waves. Tsunamis, or tidal waves, are different. They result from underwater earthquakes, volcanic eruptions, or landslides, not wind. Energy from the Core Another source of Earth’s energy comes from Earth’s core. We distinguish four main layers of Earth: the inner core, the outer core, the rocky mantle, and the crust. The inner core is a solid mass of iron with a temperature of about 7,000° F. Most likely, the high temperature is caused by radioactive decay of uranium and other radioactive elements. The inner core is approximately 1,500 miles in diameter. The outer core is a mass of molten iron that surrounds the solid inner core. Electri- cal currents generated from this area produce the earth’s magnetic field. The rocky mantle is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium and is about 1,750 miles thick. This mantle accounts for most of the Earth’s mass. When parts of this layer become hot enough, they turn to slow moving molten rock, or magma. The Earth’s crust is a layer from four to 25 miles thick, consisting of sand and rock. The upper mantle is rigid and is part of the litho- sphere (together with the crust). The lower mantle flows slowly, at a rate of a few centimeters per year. The crust is divided into plates that drift slowly (only a few cen- timeters each year) on the less rigid mantle. Oceanic crust is thinner than continental crust. This motion of the plates is caused by convection (heat) currents, which carry heat from the hot inner mantle to the cooler outer mantle. The motion results in earthquakes and volcanic eruptions. This process is called plate tectonics. Tectonics Evidence suggests that about 200 million years ago, all continents were a part of one landmass, named Pangaea. Over the years, the continents slowly separated through the movement of plates in a process called continental drift. The movement of the plates is attributed to con- vection currents in the mantle. The theory of plate tec- tonics says that there are now twelve large plates that slowly move on the mantle. According to this theory, earthquakes and volcanic eruptions occur along the lines where plates collide. Dramatic changes on Earth’s land- scape and ocean floor are caused by collision of plates. These changes include the formation of mountains and valleys. Geochemical Cycles Water, carbon, and nitrogen are recycled in the bios- phere. A water molecule in the cell of your eye could have been, at some point, in the ocean, in the atmosphere, in a leaf of a tree, or in the cell of a bear’s foot. The circula- tion of elements in the biosphere is called a geochemical cycle. Water Oceans cover 70% of the Earth’s surface and contain more than 97% of all water on Earth. Sunlight evapo- rates the water from the oceans, rivers, and lakes. Living beings need water for both the outside and the inside of their cells. In fact, vertebrates (you included) are about 70% water. Plants contain even more water. Most of the water passes through a plant unaltered. Plants draw on water from the soil and release it as vapor through pores in their leaves, through a process called transpiration. Our atmosphere can’t hold a lot of water. Evaporated water condenses to form clouds that produce rain or snow on to the Earth’s surface. Overall, water moves from the oceans to the land because more rainfall reaches the land than is evaporated from the land. (See the figure on the next page.) Carbon Carbon is found in the oceans in the form of bicarbon- ate ions (HCO 3 − ), in the atmosphere, in the form of car- bon dioxide, in living organisms, and in fossil fuels (such as coal, oil, and natural gas). Plants remove carbon diox- ide from the atmosphere and convert it to sugars through photosynthesis. The sugar in plants enters the food chain, first reaching herbivores, then carnivores, and finally scavengers and decomposers. All these organ- isms release carbon dioxide back into the atmosphere when they breathe. The oceans contain 500 times more carbon than the atmosphere. Bicarbonate ions (HCO 3 – ) settle to the bottoms of oceans and form sedimentary rocks. Fossil fuels represent the largest reserve of carbon on Earth. Fossil fuels come from the carbon of organisms that had lived millions of years ago. Burning fossil fuels releases energy, which is why these fuels are used to power human contraptions. When fossil fuels burn, car- bon dioxide is released into the atmosphere. Since the Industrial Revolution, people have increased the concentration of carbon dioxide in the atmosphere – EARTH AND SPACE SCIENCE– 240 30% by burning fossil fuels and cutting down forests, which reduce the concentration of carbon dioxide. Car- bon dioxide in the atmosphere can trap solar energy—a process known as the greenhouse effect. By trapping solar energy, carbon dioxide and other greenhouse gases can cause global warming—an increase of temperatures on Earth. In the last 100 years, the temperatures have increased by 1° C. This doesn’t seem like much, but the temperature increase is already creating noticeable cli- mate changes and problems. Many species are migrating to colder areas, and regions that normally have ample rainfall have experienced droughts. Perhaps the most dangerous consequence of global warming is the melting of polar ice. Glaciers worldwide are already melting, and the polar ice caps have begun to break up at the edges. If enough of this ice melts, coastal cities could experience severe flooding. Reducing carbon dioxide concentrations in the atmosphere, either by finding new energy sources or by actively removing the carbon dioxide that forms, is a challenge to today’s scientists. (See the figure on the next page.) Nitrogen The main component of air in the atmosphere is nitro- gen gas (N 2 ). Nitrogen accounts for about 78% of the atmosphere. However, very few organisms can use the form of nitrogen obtained directly from the atmosphere. This is because the bond between two atoms in the nitro- gen gas molecule is tough to break, and only a few bac- teria have enzymes that can make it happen. These bacteria can convert the nitrogen gas into ammonium ions (NH 4 + ). Bacteria that do this are called nitrifying or nitrogen-fixing bacteria. – EARTH AND SPACE SCIENCE– 241 Run-off from glaciers, snow rivers, and lakes Precipitation Precipitation Evaporation and transpiration Ocean Groundwater flow Another source of nitrogen for the non-nitrogen-fixing organisms is lightning. Lightning