The Earth
The Earth is the third planet from the Sun. The average distance from Earth to the Sun is 150 million kilometers, or one astronomical unit (AU). Unlike other planets, surface temperatures on Earth allow water to exist as a solid, liquid, and gas. Earth's atmosphere causes most meteors to burn up before they reach the surface, and it protects life-forms from the effects of the Sun's intense radiation. It is currently the only known planet to harbor life.
Physical Properties of Earth
Diameter (pole to pole) 12,714 km
Diameter (equator) 12,756 km
Circumference (pole) 40,008 km
Circumference (equator) 40,075 km
Mass 5.98 X 10(to the 24 power) kg
Average Density 5.52 g/cm(to the 3 power or cubed)
Average Distance to the Sun 149,600,000 km
Period of Rotation (1day) 23 h, 56 min
Period of Revolution (1 year) 365 days, 6 h, 9 min
Diameter (equator) 12,756 km
Circumference (pole) 40,008 km
Circumference (equator) 40,075 km
Mass 5.98 X 10(to the 24 power) kg
Average Density 5.52 g/cm(to the 3 power or cubed)
Average Distance to the Sun 149,600,000 km
Period of Rotation (1day) 23 h, 56 min
Period of Revolution (1 year) 365 days, 6 h, 9 min
Properties of Earth
You awaken at daybreak to catch the Sun "rising" from the dark horizon. Then it begins its daily "journey" from east to west across the sky. Finally the Sun "sinks" out of view as night falls. Is the Sun moving-or you?
It wasn't long ago that people thought Earth was the center of the universe. It was widely believed that the Sun revolved around Earth, which stood still. It is now common knowledge that the Sun only appears to be moving around Earth. Because Earth spins as it revolves around the Sun, it creates the illusion that the Sun is moving across the sky.
Another mistaken idea about Earth concerned its shape. Even as recently as the days of Christopher Columbus, many people believed Earth to be flat. Because of this, they would fall off the edge of the world. How do you know this isn't true? How have scientists determined the true hape of Earth?S
Spherical Shape
A round, three-dimensional object is called a sphere. Its surface is the same distance from its center at all points. Some common examples of spheres are basketballs and tennis balls.
In the late twentieth century, artificial satellites and space probes sent back pictures showing that the Earth is spherical. Much earlier, Aristotle, a Greek astronomer and philosopher who lived around 350 B.C., suspected that Earth was spherical. He observed that Earth cast a curved shadow on the Moon during an eclipse.
In addition to Aristotle, other individuals made observations that indicate Earth's spherical shape. Early sailors, for example, noticed that approaching ships came into view a little at a time.
It wasn't long ago that people thought Earth was the center of the universe. It was widely believed that the Sun revolved around Earth, which stood still. It is now common knowledge that the Sun only appears to be moving around Earth. Because Earth spins as it revolves around the Sun, it creates the illusion that the Sun is moving across the sky.
Another mistaken idea about Earth concerned its shape. Even as recently as the days of Christopher Columbus, many people believed Earth to be flat. Because of this, they would fall off the edge of the world. How do you know this isn't true? How have scientists determined the true hape of Earth?S
Spherical Shape
A round, three-dimensional object is called a sphere. Its surface is the same distance from its center at all points. Some common examples of spheres are basketballs and tennis balls.
In the late twentieth century, artificial satellites and space probes sent back pictures showing that the Earth is spherical. Much earlier, Aristotle, a Greek astronomer and philosopher who lived around 350 B.C., suspected that Earth was spherical. He observed that Earth cast a curved shadow on the Moon during an eclipse.
In addition to Aristotle, other individuals made observations that indicate Earth's spherical shape. Early sailors, for example, noticed that approaching ships came into view a little at a time.
Continental Drift
If you look at a map of Earth's surface, you can see that the edges of some continents look as though they could fit together like a puzzle. Other people also have noticed this fact. For example, Dutch mapmaker Abraham Ortelius noted that fit between the coastlines of South America and Africa more than 400 years ago.
Pangaea
German meteoorologist Alfred Wegener (VEG nur) thought that the fit of the continents wasn't just a coincidence, some time in the past. In a 1912 lecture, he proposed the hypothesis of continental drift. Accouding to the hypothesis of continental drift, continents have moved slowly to their current locations. Wegener suggested that all continents once were connected as one large landmass that broke apart about 200 million years age. He called this large landmass Pangaea (pan JEE uh), which means "all land."
Fossil Clues
Besides the puzzlelike fit of the continents, fossils provided support for continental drift. Fossils of the reptile Mesosaurus have been found in South America and Africa. This swimming reptile lived in freshwater and on land. How could fossils of Mesosaurus be found on land areas separated by a large ocean of salt water? It probably couldn't swim between the continents. Wegner hypothesized that this reptile live on both continents when they were joined.
How could continents drift?
