Age of the Earth
The Earth is 4.6 billion years old. Since 1 million years (MY) is a standard unit of geologic time, it is appropriate to remember that the Earth is 4,600 MY old.
Geologic rates
Geologic rates vary from extremely slow to instantaneous. Most rates, motion of plates, erosion, and uplift are slow, averaging only millimeters per year. On the other extreme, 65 MY ago, an asteroid changed the history of Earth and life in an afternoon.
Slow rates can have great effects in relatively small amounts of the 4,600 MY history of the Earth. For example, the Sierra Nevada mountains have been uplifted at about 1 mm/year, and have only taken 5 MY to form mountains nearly 5 km in height.
1 mm/year = 1 meter/thousand years = 1 kilometer/MY ; be able to use this relationship in simple problems.
Highlights of Earth History
4,000 MY
The oldest existing rock is 4,000 MY old. Clearly there were rocks earlier in Earth history. After its initial assembly from smaller, asteroid size bodies, the final stage of the assembly of the Earth was a massive collision with a body approximately the size of Mars (about 1/2 the mass of the Earth). This final collision flung large amounts of vaporized material into orbit around the Earth. This material consolidated to form the Moon, and the Earth separated into compositional layers including the core, mantle and crust. By about 4,500 MY ago, the Earth must have had a solid surface, and undergone a period of intense bombardment by meteorites, like the Moon and Mars. Unlike those bodies, the Earth had liquid water and a substantial atmosphere, and thus erosion began. So while we can find 4,500 MY old rocks on the Moon (and did!), rocks from the first 500 MY of Earth's history have not survived.
3,500 MY
The oldest evidence of life on Earth is about 3,500 MY old. These "fossils" are impressions of single celled blue-green algae (cyanobacteria) which lived in Earth's oceans, and perhaps in mat-like sheets near shore. These bacteria survive in an oxygen-free atmosphere, and produce oxygen as a waste product. Over hundreds of millions of years, these organisms dumped their waste oxygen in to the atmosphere, "polluting" their world to create the Earth we live on. Today, these bacteria are relegated to environments poor in oxygen, such as tidal mud flats.
2,500 MY
This time marks a dramatic change in the Earth. Before this, the atmosphere was poor in oxygen, and the Earth had a thin lithosphere (because it was hotter) that did not become dense enough to subduct easily. That first part of Earth history is known as the Archean. Beginning about 2,500 MY ago, oxygen began to become available in the atmosphere. We know this because at about this time the first red iron oxides (rust) began to appear in rocks exposed at the Earth's surface. This is also the time that modern plate tectonics, controlled in large part by the subduction of oceanic lithosphere, begins. We know this because rock systems after this time resemble current rock systems formed by modern plate tectonics.
1,200 MY
At this point in Earth's history, all of the small continents were colliding to form a supercontinent (there had probably been at least one earlier supercontinent as well). One of these collisions was taking place in the area that is now central Texas. A large mountain belt had formed due to this collision. The remains of core of those mountains is exposed in the granites of Enchanted Rock near Fredricksburg. This collision creates the continental crust that underlies central Texas... before this, there was no central Texas! Up until this time, the only life on Earth had been single celled organisms. At 1200 MY, there are cells with nuclei, and the first multi-celled organisms are beginning to form, although they are only very primitive colonies of single celled organisms.
600 MY
This time marks a dramatic change in life on Earth. Since the development of the first multi-celled organisms, life has become more complex. By now, soft bodied multi-celled organisms with tissues and organs have developed. These life forms resembled flat worms and jellyfish. Suddenly, at about 600 MY ago, organisms began to produce hard skeletal material. The first corals, sponges and shelled creatures appear. Trilobites, resembling horseshoe crabs, now crawl on the ocean floors. Life explodes into a wide variety of forms which are the ancestors of all modern life forms. Despite this explosion of life forms, there is still nothing living on land, no plants or insects. Since there are no plants, no true "soil" has yet formed on land. By this time, the previous supercontinent has broken apart into small continents which have "drifted" apart due to plate tectonics.
300 MY
By 300 MY ago, fish inhabit the seas, and plants, insects and amphibians have invaded the land. Large primitive forests grow along coastline in what is today Pennsylvania and West Virginia. When these trees die and collapse, they will be buried in muds and become the rich coal deposits that are found there today. The continents are again colliding to form a new supercontinent. That supercontinent will be called "Pangaea". One of those collisions was taking place in central Texas, forming another mountain range here. The folded remains of that mountain range can be seen in well drillings from beneath San Antonio, or at the surface in the west Texas area known as the Marathon Basin. The formation of Pangaea also creates the mountain range whose roots remain as today's Appalachian Mountains of the eastern U.S. In Colorado, the "Ancestral Rockies" had formed. Located in nearly the same area as today's Rockies, these Ancestral Rockies would be eroded away long before today's Rockies were formed. The remains of the Ancestral Rockies are seen today in the red sandstones of Garden of the Gods in Colorado Springs, Red Rocks Ampitheater in Denver, and the reddish rocks of the famous Maroon Bells peaks near Aspen. On another part of Pangea, an Ice Age is taking place as glaciers cover what is today northern Africa.
150 MY
By 150 MY ago, Pangea is breaking apart, and the modern Atlantic ocean is forming as the Americas separate from Europe and Africa. The mountains in central Texas, and the Ancestral Rockies have been removed by erosion. The area of the western US in Utah and Arizona is covered with large sand dunes. Dinosaurs have appeared and grown to enormous size. During this period, dinosaurs like Brontosaurus (Apatosaurus) and Stegosaurus have appeared. No T-Rex yet!
75 MY
At 75 MY ago, dinosaurs still rule, but their time is running out. Now, dinosaurs like T-Rex, and the large herds of Triceratops and duck-billed species inhabit the western U.S. A large shallow tropical sea covers most of the central U.S. In central Texas, the carbonate skeletal remains of microscopic plankton collect on the bottom of this warm shallow sea. These "oozes" will be compressed and will form the limestones that are exposed in the quarry face on the Trinity campus. This sea extends all the way to Canada, and the area around Denver is a shore much like coastal Texas today. There are no mountains in Colorado or Utah... the dinosaurs roam a coastal plain there. The Ancestral Rockies are long gone, and the modern Rockies have yet to form. At 65 MY ago, the last dinosaurs will perish when an asteroid collides with the Earth, and Bruce Willis is not there to stop it!
37 MY
At 37 MY ago, large mammals have evolved to replace the dinosaurs in the ecology. Some mammals are evolving back to sea dwelling creatures as the early ancestors of modern whales. The modern Rocky mountains have formed, and volcanic activity covers the western U.S. including what is today Big Bend National Park.
18 MY
At 18 MY ago, central Texas is rocked by earthquakes as the Balcones faults move. Today, these faults mark the edge of the Texas hill country, and form the recharge zone of the Edwards aquifer along highway 35 from here to Austin. Today, these faults are inactive!
9 MY
At 9 MY ago, a large low-lying area around northern Arizona is just beginning a long period of uplift. A river meanders through this flat region. During the uplift to come, this river will hold its place, eroding downward as the land around it rises. This river is the Colorado river, and it will eventually carve the Grand Canyon.
4 MY
At 4 MY ago, the Sierra Nevada mountains, and the Grand Tetons are beginning to be formed as they are uplifted along large faults. Early hominids are beginning to stand on two legs in Africa.
2 MY
At 2 MY ago, the most recent Ice Ages are just beginning, and the first members of our genus appear in Africa.