A Walk Through Time

Rock

Not Always A Hard Place

4300 Million Years Ago

Earth processes cycle so powerfully that little evidence remains of early planetary faces and flows. Critical and intriguing clues lie in the formation of Earth rocks and minerals when compared with those on the Moon and other planets.

The oldest mineral crystals on Earth, zircons from western Australia, date back 4,300 million years. The oldest moon rock brought back by Apollo astronauts is 4,200 million years old. Granite rocks found near Canada's Great Slave Lake go back 3,960 million years. Scientists believe these evolved from even older crustal material, melted and remelted by the restless Earth.

Explorers have not found the rocks that held the early zircons. Perhaps they metamorphosed (changed in form) in the continual cycling of Earth's mantle and crust. Rocks seem like such a "hard place" from our human time perspective. As inflexible as they appear, even rocks change form with heat, pressure, and time. Continual transformation in the geological cycle foreshadows the patterns of life. (Photograph by Lois Brynes)

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