Mapping Symbols

          A large scale map can show a relatively high level of detail, whereas a small scale map contains more information in relation to its size. The scale of a map is expressed as the ratio between the size of the map and the size of the corresponding area on the ground. This can be expressed as a simple ratio, for example 1:100, where one unit on a map is equal to 100 of the same units on the ground.  Alternatively, the ratio may be expressed using different units, for example 1cm: 1km, in which one centimeter on the map represents one kilometer on the ground. Many maps also show scale in the form of a scale bar which can be used to calculate any distance on the map. The scale dictates the level of detail shown by the map. Because maps are smaller than the areas they represent, symbols are used to indicate the features on the ground. Many symbols do not resemble the things the things they represent and so they are often explained in a key.

Origin of the Earth

Earth science examines our planet’s structure and the geological processes at work within it and on its surface. It considers rock formation and composition, and the geological evolution of the earth. Land sculpting agents, weather and mapping conclude the study.

Planet earth is a small, dense, rocky planet that is unusual in several aspects. It has free oxygen in its atmosphere; water is present in three phases- ice, liquid and vapor, and surface temperatures are maintained in a relatively narrow range. Life forms play a key role in maintain the balance of conditions on Earth. The interior is layered. Radioactive decay in the interior releases heat and which slowly rises to the surface. Convection currents within the semi solid mantle produce volcanic hot spots. The currents create new ocean crust and push the continents around like floating rafts. About 4700 million years the young sun began to shine, blowing the remaining material of the solar nebular into a ring of dust, gas and ice. Rocky silicates condensed out nearest the sun; gases and ices were driven further.

Gravity and impacts helped the rocky material clump together. Radio activity and the heat of impacts melted the larger proto planets so that iron could sink and form a core; So that the iron could sink and form a core. Depression in the young earth’s surface that became the ocean basins may have been created by impacting comets.  The first atmosphere was composed of gases expelled by volcanoes. A Mars sized object hitting the planet may have produced moon.

The Earth’s Magnetism

The earth as a strong bipolar magnetic field, as if there was a large bar magnet in its core. In fact, electrical currents in the churning, liquid outer core produce a self sustaining dynamo. The influence of the magnetic field extends beyond the planet. The field meets charged particles streaming from the Sun in a shock wave about 50,000 Km above the Earth and slows hem from 400 to 250 Km per second. The solar wind, as these particles are known sweeps the Earth’s magnetic field into a six million Km tail. Solar flares on the surface of the Sun result in magnetic storms on Earth to days later. There are belts of solar particles trapped in the magnetic field about 3,000 and 25,000Km above the equator, called the Van Allen belts. Aurorae occur where charged particles trapped by the Earth’s magnetic field stream towards the poles.

The earth’s magnetic axis is 11 degree west of the North Pole and is slowly wandering. Eddy currents in the core and magnetism trapped in rocks produce local anomalies. In the geological past, the magnetic field have reversed many times. Between 118 and 83 million years ago it did not reverse at all. For the last few million years it has reversed about once every 220,000 years, revealed by the magnetic alignment in volcanic rocks. We are overdue for another reversal.