Archean

A long time ago, Earth was very different.

It was a world of deep water. Most of the land was under the sea. The air did not have the oxygen we breathe.

Earth was also much hotter then. Large rocks fell from space and hit the ground.

Tiny living things began to grow in the water. They lived in mats called stromatolites. These tiny things helped change the air.

Life started to grow in the warm sea. It is amazing to think about our early world!

The Archean was a very different time for Earth. It began about 4,031 million years ago.

During this time, Earth was mostly a water world. Most of the land was under deep oceans. The air was also different. It had very little oxygen. Instead, the air was rich in methane. This made the Earth look like a pale orange dot.

Earth was much hotter then. The heat came from the core and from decaying elements. This heat made the mantle, or the layer under the crust, very hot. Large rocks from space also hit the Earth often.

Life began to grow in the oceans. These were simple living things called prokaryotes. They were single cells without a nucleus. Some lived in mats called stromatolites.

Tiny life forms called cyanobacteria appeared later. They used a way of making food called photosynthesis. This process let out oxygen. This changed the ocean and the air forever.

The Archean was a very special time in Earth's long history. It is the second of the four geologic eons. This eon began about 4,031 million years ago.

Scientists use the age of the oldest intact rocks to mark its start. This period follows the Hadean Eon. It is followed by the Proterozoic Eon. The name comes from a Greek word meaning "beginning" or "origin." It was a time when our planet started to look more familiar.

Earth was mostly a water world during this time. There was some continental crust, but it was often under deep oceans. The atmosphere was very different from the air we breathe now. It was a reducing atmosphere, which means it was rich in methane. It lacked free oxygen, which is the gas we need to live. Because of the methane, the Earth might have looked like a pale orange dot.

This hazy air helped keep the planet warm even though the Sun was dimmer.

Our planet was also much hotter during the Archean. The heat flow was nearly three times as high as it is today. This heat came from the formation of the metallic core. It also came from the decay of radioactive elements. This extra heat made the mantle, the layer under the crust, much hotter. This heat caused lots of volcanic activity. Some volcanoes even erupted unusual types of lava called komatiite. Large rocks from space also hit the Earth often during this time.

Life began to appear in the oceans during this eon. The earliest life forms were simple prokaryotes. These are single-celled living things that do not have a nucleus.

Some of these lived in mats called stromatolites. These mats were formed by tiny organisms in shallow water. Scientists found 3.48 billion-year-old stromatolites in Western Australia. These are the first direct fossil traces of life on Earth. They show us how life started to take hold.

Later in the Archean, tiny organisms called cyanobacteria appeared. They used a process called photosynthesis to make food.

This process released oxygen into the environment. This caused a permanent chemical change in the ocean and the air. It set the stage for much bigger living things to exist later. You can think of these tiny bacteria as the builders of our modern air. Without them, the Earth would still be a very different place.

The Archean is the second of Earth's four geologic eons. Its name comes from the Greek word meaning "beginning" or "origin." This eon began approximately 4,031 million years ago. This starting point is defined by the age of the oldest intact rock formations on our planet. The Archean followed the Hadean Eon and preceded the Proterozoic Eon. It represents a fundamental era when the Earth began to form its lasting features.

During this time, Earth was mostly a water world. There was some continental crust, but much of it sat under oceans deeper than those we have today. The atmosphere was also very different from the air we breathe now. It was a reducing atmosphere, which means it was rich in methane. It lacked free oxygen, which is the gas necessary for most modern life. Because of the methane, the Earth may have appeared as a pale orange dot.

This hazy atmosphere likely helped keep the planet warm even though the Sun was much dimmer than it is today.

Earth's internal heat was much higher during the Archean. The heat flow was nearly three times as high as it is today. This heat came from several sources. It was partly remnant heat from the formation of the planet. It also came from the formation of the metallic core. Finally, it arose from the decay of radioactive elements. This extra heat made the mantle, the layer beneath the crust, much hotter than it is now. This resulted in very high volcanic activity. Some volcanoes erupted unusual types of lava, such as komatiite.

Geology in the Archean was shaped by moving plates. Plate tectonics likely started in the Hadean but slowed down during the Archean. This slowdown may have happened because the mantle became more viscous, or thick, as water moved out of it. These moving plates helped create large amounts of continental crust. However, the deep oceans likely covered most of the continents. It was only toward the end of the Archean that continents began to emerge from the sea. This emergence caused weathering that changed the chemistry of the seawater.

Scientists have found Archean rocks in many places around the world. These include Greenland, Siberia, Canada, Brazil, and Australia. Many of these rocks are granitic, meaning they are made of granite. Other rocks include metamorphosed sediments like mudstones and volcanic rocks. One common formation is called a greenstone belt. These belts consist of alternating layers of different types of rock. They often represent the places where early continents joined together. Archean rocks make up only about 8% of the continental crust we see today. Most of the original Archean crust has been recycled by the Earth's processes.

Life began to emerge during this eon. The earliest evidence of life includes graphite found in 3.7 billion-year-old rocks in Greenland. The most famous fossils from this time are stromatolites.

Stromatolites are microbial mats formed by tiny organisms in shallow water. These organisms were prokaryotes, which are simple, single-celled life forms like bacteria and archaea. They lacked a nucleus, which is a central part of more complex cells. The oldest identifiable stromatolite fossils were found in 3.48 billion-year-old sandstone in Western Australia.

As the Archean progressed, a major change occurred through photosynthesis. Early cyanobacteria began using sunlight to make food. This process released oxygen as a byproduct. While oxygen levels remained very low for most of the eon, these tiny organisms changed the chemistry of the oceans and the atmosphere. This set the stage for the Great Oxygenation Event that followed. Even though asteroid impacts were frequent, life continued to establish itself. These tiny biological processes eventually transformed the entire planet.