history of earth

• Most rocks are sedimentary rocks
• They are formed from older rocks that have been broken down by water or wind.
• The older rocks become sedimentary particles such as gravel, sand, and mud.
• These particles can also bury dead plants and animals. As time goes by, the particles accumulate, and those that are at the bottom of the pile become rocks.
• Gravel becomes conglomerate; sand becomes sandstone; and mud becomes shale or mudstone
• The animals or plants buried with them become fossils
• • Relative dating is a method of arranging geological events based on the rock sequence.
• • Absolute dating is a method that gives an actual date of the rock or period of an event.
• Relative dating cannot provide actual numerical dates of rocks. It only tells that one rock is older than the other but does not tell how old each of the rock is.
• In the early mid-1600’s, a Danish scientist, Nicholas Steno, studied the relative positions of sedimentary rocks. He discovered that they settle based on their relative weight or size in a fluid.
• Any slight changes in the particle size or composition may result in the formation of layers called beds.
• Layering or bedding is a distinct quality of sedimentary rocks.
• The layered rocks are also called strata.
• The law of superposition states that, in any sequence of layered sedimentary rocks, the top layer is younger than the bottom layer.
• The law of original horizontality states that most sediments were originally laid down horizontally.
• The law of lateral continuity states that rock layers extend laterally or out to the sides
• The law of cross-cutting relationship states that fault lines and igneous rocks are younger features that cut through older features of rocks.
• Absolute dating or radiometric dating is a method used to determine the age of rocks by measuring its radioactive decay.
• • Radiocarbon dating for organic remains could date up to 60 000 years.
• • K-Ar dating and U-Pb dating for volcanic rocks could date up to five billion years.
• geologic time scale shows the geologic time intervals based on the geologic rock records, which describe the relationships between the events that happened throughout the Earth’s history.
• When plants and animals die, their remains and imprints are buried in rocks or sediments. These preserved remains or traces are called fossils.
• Fossils are pieces of evidence that life has happened in the past. Information from these fossils are used to construct the geologic time scale.
• geologic time scale is a record of the geologic history of the Earth. It is made up of time units that divide Earth’s history based on the appearance or disappearance of life forms (supported by fossil remains) in specific times.
• An eon, the largest division of the geologic time scale, spans hundreds to thousands of millions of years.
• There are three major eons, the Archean, Proterozoic, and Phanerozoic eons.
• • The Phanerozoic eon is the one we are in today.
• An era is hundreds of millions of years long.
• The three major eras in the Phanerozoic eon are the Paleozoic, Mesozoic, and Cenozoic eras.
• • We are in the Cenozoic era, which began 65 million years ago, after the extinction of the dinosaurs.
• A period is tens of millions of years long. It is based on the forms of life existing at that time.
• • the tertiary and quaternary periods comprise the Cenozoic Era. The tertiary period is the beginning of the age of mammals while the quaternary period is considered the age of humans
• An epoch is several million years long. It is the division of the most recent periods.
• , is divided into two epochs, the Pleistocene and Holocene epochs.
• The subdivisions of the geologic time scale are identified through marker fossils, or guide fossils.
• A marker fossil is a fossil of a plant or an animal that existed for a relatively short period of time
• Primitive life forms existed on Earth during Precambrian time and the Paleozoic era. They continue to evolve through the Mesozoic Era and the current Cenozoic Era.