earth’s magnetic field

Worldwide seismicity from 1 January 2007 to 1 June 2007. Source: USGS National Earthquake Information Center.

Earth’s Magnetic

Field

http://www.earthweek.com/2011/ew110311/ew110311h.jpg

http://science.nasa.gov/media/medialibrary/2003/12/22/29dec_magneticfield_resources/nmppath2001_med.gif

Magnetization of Magnetite

Lavas record magnetic reversals

Magnetic reversals

over the past 20 million

years

Magnetic time scale • 0-700,000 -- Normal• 700,000 - 2.5 my -- Reversed• > 2.5 my -- Normal• Ko`olau lavas mostly reversed in polarity,

so they must be older than 700,000 yrs, but younger than 2.5 my

• Lavas on Kaua`i and in Wai`anae Range show normal polarity, so they must be older than 2.5 my

__________ 1. According to the concepts of plate tectonics, the continents of Africa and South America are gradually approaching each other.

__________ 2. Most geologists believe that continents are larger now than they were in the past.

__________ 3. The size of the earth is gradually increasing over time because of seafloor spreading.

__________ 4. Tectonic plates drift in oceans of melted magma just below the surface of the earth.

__________ 5. When two oceanic plates collide, the colder denser older plate is stronger and subducts the newer warmer plate.

Maps recording the locations of earthquakes around the world show that

1. earthquakes are evenly distributed.

2. most earthquakes follow the border of the Atlantic Ocean.

3. most earthquakes occur in the interior of continents.

4. most earthquakes surround the Pacific Ocean.

The lithosphere is composed of:

1. Core and mantle2. Crust and asthenosphere3. Crust and uppermost mantle4. Core and asthenosphere

Sea-floor spreading suggests that1. the sea floor moves away from the mid-ocean ridge.2. the sea floor moves toward the trenches.3. the sea floor age increases from mid-ocean ridge to the continents.4. all of the above are true.

Most plates move at an average rate that is best described by which unit rate?1. cm/year.2. m/year.3. km/year.4. plates don’t move.

If you dredged a sample of volcanic rock from locality X, the most likely age of the rock (in other words, the amount of time elapsed since the rock cooled from a magma) would be:1. 1 year 2. 1 million years 3. 10 million years 4. 100 million years