of the same zircon shows pits caused by SHRIMP analysis.

This photograph of
the same zircon shows
pits caused by SHRIMP analysis
© Geoscience Australia

In the dark igneous rocks of Mordor Pound, uranium and lead accumulations in a mineral called zircon (pictured at right) have allowed geologists at Geoscience Australia to confidently measure their ages.

The uranium-lead dating of rocks is done in Canberra at the 'SHRIMP' (Sensitive High-Resolution Ion Microprobe laboratory), a joint facility of the Australian National University and Geoscience Australia.

Why is Mordor of interest to exploration companies?

During the past three decades, the igneous rocks of Mordor Pound have generated considerable exploration interest for nickel, copper, chromium, gold, platinum-group elements, diamonds, mica, uranium, and rare-earth elements. This extensive list of commodities reflects the wide range of compositions of the rocks. Trace to minor amounts of these commodities have been found by mining companies, but no economic resources have been delineated to date.

Topic contact: media@ga.gov.au Last updated: October 4, 2013