Although Wegener provided evidence to support his hypothesis of continental drift, he couldn't explain how, when, or why these changes took place. The idea suggested that lower-density, continental material somehow had to plow through higher-density, ocean-floor material. The force behind this plowing was thought to be the spin of Earthh on its axis, a notin that was quickly rejected by physicists. Because other scientists could not provide explanations either, Wegener's idea of continental drift was initially rejected. The idea was so radically different at that time that most people clossed their minds to it.
Seafloor Spreading
During the 1940s and 1950s, scientists began using sound waves on moving ships to map large areas of the ocean floor in detail. Sound waves echo off the ocean botton, the longer the sound waves take to return to the ship, the deeper the water is. Using sound waves, researchers discovered an underwater sysem of ridges, or mountains, and valleys like those found on the continents. In the Atlantic, the Pacific, and in other oceans around the world, a system of ridges, called the mid-ocean ridges, is present. These underwater mountain ranges stretch along the center of much of Earth's ocean floor. This discovery raised the curiosity of many scientists. What formed these mid-ocean ridges?
In the early 1960s, Princeton University scientist Harry Hess suggested an explanaaation. His now-famous theory is known as seafloor spreading. Hess proposed that hot, less dense material below Earth's crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions.
As the seafloor spreads apart, magma moves upward and flows from the cracks. It becomes solid as it cools and forms new seafloor. As new seafloor moves away from the mid-ocen ridges, it cools, contracts, and becomes denser. This denser, colder seafloor sinks, helping to form the ridge.
In the early 1960s, Princeton University scientist Harry Hess suggested an explanaaation. His now-famous theory is known as seafloor spreading. Hess proposed that hot, less dense material below Earth's crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions.
As the seafloor spreads apart, magma moves upward and flows from the cracks. It becomes solid as it cools and forms new seafloor. As new seafloor moves away from the mid-ocen ridges, it cools, contracts, and becomes denser. This denser, colder seafloor sinks, helping to form the ridge.
Theory of Plate Tectonics
The idea of seafloor spreading showed that more than just continents were moving, as Wegener had thought. It was now clear to scientists that sections of the seafloor and continents move in relation to one another.
Plate Movements
In the 1960s, scientists developed a new theory that combined continental drift and seafloor spreading. According to the theory of plate tectonics, Earth's crust and part of the upper mantle are broken into sections. These sections, called plates, move on a plasticlike layer of the mantle. The plates can be thought of as rafts that float and move on this layer.
Composition of Earth's Plates
Plates are made of the crust and a part of the upper mantle. These two parts combined are the lithosphere (LIH thuh sfihr). This rigid layer is about 100 km thick and generally is less dense than material underneath. The plasticlike layer below the lithosphere is called the asthenosphere (as THE nuh sfihr). The rigid plates of the lithosphere float and move around on the asthenosphere.
Plate Movements
In the 1960s, scientists developed a new theory that combined continental drift and seafloor spreading. According to the theory of plate tectonics, Earth's crust and part of the upper mantle are broken into sections. These sections, called plates, move on a plasticlike layer of the mantle. The plates can be thought of as rafts that float and move on this layer.
Composition of Earth's Plates
Plates are made of the crust and a part of the upper mantle. These two parts combined are the lithosphere (LIH thuh sfihr). This rigid layer is about 100 km thick and generally is less dense than material underneath. The plasticlike layer below the lithosphere is called the asthenosphere (as THE nuh sfihr). The rigid plates of the lithosphere float and move around on the asthenosphere.
Plate Boundaries
When plates move, they can interact in several ways. They can move toward each other and converge, or collide. They also can pull apart of side alongside one another. When the plates interact, the result of their movement is seen at the plate boundaries.
Movement along any plate boundary means that changes must happen at other boundaries. What is happening to the Atlantic Ocean floor between the North American and African Plates? Compare this with what is happening along the western margin of South America.
Plates Moving Apart
The boundary between two plates that are moving apart is called a divergent boundary. In the Atlantic Ocean, the North American Plate is moving away from the Eurasian and the African Plates. That divergent boundary is called the Mid-Atlantic Ridge. The Great Rift Valley in eastern Africa might become a divergent plate boundary. There, a valley has formed where a continental plate is being pulled apart.
Plates Moving Together
Movement along any plate boundary means that changes must happen at other boundaries. What is happening to the Atlantic Ocean floor between the North American and African Plates? Compare this with what is happening along the western margin of South America.
Plates Moving Apart
The boundary between two plates that are moving apart is called a divergent boundary. In the Atlantic Ocean, the North American Plate is moving away from the Eurasian and the African Plates. That divergent boundary is called the Mid-Atlantic Ridge. The Great Rift Valley in eastern Africa might become a divergent plate boundary. There, a valley has formed where a continental plate is being pulled apart.
Plates Moving Together