FAQ's about Paleontology

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.

Sediments eroded from the Appalachian and other inland highlands were carried east and southward by streams and gradually covered the faulted continental margin, burying it under a wedge, thousands of feet thick, of layered sedimentary and volcanic debris. Today most Mesozoic and Cenozoic sedimentary rock layers that lie beneath much of the coastal plain and fringing continental shelf remain nearly horizontal or tilt gently toward the sea.

Excerpt and Graphic from: USGS/NPS Geology in the Parks Website, Atlantic Plain Province, August 2001

USGS Field Records Library

Box 25046, MS 914

Denver Federal Center

Denver, CO 80225-0046

Telephone: 303-236-1000

FAX: 303-236-0015

TDD: 303-236-0998

E-mail: den_lib@usgs.gov

Jill Schneider

USGS Mineral Resources Surveys

Alaska Section

4200 University Drive

Anchorage, AK 99508-4667

Telephone: 907-786-7457 (Office) and 907-786-7007 (Library)

FAX: 907-786-7401

E-mail: jschnidr@usgs.gov

Two other sources of field records are:

1. National Archives and Records Administration (NARA) in College Park, Maryland, which keeps field record materials in their Archives II facility. Refer to the Guide to Federal Records in the National Archives of the United States (Washington, D.C.: NARA, 1995).

Inventory of the Records of the United States Geological Survey, Record Group 57, in the National Archives, part of USGS Circular 1179 (2000, CD-ROM): Records and History of the United States Geological Survey, contains information on USGS and related records accessioned by NARA through 1997 and held at NARA-II. Appendices in this inventory list field records held at NARA-II and by the USGS Field Records Library at Denver.

National Archives and Records Administration

8601 Adelphi Road

College Park, MD 20740-6001

Telephone: 301-713-6800 (General Reference Information)

Telephone: 1-866-272-6272 (Toll Free)

E-mail: inquire@nara.gov

URL: http://www.archives.gov/index.html

The Smithsonian Institution Archives

Arts and Industries Building, Room 2135

900 Jefferson Drive, S.W.

Washington, D.C. 20560

Telephone: 202-357-1420

E-mail: OSIAREF@OSIA.SI.EDU

Rocky Mountain Mapping Center

USGS National Mapping Division

Box 25046, MS 506, Federal Center

Denver, CO 80225-0046

Telephone: 303-202-4394

Mid-Continent Mapping Center

USGS National Mapping Division

MS 309

1400 Independence Road

Rolla, MO 65401

Telephone: 573-308-3663

Its pieces then spread across the globe into a nearly modern arrangement by a process called plate tectonics. Volcanoes, earthquakes, mountain building, and sea-floor spreading are all part of plate tectonics, and this process is still changing our modern Earth.

Relative positions of continents during the age of dinosaurs.

Yes. We know this from an examination of the geological record. When lavas are deposited on the Earth's surface, and subsequently freeze, and when sediments are deposited on ocean and lake bottoms, and subsequently solidify, they often preserve a signature of the ambient magnetic field at the time of deposition. This type of magnetization is known as 'paleomagnetism'. Careful measurements of oriented samples of faintly magnetized rocks taken from many geographical sites allow scientists to work out the geological history of the magnetic field. We can tell, for example, that the Earth has had a magnetic field for at least 3.5 billion years, and that the field has always exhibited a certain amount of time-dependence, part of which is normal secular variation, like that which we observe today, and part of which is an occasional reversal of polarity. Incredible as it may seem, the magnetic field occasionally flips over! The geomagnetic poles are currently roughly coincident with the geographic poles, because the rotation of the Earth is an important dynamical force in the core, where the main part of the field is generated. Occasionally, however, the secular variation becomes sufficiently large such that the magnetic poles end up being located rather distantly from the geographic poles; we say that the poles have undergone an 'excursion' from their preferred state. Now, we know from physics that the Earth's dynamo is just as capable of generating a magnetic field with a polarity like that which we have today as it is capable of generating a field with the opposite polarity. The dynamo has no preference for a particular polarity. Therefore, after an excursional period of enhanced secular variation, the magnetic field, upon returning to its usual state of rough alignment with the Earth's rotational axis, could just as easily have one polarity as another. The consequences of polarity reversals for the compass are dramatic. Nowadays, the compass points roughly north, or, more precisely, the north end of the compass points roughly north at most geographical locations. However, before the last reversal, which was about 780,000 years ago, the polarity was reversed compared to today's, and the compass would have pointed roughly south, and before that reversed state the polarity was like that which we have today, and the compass would have pointed roughly north, and so on. The timings of reversals forms the so-called 'geomagnetic polarity timescale', shown here at the right. During a reversal, between polarities, the geometry of the magnetic field is much more complicated than it is now, and a compass could point in almost any direction depending on one's location on the Earth and the exact form of the mid-transitional magnetic field. One of the things that is interesting about reversals is that there is no apparent periodicity to their occurrence. Reversals are random events. They can happen as often as every 10 thousand years or so, and as infrequently as every 50 million years or more. Questions about reversals are very popular with the general public, and further information can be found in the references given in the Further Reading page of the Geomagnetism website.

The magnetic field of the Earth does protect us from fast-moving charged particles streaming from the Sun, but so does the atmosphere. It is not clear whether or not the radiation that would make it to the Earth's surface during a polarity transition, when the magnetic field is relatively weak, is sufficient to affect evolution, either directly or indirectly, and cause extinctions, such as that of the dinosaurs. But it seems that the radiation is probably insufficient. This conclusion is supported by the fact that reversals happen rather frequently, every million years or so, compared to the occurrence of mass extinctions, every hundred million years or so. In other words, many reversals and, in fact, most reversals, appear to be of no consequence for extinctions.

It is the period of time extending from the formation of the earth to the present. For more information visit our Geologic Time Scale Web page.

Dinosaur communities were separated by both time and geography. The "age of dinosaurs" (the Mesozoic Era) included three consecutive geologic time periods (the Triassic, Jurassic, and Cretaceous Periods). Different dinosaur species lived during each of these three periods. For example, the Jurassic dinosaur Stegosaurus already had been extinct for approximately 80 million years before the appearance of the Cretaceous dinosaur Tyrannosaurus. In fact, the time separating Stegosaurus and Tyrannosaurus is greater than the time separating Tyrannosaurus and you. At the beginning of dinosaur history (the Triassic Period), there was one supercontinent on Earth (Pangea). Many dinosaur types were widespread across it. However, as Pangea broke apart, dinosaurs became scattered across the globe on separate continents, and new types of dinosaurs evolved separately in each geographic area.

Explore the variety of USGS resources on polar research, from maps and fact sheets to photographs and databases. Especially note Fact Sheet 2007-3013, "International Polar Year: Science at the Ends of the Earth".