Acasta Gneiss: Canada's 4-Billion-Year-Old Rock Formation

The Acasta Gneiss is a body of ancient rock along the Acasta River in Canada’s Northwest Territories, and parts of it, about 4.03 billion years old, are the oldest securely dated intact rocks on Earth. They formed only about 500 million years after the planet itself. Studying them helps geologists understand how the first continental crust formed during the transition from the Hadean to the Archean eon.

Background: hunting for the oldest rocks

Earth is about 4.54 billion years old, but rocks from its first few hundred million years have almost entirely disappeared. Plate tectonics, erosion and remelting have recycled the early crust. The oldest surviving pieces are found in cratons, the stable, ancient cores of continents.

The Slave Craton of northern Canada is one such core. It covers a large area between Great Slave Lake and the Arctic coast and contains rocks spanning well over a billion years of early Earth history.

Discovery and dating

Geological mapping of the western Slave Craton in the 1980s drew attention to the gneisses near the Acasta River, some of which appeared unusually old. In 1989 the American geologist Samuel Bowring, together with Ian Williams and Bill Compston of the Australian National University, reported ages of about 3.96 billion years. At the time, these were the oldest dated rocks known.

They used SHRIMP, an ion microprobe developed in Canberra, to measure uranium and lead in individual zircon crystals. Zircon takes in uranium but almost no lead when it grows, so the lead it contains comes from radioactive decay and acts as a clock.

In 1999 Bowring and Williams published further results showing that some of the gneisses contained zircons about 4.00 to 4.03 billion years old. This age, recording when the original molten rock cooled and crystallised, made the Acasta Gneiss the oldest known intact rock formation.

What the rocks are

A gneiss is a metamorphic rock: one that has been changed by heat and pressure without fully melting. Its minerals are typically arranged in light and dark bands. The oldest Acasta rocks were originally tonalites and similar granite-like igneous rocks, formed when magma cooled underground. Over billions of years they were repeatedly buried, heated, squeezed and folded, which is why they look so streaky today.

Some zircons in the rocks contain even older cores. In 2006 a Japanese-led team reported a zircon core of about 4.2 billion years, a fragment of still older crust caught up in the younger magma.

An Iceland-like origin

In 2014 a team led by Jesse Reimink of the University of Alberta described a 4.02-billion-year-old unit within the complex, which they named the Idiwhaa gneiss, from a word for “ancient” in the local Tłı̨chǫ language. Its chemistry resembles rocks formed today in Iceland, where shallow melting of basaltic crust over a hot spot produces lighter, granite-like rocks. The study suggested that some of Earth’s earliest evolved crust formed in this way, rather than through modern-style plate tectonics.

Rivals for the title

The Acasta Gneiss is not the oldest material of Earth origin. Individual Jack Hills zircons from Western Australia are about 4.4 billion years old, but they survive as grains in a much younger sedimentary rock.

In 2008 researchers reported that rocks of the Nuvvuagittuq greenstone belt in northern Quebec might be about 4.28 billion years old, based on neodymium isotopes. Other scientists argue that these rocks are closer to 3.8 billion years old, the age given by their zircons, and the question remains open. Rocks of about 3.8 billion years are also known from the Isua Greenstone Belt in Greenland.

What the rocks tell us

The Acasta Gneiss shows that by about 4 billion years ago, Earth already had patches of evolved crust, rock richer in silica and lighter than the basalt that makes up the ocean floor. Such crust is the raw material of continents, because it is too buoyant to sink easily back into the mantle.

The rocks also record a long and complicated history. Different parts of the complex span hundreds of millions of years, from about 4.03 billion years onwards, showing that crust was formed, melted and reworked many times in the same region. This supports the idea that the earliest continents grew slowly, piece by piece, rather than appearing all at once.

By the numbers

Timeline

• c. 4,200 million years ago: Older crust forms, later preserved as zircon cores within the Acasta rocks.
• c. 4,030–4,000 million years ago: The oldest Acasta igneous rocks crystallise.
• Over the following billion years and more: Repeated heating and deformation turn the rocks into gneiss.
• AD 1989: Bowring, Williams and Compston report ages of about 3.96 billion years.
• AD 1999: Bowring and Williams report zircon ages of 4.00–4.03 billion years.
• AD 2006: A 4.2-billion-year-old zircon core is reported from the Acasta Gneiss.
• AD 2014: The Idiwhaa gneiss is described as Iceland-like early crust.

Why it matters

The Acasta Gneiss is a rare survivor from the dawn of the Archean, when Earth’s first lasting continental crust was taking shape. Together with ancient zircons, it lets geologists test ideas about the young planet described in the formation of the Earth, including how early continents formed and when conditions became suitable for life.