Geology

What was Pangea?

From about 280-230 million years ago (Late Paleozoic Era until the Late Triassic), the continent we now know as North America was continuous with Africa, South America, and Europe. They all existed as a single continent called Pangea. Pangea first began to be torn apart when a three-pronged fissure grew between Africa, South America, and North America. Rifting began as magma welled up through the weakness in the crust, creating a volcanic rift zone. Volcanic eruptions spewed ash and volcanic debris across the landscape as these severed continent-sized fragments of Pangea diverged. The gash between the spreading continents gradually grew to form a new ocean basin, the Atlantic. The rift zone known as the mid-Atlantic ridge continued to provide the raw volcanic materials for the expanding ocean basin.

Meanwhile, North America was slowly pulled westward away from the rift zone. The thick continental crust that made up the new east coast collapsed into a series of down-dropped fault blocks that roughly parallel today's coastline. At first, the hot, faulted edge of the continent was high and buoyant relative to the new ocean basin. As the edge of North America moved away from the hot rift zone, it began to cool and subside beneath the new Atlantic Ocean. This once-active divergent plate boundary became the passive, trailing edge of westward moving North America. In plate tectonic terms, the Atlantic Plain is known as a classic example of a passive continental margin.

Today, the Mesozoic and Cenozoic sedimentary rock layers that lie beneath much of the coastal plain and fringing continental shelf remain nearly horizontal.

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Where did dinosaurs live?

Dinosaurs lived on all of the continents. At the beginning of the age of dinosaurs (during the Triassic Period, about 230 million years ago), the continents were arranged together as a single supercontinent called Pangea. During the 165 million years of dinosaur existence this supercontinent slowly broke apart. Its pieces then spread across the...

When did dinosaurs become extinct?

Dinosaurs went extinct about 65 million years ago (at the end of the Cretaceous Period), after living on Earth for about 165 million years. If all of Earth time from the very beginning of the dinosaurs to today were compressed into 365 days (one calendar year), the dinosaurs appeared January 1 and became extinct the third week of September. (Using...

Did people and dinosaurs live at the same time?

No! After the dinosaurs died out, nearly 65 million years passed before people appeared on Earth. However, small mammals (including shrew-sized primates) were alive at the time of the dinosaurs. Many scientists who study dinosaurs (vertebrate paleontologists) now think that birds are direct descendants of one line of carnivorous dinosaurs, and...

Did all the dinosaurs live together, and at the same time?

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What is declination?

At most places on the Earth's surface, the compass doesn't point exactly toward geographic north. The deviation of the compass from true north is an angle called "declination" (or "magnetic declination"). It is a quantity that has been a nuisance to navigators for centuries, especially since it varies with both geographic location and time . It...

Image: Trilobite Fossil (Phacops rana africana)

Do any mass extinctions correlate with magnetic reversals?

No. There is no evidence of a correlation between mass extinctions and magnetic pole reversals. Earth’s magnetic field and its atmosphere protect us from solar radiation. It’s not clear whether a weak magnetic field during a polarity transition would allow enough solar radiation to reach the Earth's surface that it would cause extinctions. But...

Are there geologic maps or publications for where I live?

Detailed geologic mapping has not been completed for the entire United States, but maps are available for most locations. Geologic maps at many scales and from many sources are listed in the National Geologic Map Database . Some geologic maps can be purchased in hard copy through the USGS Store . Download digital geologic maps for entire states...

Is it true that Earth's magnetic field occasionally reverses its polarity?

Yes. We can see evidence of magnetic polarity reversals by examining the geologic record. When lavas or sediments solidify, they often preserve a signature of the ambient magnetic field at the time of deposition. Incredible as it may seem, the magnetic field occasionally flips over! The geomagnetic poles are currently roughly coincident with the...

This Placenticeras meeki ammonite specimen from our Campanian Pueblo, Colorado Pierre Shale collection of the CRC website

Can USGS fossil data sets be downloaded or viewed online?

Not yet. That is our goal. Presently, one can view brief descriptions of the collections . Some fossil photos can be viewed and downloaded from the USGS Photographic Library . Also, fossil photos can be seen as published plates within many online USGS publications. Visit the USGS Publications Warehouse to search for publications. The best keywords...