No complete rocks have survived to tell of the formative years following Earth’s formation some 4.56 billion years ago. The material that would have existed at that time has been broken apart by the power of wind and water. It has melted and metamorphosed under the immense pressure and heat deep within Earth’s interior. It has been recycled back on to the surface. It has existed at every stage of the rock cycle many, many times over.
Despite the thousands of millions of years Earth’s earliest material has encountered, tiny pieces of some of these rocks still exist. Small microscopic grains of a mineral, once part of rocks that would have witnessed Earth as it existed just a couple of hundred million years after its formation, survive to this day.
The oldest known terrestrial material is a single grain of a mineral called zircon which was found in the Jack Hills formation in western Australia. It is 4.4 billion years old. The grain itself was part of a rock composed of the broken and eroded bits of other ancient material that itself has been subject to billions of years of geologic reworking.
Zircon is an extremely rugged mineral made up of silicon and the obscure element zirconium. Its tenacity in the face of time and its ability to provide scientists with enough information to figure out the age when it was formed are among the many reasons it is exciting to geologists. Read the rest of this entry »