GEOLOGIC TIME SCALE

The GEOLOGIC TIME SCALE is a record of the history of the Earth, based major geologic & biologic events.

How is the geologic time scale divided? The geologic time scale is divided into four parts: EONS ERAS PERIODS EPOCHS

What is the largest unit of time in Earth’s history? The largest unit of time is an eon. Earth’s 4.6 billion year history is divided into four eons. Three out of the four eons are grouped together to form the Precambrian Time

What is the Precambrian Time? The Precambrian time represents 88% of the Earth’s history. The Precambrian time began 4.6 billion years ago and ended 544 million years ago. It lasted about 4.05 billion years. The Precambrian Time is so long because there is very little evidence (fossils, etc.) to determine any changes in geology or in forms of life.

The other 12% of the Earth’s history is the Phanerozoic Eon which is further divided into eras, periods, and epochs.

The most recent eon, the Phanerozoic, is divided into three eras: Paleozoic – ancient life Mesozoic - middle life Cenozoic - recent life

Earth’s Geologic History

The Precambrian began 4.6 BILLION YEARS AGO and ended 544 million years ago.

Precambrian Time: Earth was still cooling from the collision between the proto-earth and a massive asteroid that formed the Earth and the Moon. The Earth was covered with volcanoes that spewed out water vapor that would eventually condense and form the Earth’s oceans.

The first life on Earth appeared in the seas about 3 The first life on Earth appeared in the seas about 3.5 billion years ago. These early life forms were single-celled organisms, such as cyanobacteria (blue-green algae).

Late Precambrian Later in the Precambrian Time multi-cellular organisms, such as jellyfish, appeared. The end of the Precambrian Time is marked with an explosion of life.

This is the beginning of the Paleozoic Era. The Paleozoic Era began 544 million years ago. The Paleozoic Era is known as the Age of INVERTEBRATES (Animals without backbones).

The first part of the Paleozoic Era is called the CAMBRIAN period. This is called the Cambrian Explosion. A great “explosion” of invertebrate life in the Earth’s seas.

Next is the ORDOVICIAN period. The first vertebrates (animals with backbones) appear here. The first vertebrate was the jawless fish.

The third period is the SILURIAN Period Life appears on land. The first life on land included plants and insects.

The fourth period is the DEVONIAN Period. Age of Fish Sharks and fish with scales and bony skeletons become common. Lungfish develop First amphibians reach land.

The fifth period is the CARBONIFEROUS Period. During this time great swamp forests covered much of North America and Europe. These forest form the great coal reserves found on Earth today.

The Paleozoic Era’s last period is the PERMIAN Period. Reptiles become dominant on land. Pangaea forms.

The end of the Paleozoic Era is marked by the mass extinction of many marine invertebrates including the trilobite. The Paleozoic era ended 245 million years ago.

Next is the Mesozoic Era which is also known as Age of Dinosaurs or Reptiles, began 244 million years ago.

The 1st period of the Mesozoic era is the TRIASSIC Period. During the Triassic period the first dinosaurs evolved. The first mammals appeared. It is believed they evolved from warm-blooded reptiles.

The second period of the Mesozoic era is the JURASSIC Period. During the Jurassic period the first birds appear, and flying reptiles appear. Large dinosaurs, called sauropods thrive. Pangaea is breaking apart.

The third period of the Mesozoic era is the CRETACIOUS Period. The first flowering plants appear. The first snakes appear. Dinosaurs dominate, including Tyrannosaurus rex and Triceratops.

The end of the Mesozoic Era is marked by the extinction of the dinosaurs. Some scientists believe an asteroid impact caused the mass extinction. It ended 65 million years ago

Next is the Cenozoic era. The Cenozoic Era is known as the Age of Mammals.

The first period of the Cenozoic era is the TERTIARY Period. During the Tertiary period the continents move into their present day positions. The Rocky Mountains, Alps, Andes, and Himalayas form. The first grasses appear. Ancestors of humans evolve.

The second period of the Cenozoic era is the Quaternary Period. The Quaternary period stretches from about 2 million years ago to the present. During the Quaternary period the Earth’s climate cooled, causing a series of ice ages. Giant mammals flourish until the ice age ends about 10,000 years ago. Modern Humans evolve about 100,000 years ago.

The Cenozoic Era, Quaternary Period, & Recent Epoch has not ended The Cenozoic Era, Quaternary Period, & Recent Epoch has not ended. We still live in it today!!!