Geologic Time Scale

In the 1700s, excavations unearthed similar fossil sequences in distant places. Scholars began to view these findings as evidence of the connection between Earth history and the history of life.

This animation (Audio - Important) describes the geologic time scale.

Continental drift is the idea that the continents were originally joined and have since "drifted" apart. Initially this was based on the shapes of the continents and the observation that they could fit together like a puzzle. Wegener refined the hypothesis and named the theoretical supercontinent Pangea. Pangea was a single world continent that extended from pole to pole surrounded by a single huge ocean. It is thought that an even earlier supercontinent than Pangea existed. It is called Gondwana and was part of a drifting land mass that collided with others to form Pangea.

The discovery of seafloor spreading provided a possible mechanism for continental drift. According to plate tectonic theory, enormous slablike plates of the earth's crust move apart and crunch together at their margins causing earthquakes, volcanic eruptions, and lava flows. The Earth's crust is fractured into plates. The movement of plates is driven by upwelling of molten rock at mid-oceanic ridges. As the seafloor spreads, older rock is forced down into trenches. When the land masses separated, it was possible for speciation to occur. When the land masses collided, diversity declined.

This animation (Audio - Important) describes plate tectonics

The major geological periods are (don't panic, you don't need to memorize these, but do note the importance of mass extinctions as you read through this information):

• Archean eon (oldest interval)
• Proterozoic eon
• Paleozoic era
• Mesozoic era
• Cenozoic era (most recent)

• Cambrian
• Ordovician
• Silurian
• Devonian
• Carboniferous
• Permian

The Silurian Period saw the recovery of ocean reef communities and the origin of the first land plants.

In the Devonian Period, jawed fishes arose, and diversified. Ancestors of amphibians moved onto land and the radiation of amphibians began. The period ended with another mass extinction.

During the Carboniferous Period, sea levels swung widely, amphibians diversified, and the first reptiles evolved. Seedless vascular plants and gymnosperms thrived.

In the Permian Period, insects, amphibians, and early reptiles thrived in swamp forests. The period ended with the greatest known mass extinction and the formation of Pangea.

The Mesozoic Era is Divided into three periods:

• Triassic
• Jurassic
• Cretaceous

This was the "Age of the Reptiles". The major geologic event was the breakup of the supercontinent Pangea.

In the Triassic Period, the seas repopulated after the Permian extinction. The first dinosaurs and mammals appeared. The period ends with another mass extinction.

The Jurassic Period saw the radiation of the dinosaurs. It ended with a mass extinction that ended many dinosaur lineages.

In the Cretaceous Period, the surviving dinosaurs diversify. Seedless plants and gymnosperms begin to decline.

During the Cenozoic Era, continents collided and mountain ranges arose. Mammals underwent adaptive radiation. Tropical forests gave way to woodlands and grasslands. The most recent Ice Age occurred.

REVIEW: The large land mass that contained all the continents was called
Answer Pangea.