carries tremendous energy, which is able to cause nitrogen gas to convert to ammonium ions (NH 4 + ) and nitrate ions (NO 3 − )—fixed nitrogen. Plants, animals, and most other organisms can only use fixed nitrogen. Plants obtain fixed nitrogen from soil and use it to synthesize amino acids and proteins. Ani- mals obtain fixed nitrogen by eating plants, or other animals. When they break up proteins, animals lose nitrogen in the form of ammonia (fish), urea (mam- mals), or uric acid (birds, reptiles, and insects). Decom- posers obtain energy from urea and uric acid by converting them back into ammonia, which can be used again by plants. The amount of fixed nitrogen in the soil is low, because bacteria break down most the ammonium ion into another set of molecules (nitrite and nitrate), through a process called nitrification. Other bacteria con- vert the nitrite and nitrate back into nitrogen gas, which is released into the atmosphere. This process is called denitrification. This limited amount of nitrogen has kept organisms in balance for millions of years. However, the growing human population presents a threat to this stability. In order to increase the growth rate of crops, humans man- ufacture and use huge amounts of fertilizer, increasing the amount of nitrogen in the soil. This has disrupted whole ecosystems, since, with extra nitrogen present, some organisms thrive and displace others. In the long run, too much nitrogen decreases the fertility of soil by depriving it of essential minerals, such as calcium. Burning fossil fuels and forests also releases nitrogen. All forms of fixed nitrogen are greenhouse gases that cause global warming. In addition, nitric oxide, a gas released when fossil fuels are burned, can convert into nitric acid, a main component of acid rain. Acid rain destroys habitats. People are already suffering the consequences of the pollution they have caused. Preventing further damage to the ecosystem and fixing the damage that has been done is another challenge for today’s scientists. – EARTH AND SPACE SCIENCE– 242 CO 2 in atmosphere Photosynthesis (land) Photosynthesis (water) Burning fossil fuels Burning forests Respiration (organisms on land and in water) Origin and Evolution of the Earth System Earth Basics Most people know that the Earth is round and revolves around its axis in about 24 hours. It is a part of the solar system, with the sun in its center. Eight other planets and their moons orbit the sun as well. These planets include Mercury and Venus, which are closer to the sun than the Earth is, and Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, which are further away from the sun. It takes about one year for the Earth to complete its orbit around the sun. The rotation of the Earth around its axis causes the change between day and night. The tilt in the Earth’s axis gives rise to seasons. Rocks and Rock Cycles Rocks are made up of one or more minerals, homoge- neous inorganic materials. Three types of rocks are igneous, sedimentary, and metamorphic. Igneous rocks result from cooling of molten rock. If the cooling from molten rock occurred quickly on or near the earth’s sur- face, it is called volcanic igneous rock. If the cooling took place slowly, deep beneath the surface, it is called plutonic igneous rock. Sedimentary rocks are formed in layers in response to pressure on accumulated sediments. Meta- morphic rocks are formed when either igneous or sedi- mentary rocks are under intense heat and pressure deep beneath the earth’s surface. Rock cycle is the transformation of one rock type into another. Molten rock material cools and solidifies either at or below the surface of the earth to form igneous rocks. Weathering and erosion break the rocks down into smaller grains, producing soil. The soil is carried by wind, water, and gravity and is eventually deposited as sediment. The sediments are deposited in layers and become pressed firmly together and cemented or lithi- fied, forming sedimentary rocks. Variations in tempera- ture and pressure can cause chemical and physical changes in igneous and sedimentary rocks to form meta- morphic rocks. When exposed to higher temperatures, metamorphic rocks may be partially melted, resulting in the creation once again of igneous rocks, starting the cycle all over again. Molten material from inside the earth often breaks through the floor of the ocean and flows from fissures where it is cooled by the water, resulting in the formation of igneous rocks. As the molten material flows from the fissure, it forms ridges adjacent to it. Origin of the Earth and the Solar System The sun, the Earth, and the rest of the solar system formed 4.6 billion years ago, according to the solar neb- ula theory. This theory states that the solar system was initially a large cloud of gas and dust, which most likely originated from the explosions of nearby stars. This cloud is named the solar nebula. The sun formed at the central, densest point of the nebula. One argument that supports this hypothesis is that planets closer to the sun are composed of heavier elements, while light, gaseous planets are farthest from the sun. The solar nebula the- ory also states that planets form in conjunction with stars. This component of the theory is supported by the fact that other stars have planets and that the age of moon rocks is comparable to the age of the Earth. Origin and Evolution of the Universe Nobody knows for sure how the universe originated. According to the Big Bang theory, the universe began in a hot, dense state under high pressure between ten and 20 billion years ago. The Big Bang theory also postulates that the universe has been expanding since its origina- tion. The universe is still expanding and cooling. Some data suggest that the rate of expansion of the universe is increasing. Whether the universe will continue to expand forever, eventually reach an equilibrium size, or shrink back into a small, dense, hot mass is unknown. Stars are formed by the gravitational attraction of countless hydrogen and helium molecules. The stars became gravitationally bound to other stars, forming galaxies. The solar system is part of the Milky Way galaxy, which, in addition to the sun, contains about 200 billion other stars. The energy of stars stems from nuclear reactions, mainly the fusion of hydrogen atoms to form helium. Nuclear processes in stars lead to the formation of elements. – EARTH AND SPACE SCIENCE– 243