12. How much of Earth's surface is covered by water or ice?
Seventy-five percent of Earth's surface is covered by oceans
(and most of the water is salty). About five percent of Earth's water is frozen solid and exists as glaciers covering about ten percent of the land surface. The glaciers are are generally located in high mountain ranges but there are huge ice sheets in Greenland and Antarctica.
Ten or twenty thousand years ago a large glacier or ice sheet covered most of North America.
13. What is a desert?
A desert is any area of the world that receives less than one inch of annual rainfall. Quite often deserts exist in the rain-shadow of great mountain ranges. The Mojave Desert of California is one example and the Sahara Desert of Africa is another.
We often equate deserts with sand dunes and of course many desert areas are sandy but deserts more commonly consist of bare, arid ground with small patches of poor vegetation.
Climates as well as landforms have changed over the eras of geologic time and there is evidence that the Sahara Desert was covered by a glacier several hundred million years ago.
14. All rocks are not the same.
There are three main types of rocks - igneous (fire-formed),
sedimentary (water-deposited), and metamorphic (changed). This classification is based on the way in which the rocks were formed rather than on their structure or appearance.
About 75 percent of rocks on land surfaces of earth are sedimentary, most of the rest is igneous and very little is metamorphic. On the other hand, most of the rock of deep ocean floors is igneous.
15. Are all igneous rocks the same?
No. Igneous rocks are formed by the cooling and crystallization of magma from beneath Earth's mantle.
Igneous rocks that form slowly at great depth are called plutonic and they have rather coarse structure like granite, diorite or gabbro.
Igneous rock that forms from the magma extruded onto Earth's surface by volcanic eruption is called volcanic and because it cools quite rapidly it is fine-grained like basalt and sometimes glassy like obsidian.
Some igneous rock is formed at shallower depths than the plutonic rocks and is called hypabyssal. Such rock is medium grained like diorite.
Igneous rocks can also be classified chemically according to their silica content. Those with high (above 66 percent) silica content are called acidic, and those with low (less than 45 percent) silica content are called ultrabasic .
16. How are sedimentary rocks formed?
This sort of rock is formed from sediments that have accumulated and over long periods of time have consolidated into rock. The sediments are often fragments that have been worn away from pre-existing rocks by a mechanical process such as the abraiding action of wind, water or ice. Examples of sedimentary rock formed from such fragments are conglomerates, sandstones, and shales.
Sedimentary rocks are also formed from sediments (precipitates) that are the result of chemical action. Examples are evaporites and sedimentary iron ores.
Organic sedimentary rocks are formed from the remains of plants and animals. Examples are coal and limestones.
17. How are metamorphic rocks formed?
This type of rock is rock that has been changed from its original form by high temperatures, pressures, or chemical action in such a way that its structure is changed. For example, a fine-grained limestone, subjected to increased pressure and temperature, can change over time into marble. Two other metamorphic rocks are slate and quartzite. Slate is a dark, smooth rock that breaks into smooth flat sheets of rock - it was once shale; quartzite is usually yellowish brown and one of the toughest rocks - was once sandstone
Marble can be many colors depending upon the types of
minerals that were in the particular sedimentary limestone from which it was made. Marble is often a beautiful rock and has been a favorite material for sculptorrs and builders over the ages.
18. What are rocks made of?
All rocks are made of chemical compounds called minerals. Most minerals are crystalline and there are several thousand know minerals each containing specific elements and having a particular molecular structure. Quartz is a mineral and is a common constituent of granite (an igneous rock) where it exists as small crystalline pieces fused together with other minerals such as feldspar and mica. Quartz, which is silicon dioxide (SiO2). can be also be found as large, colorless, transparent, six-sided crystals.
Even rocks, like chalk, that are made up of the remains of once-living organisms, are made of minerals because the animal remains are fossilized and consist largely of calcium carbonate (CaC03).
By the way, although chalk rock will make the familiar white marks on a blackboard, commercial blackboard chalk is made of gypsum which is calcium sulphate.