EarthHistory

Earth is approximately 4.5-4.8 billion years old.

__**Earth's Geologic Time Scale**__ Split into Eons, Eras, Periods, and Epochs. (Eons being the longest) The **Precambrian** period of Earth History comprises 87% of the geologic time scale. This is an important time distinction. At the beginning of the **Cambrian Period (570 mya)**, the __Cambrian Explosion__ occurred. This was the point where most of the diverse life forms on Earth originated (prior to the Cambrian Explosion, Earth was only populated by bacteria and some protists, plants, and fungi. The Cambrian Explosion was the point where animals increased in number and diversity.

__**Categorizing Life**__ Life on Earth is categorized into __5 Kingdoms__
 * Animalia (Animals)
 * Plantae (Plants)
 * Fungi (Fungus)
 * Protista (Protists
 * Can be "animal-like", "plant-like," or "fungus-like" but aren't quite either of these groups
 * Bacteria
 * Split into Archaea (oldest organisms on Earth- also the group that does chemosynthesis) and Monera

__**The Benefit Of Extinction**__ Extinctions, while they often mean major loss of immediate organismal diversity, extinctions allow for other organisms previously oppressed to take over available niches and diversify. While extinctions do mean momentary loss of diversity, they allow for potential gains in diversity later on. A famous mass extinction was the K-T extinction at the end of the Cretaceous Period (this was the end of the dinosaur era).
 * A large asteroid hit the Yucatan penninsula, causing seismic activity throughout the entire globe and a tsunami
 * The asteroid triggered not only earthquakes but volcanoes
 * Tsunamis coastal destruction
 * Volcanic action led to a worldwide spreading of ash clouds, creating a "nuclear winter", blocking out sunlight and preventing warm temperatures and photosynthesis.
 * Dinosaurs, being cold blooded, could not survive the cold temperatures
 * Herbivores were left with no food source as plants could not photosynthesize
 * Eventually carnivores ran out of food supplies
 * Pressure on dinosaurs (cold temps) was not an issue for mammals, which were much smaller and scarce at the time
 * Dinosaur die-off paired with temperature change allowed for mammal population to explode and diversify.

__**Rock Formation**__ There are 3 Types of rocks on Earth
 * 1) **Sedimentary-** rocks formed by sediments- Tennessee rock types (siltstone, sandstone, limestone) are examples
 * 2) **Metamorphic**- rocks formed at the surface but altered by intense heat and pressure below ground. An example is slate.
 * 3) **Igneous-** formed by intense heat and then solidifies- volcanic rocks like basalt and marble are examples

//The Rock Cycle// See the diagram on page 31 of the Princeton review for a diagram of the rock cycle.