The geologic time scale (GTS) is a system of chronological measurement that relates stratigraphy to time, and is used by geologists, paleontologists, and other earth scientists to describe the timing and relationships between events that have occurred throughout Earth's history. The table of geologic time spans presented here agrees with the dates and nomenclature set forth by the International Commission on Stratigraphy standard color codes of the International Commission on Stratigraphy.
Evidence from radiometric dating indicates that the Earth is about 4.54 billion years old. The geology or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the GTS are usually delimited by changes in the composition of strata which correspond to them, indicating major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the dinosaurs and many other groups of life. Older time spans which predate the reliable fossil record (before the Proterozoic Eon) are defined by the absolute age.
Historical geology is the use of the principles of geology to reconstruct and understand the history of the Earth. It focuses on geologic processes that change the Earth's surface and subsurface; and the use of stratigraphy, structural geology and paleontology to tell the sequence of these events. It also focuses on the evolution of plants and animals during different time periods in the geological timescale. The discovery of radioactivity and the development of a variety of radiometric dating techniques in the first half of the 20th century provided a means of deriving absolute versus relative ages of geologic history.
Economic geology, the search for and extraction of energy and raw materials, is heavily dependent on an understanding of the geological history of an area. Environmental geology, including most importantly the geologic hazards of earthquakes and volcanism, must also include a detailed knowledge of geologic history.
Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building") is a scientific theory that describes the large-scale motions of Earth's lithosphere. The model builds on the concepts of continental drift, developed during the first few decades of the 20th century. The geoscientific community accepted the theory after the concepts of seafloor spreading were developed in the late 1950s and early 1960s.
The lithosphere is broken up into tectonic plates. On Earth, there are seven or eight major plates (depending on how they are defined) and many minor plates. Where plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative movement of the plates typically varies from zero to 100 mm annually.
The Central Pangean Mountains were an extensive northeast-southwest trending mountain range in the central portion of the supercontinent Pangaea during the Triassic period. They were formed as a result of collision between the minor supercontinents Laurussia and Gondwana during the formation of Pangaea. Remnants of this massive mountain range include the Appalachian Mountains of North America and the Little Atlas of Morocco, Africa.
A number of mountain building periods were involved in the formation of the Central Pangean Mountains, including the Acadian, Caledonian and Alleghenian orogenies.
Pangaea Ultima (also called Pangaea Proxima, Neopangaea, and Pangaea II) is a possible future supercontinent configuration. Consistent with the supercontinent cycle, Pangaea Ultima could occur within the next 250 million years. This potential configuration, hypothesized by Christopher Scotese, earned its name from its similarity to the previous Pangaea supercontinent. The concept was based on examination of past cycles of formation and breakup of supercontinents, not on current understanding of the mechanisms of tectonic change, which are too imprecise to project that far into the future. "It's all pretty much fantasy to start with", Scotese has said. "But it's a fun exercise to think about what might happen. And you can only do it if you have a really clear idea of why things happen in the first place."
Supercontinents describe the merger of all, or nearly all, of the Earth's landmass into a single contiguous continent. In the Pangaea Ultima scenario, subduction at the western Atlantic, east of the Americas, leads to the subduction of the Atlantic mid-ocean ridge followed by subduction destroying the Atlantic oceanic basin, causing the Atlantic Ocean to close, bringing the Americas back together with Africa and Europe. As with most supercontinents, the interior of Pangaea Proxima would probably become a semi-arid desert prone to extreme temperatures.
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the west and south by the Pacific Ocean, and to the southeast by South America and the Caribbean Sea.
North America covers an area of about 24,709,000 square kilometers (9,540,000 square miles), about 4.8% of the planet's surface or about 16.5% of its land area. As of July 2008, its population was estimated at nearly 529 million people across 23 independent states, representing about 7.5% of the human population. Most of the continent's land area is dominated by Canada, the United States, and Mexico, while smaller states exist in the Central American and Caribbean regions. North America is the third-largest continent by area, following Asia and Africa, and the fourth by population after Asia, Africa, and Europe.