The Earth is a fascinating planet with a tumultuous history. Forming a long time ago, the cluster of rocks floating around the solar system coalesced into the third planet from the sun. The molten ball erupted with noxious gas and hot metals for millions of years. Eventually, the heavier metals began to sink down to the center of the Earth in a process called differentiation. Low density rocks were pushed up from the center, creating the layering of Earth we see today. Fast forward a couple more millions of years, and a giant asteroid skims the side of Earth, blowing off a chunk that will eventually become our moon. As the eras pass, the Earth begins to cool and harden on the surface, allowing water (most likely brought by comets) to condensate and form the vast oceans. In an instant, life begins to form almost 3.5 billion years ago. Tiny bacteria quickly transformed into multi-cellular animals, which soon became the primates with opposable thumbs and large brains we know and love today (National Geographic, 2014).
With such an extensive history, it’s hard to fully comprehend how scientists are even able to figure all of this out. The Earth is constantly moving and evolving, with little to no apparent preservation. Although this change is unnoticeable to the naked eye, the Earth is its own silicon-based organism as it changes through geologic time. During the Archean Eon, once the Earth finished undergoing differentiation, landmasses on the Earth’s surface began to form. These “proto-continents” shifted constantly as a result of plate tectonics, and were constantly being subducted back into the Earth’s mantle, remelted, and eventually resurfaced. This process almost guarantees that rock formed when Earth was born can no longer be dated through isotopic means (a method that uses the half-life of radioactive isotopes to determine the age of rock). Not only that, but many of the rocks found on Earth are now at its core, where heat and pressure exceed human capabilities. As a result, scientists need to use alternative methods to determine the age of the Earth (Trinity College Dublin, 2016).
During Earth’s early stages of the Hadean Eon, it underwent what geologists call heavy bombardment. The solar system 4.5 billion years ago was filled with asteroids, leaving the Earth, which lacked an atmosphere at the time, defenseless against the constant rain of space rock. The rapid impact left the surface inhospitable and constantly molten. When asteroids collide with certain types of rock, they permanently change the crystal structure. A common example of this phenomena are tektites, which are “shocked” pieces of quartzite with fractures at special angles. Another type of crystal that results from asteroid impact are zircon crystals. Although the formation of these zircon crystals is still unclear, researchers suspect that they were formed either from the collision of tectonic plates or from meteor impact craters that plagued the Earth during heavy bombardment of the Hadean Eon (Oskin, 2014). Because of these properties, researchers were able to find a zircon crystal in Western Australia’s Jack Hills that dates back to 4.4 billion years ago. This is the most recent evidence of the oldest geologic features present on Earth, meaning that it only took 100 million years for differentiation to finish so that a crustal formation could form. These crystals are extremely durable and resistant to chemical and physical processes, and typically only consist of a few atoms. As a result of their extreme rarity, they can only be detected by different techniques, such as atom-probe tomography and ion mass spectrometry (Valley, 2014).
Our understanding of Earth is constantly changing. While we may have some understanding of its processes and history, we are always being surprised. The story of Earth is not yet complete, and every new piece of evidence allows us to fill in the gaps. Although it may seem like a lofty goal, we have the beginning and end of Earth’s story, and the fun part is figuring out the rest.
- National Geographic. (2014, February 24). Earth’s Oldest Crust Dates to 4.4 Billion Years Ago. Retrieved from https://news.nationalgeographic.com/news/2014/02/140224-oldest-crust-australia-zircon-science/
- Oskin, B. (2014, February 23). Confirmed: Oldest Fragment of Early Earth is 4.4 Billion Years Old. Retrieved from https://www.livescience.com/43584-earth-oldest- rock-jack-hills-zircon.html
- Trinity College Dublin. (2016). Scientists reveal likely origin of Earth’s oldest crystals (Update). Retrieved from https://phys.org/news/2016-04-scientists-reveal-earth-oldest-crystals.html
- Valley. (2014). Jack Hills Zircon: Scientists Discover Oldest-Known … Retrieved from http://www.sci-news.com/geology/science-jack-hills-zircon-oldest-known-fragment-earth-01779.html