Ll History of the Earth 2311 Early history

The main steps in the history of the Earth are shown in Fig. 23.2. Somehow, the Earth accreted from the material in the original solar nebula. We will discuss more about the solar nebula in Chapter 27. Enough material collected together so that its own gravitational pull was able to keep most of the material from escaping. As particles fell towards a central core, they were moving closer together, so their gravitational potential energy decreased. This means that their kinetic energy increased. This kinetic energy was then available to heat the forming planet. In addition, heat was provided by the radioactive decay of potassium, thorium and uranium. Such decays led to heating, because the energetic particles -alpha, beta, gamma - were absorbed by the surrounding rock. The relatively massive alpha particles were particularly effective in this heating.

The heating resulted in a liquid, or molten, interior. Since materials are free to move in a liquid, heavier elements, such as iron and nickel,

H In this photo of the Earth from space we can really appreciate that it is just another planet, one that has a moon.This image was taken on 16 Dec 1992, by the Galilelo spacecraft from a distance of 6.2 million km. [NASA]

sank into the center, while lighter elements, such as aluminum, silicon, sodium and potassium, floated to the surface. This process is called differentiation. The iron and nickel form the current core of the Earth. So much heat was trapped by the Earth that its core is still molten. The thorium and uranium were squeezed out of the core, and carried along in crystals to the surface. These radioactive materials provide heating close to the surface.

The molten core is responsible for the Earth's magnetic field. The rapid rotation of the Earth

Fig 23.2.

Diagram showing steps in the formation and development of the Earth.

Cooled volcanic rock is an important component of the Earth's surface. [USGS]

Cooled volcanic rock is an important component of the Earth's surface. [USGS]

leads to a dynamo process. In this process, a small magnetic field plus convection creates electrical currents flowing through the fluid. These currents produce a larger magnetic field, and so on. This process takes energy from the Earth's rotation. The Earth's magnetic field is not fixed. The magnetic pole wanders irregularly. In addition, geological records indicate that the direction of the magnetic field has reversed every few hundred thousand years. The actual reversals take tens of thousands of years, and there is a period when the field is very weak. The causes of the reversals are not well understood.

The upper part of the Earth's crust is composed of several different types of rocks. There are igneous rocks, such as granite, which are formed in vol-canos (and are enriched in silicon, aluminum and potassium), and sedimentary rocks, that have been deposited gradually. Both types of rocks can be altered by high pressures, and are then called meta-morphic. The lower part of the crust is composed of gabbro and basalts. These are dark rocks containing silicates of sodium, magnesium and iron. (Silicates are compounds in which silicon and oxygen are added to the other atoms in various combinations.) Gabbros have coarser crystals than do basalts. Larger crystals result from slower cooling.

An important clue to the Earth's interior comes from volcanic activity. This activity results from the heating of the crust, producing the molten material. Volcanic activity is very important in mountain building, as shown in Fig. 23.4. It also provides a means of transporting certain materials from the interior to the surface. In particular, we think that this is the source of carbon dioxide (CO2 ), methane (CH4) and water (H2O), as well as sulfur-containing gases in our atmosphere. Visitors to active volcanoes on Earth, such as Kilauea on Hawaii, shown in Fig. 23.4, often note a strong sulfur smell.

As the water vapor was ejected into the atmosphere, the temperature was low enough for it to condense. Other gases dissolved in the water, and combined with calcium and magnesium leached from surface rocks. This had the important effect of removing most of the carbon dioxide from the Earth's atmosphere.

23.1.2 Radioactive dating

Much of what we know about the age of the Earth's crust comes from radioactive dating. In this

Fig 23.4.

Images of an active volcano, Kilauea, in Hawaii.This is an image from space, to give you an idea of the quality images we can make from orbit around other planets. (a) An overhead view from radar studies on the shuttle Endeavor.This is an inter-ferometric image. (b) A three-dimensional perspective (in false color) reconstructed from the radar images. [NASA]

t1|2, which is the time for the number in the sample to fall to half of its original value,

technique, we are studying the products of radioactive decay. In radioactive dating we take advantage of the fact that if we start with some number N0 of nuclei of a radioactive isotope, the number left after a certain time t is given by

The quantity re is the time for the number in the sample to fall to 1/e of its original value. We can also write this expression in terms of the half-life,

Comparing these two expression (see Problem 23.2), tells us that n/2 = (0.693)Te

Half-lives of many nuclei are measured in the laboratory (see Problem 23.4). Therefore, if we know N0 and N(t), we can solve for t, the time that has elapsed since the sample had N0 nuclei of the particular isotope. This is the essence of radioactive dating. It is important to choose an isotope whose half-life is comparable to the time period you are trying to measure. If the time period is much longer than the half-life, there will be too few left to measure. If the time period is much less than the half-life, very few decays will have

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