When the star that we call our Sun condensed out of a swirling cloud of hot interstellar gas, some material remained in orbit around the newly forming star. That orbiting material consisted of hot chunks of rock and frozen liquids called "planetisimals". From these were gradually formed several (nine that we know of) planets including Earth. The planets - including Earth - still move in relatively stable orbits around the sun.

Earth not only orbits the sun but spins on its own axis as it does so. At the equator the Earth is spinning on its axis at a speed of about 1,038 miles (1,670 kilometres) an hour. Earth's spin or rotation is a relic of its origin as a hot, spinning mass when it was first formed.

Earth science or geology is the science of the Earth. It is an organized study and body of knowledge about mountains, plains and oceans; about the history of life on Earth; and about the succession of physical events that have been (or are still) part of this history.

1. How was the planet Earth formed?

Chunks of rock and frozen liquids in orbit around the newly forming Sun converged and melted together to form the planets. Each time a planetisimal (see the Introduction) hit the rotating chunk of material that was to become Earth it melted in to the existing mass. The mass was hot and the metals were molten so the heavy metals like iron sank toward the center of the mass while lighter metals like silicon floated near the surface.

2. Why does the Earth have a concentric-layered structure?

During the time when Earth was forming and was in a molten state, heavy materials like iron and nickel sank to the center of the Earth's mass and lighter materials floated to the surface of the mass. The very center of what we call the core, is solid in the middle because of the increased pressure in that region. The solid, central part of the core has a liquid (molten) layer surrounding it. Outside the core, the largest mass of planet Earth is called the mantle and this is mostly solid rock. The very outermost layer of our planet is about 30 miles thick, is made of lighter rocks, and is called the crust.

3. How warm is the center of Earth?

The further you go toward the center of Earth the hotter it gets. The first hundred feet below the surface is generally quite uniformly cool. However below the first hundred feet, the rocks are warmer by several degrees celsius for each 1,000 feet of depth. Very deep mines, such as deep gold mines for example, can have temperatures of as much as 130 degrees celsius.

4. How do we know what the center of Earth is like?

No-one has ever penetrated Earth's mantle to reach the core to investigate what it is made of so much of what we "know" is theory based on intelligent deduction. Some deductions are based on good empirical (based on observation or experiment) data obtained by analyzing the seismic waves that are produced when earthquakes happen.

Seismic waves that travel through the deep interior of the Earth can sometimes be detected by seismographs in different parts of the world. Scientists observe where these waves reach the surface and how long it takes for them to get there. Comparing these observations with controlled experiments on the behavior of waves passing through different materials, scientists can then construct good theories about what materials deep in the Earth the seismic waves passed through on their way to the surface.

5. How strong is an earthquake?

Charles Richter (1900-1985), an American seismologist, created a logarithmic scale that expresses the strength of earthquake movements. An earthquake that is only detectable by very sensitive seismographic machines is strength 2.0 to 2.9; the slight vibration and swinging of hanging objects is indication of a strength 3.0 to 3.9 earthquake. In a level 7.0 to 7.9 earthquake, buildings collapse, cracks appear in the ground and landslides occur.
The Richter magnitude scale is a logarithmic scale so a level 4.0 earthquake is ten times stronger than a level 3.0. (And a level 5.0 quake is 100 times stronger than a level 3.0).
The number given to the strongest earthquake described by the Richter scale is 8.9 (An 8.9 quake is more than 79 million times stronger than a 1.0 quake!)

6. How did Earth's atmosphere develop?

When Earth was first formed it was a solid ball of molten rock with no atmosphere. As Earth began to cool and the crust and mantle began to harden, material from the inner regions would sometimes erupt through the surface bringing hot gases. Scientists theorize (no-one really knows because no one was there!) that these gases were methane, ammonia, carbon dioxide and water (steam). The water eventually condensed to form oceans but almost certainly there was at first no oxygen or nitrogen in Earth's atmosphere. It is believed that the atmosphere (air) that we now breathe was formed only after the development of the first living things, blue-green algae (now generally classified as blue-green bacteria), that happened about two billion years ago. These simple organisms used carbon dioxide during photosynthesis and gave off oxygen as a biproduct of the chemical process.

7. Where did the Moon come from?

The best, simple answer to this question is that we dont know. There are several theories: a) perhaps the Moon was torn out of Earth's crust and mantle at some time in the distant past (billions of years ago); b) perhaps the Moon was created out of planetisimals (see the Introduction and Day 1) and somehow got caught by Earth's gravitational influence and went into orbit around Earth as well as around the Sun; c) perhaps a giant meteorite hit Earth soon after it was formed and the material thrown out into space as a result of the impact coalesced into a small mass that became the Moon.

8. How can we tell the age of the rocks in Earth's crust?

There are several methods: a) We can make educated guesses - in general, the deeper the rock, the older it is. Using this generalization geologists have created a relative time scale that describes what is older or younger but does not give a specific ages in years except in the most general terms; b) It is also possible by comparing the fossils in two different rock layers to tell that they are about the same age if the same types of fossils are found in each; c) Radiometric dating such as the carbon-14 method can provide quite accurate specific ages of the remains of living things.

9. How old is Earth?

Earth's oldest rocks are believed to be about 3.8 or 3.9 billion years old. Because rocks formed quite slowly from the Earth's original molten mass, clearly Earth is older than its oldest rocks. Scientists believe that Earth is about 4.6 billion years old. (Note, in this and other explanations the US billion is the number used. In the US one billion is 1000 million.)

10. What is Plate Tectonics?

This is a relatively modern and widely accepted theory concerning the structure of Earth's crust. Plate tectonics proposes that the Earth's crust is made up of six major plates and several smaller ones. The plates consist of the crust including a thinnish layer of the upper mantle. These plates have moved throughout geologic time converting the very early land mass known as Pangaea into the present-day positions of continents. The North American plate extends from the middle of the Atlantic ocean to the West Coast of the United States ending at California's San Andreas Fault - an active earthquake zone. The Pacific plate is entirely under the Pacific Ocean and has no continental areas. Plate margins, where the six main plates roughly coincide, are generally zones of seismic or volcanic activity.

11. How have Earth's mountains been formed?

Mountains are formed in several different ways. For example the Appalachian Mountains on the east coast of the United States are fold mountains that were pushed up as ridges on Earth's crust when the Eurasian and American tectonic plates collided. These mountains are perhaps several hundred million years old.
The Sierra Nevada mountains of California were formed through a fault-block process in which large chunks of rock sank leaving other chunks standing.
The Black Hills of South Dakota are dome mountains that were pushed up by a hot spot in the mantle of that region.
The Rocky Mountains of the Western United States are thought to be quite young, perhaps only about 65 million years old. They are being studied by geologists to determine the method or methods by which they were formed.

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 sculptors 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.

19. What mineral substances are MOST rocks made of?
Most rock-forming minerals are silicates (oxides of the element silica). About 90 percent of Earth's crust is composed of silicate minerals. Quartz is the most common form of silicon dioxide. It makes up most of the sand at the seashore, occurs as a rock in the form of sandstone, and quartzite, and is an important constituent of many other rocks such as granite and gneiss.
There are many varieties of quartz and those that occur in distinct crystals are named chiefly according to their color, for example: rock crystal is clear and colorless; smoky quartz has probably darkened as a result of exposure to radioactivity; amethyst crystals are purple; citrine is a light yellow crystal; rose quartz is pale to deep pink.
Other silicate rock forming minerals, such as the feldspars (which like quartz are found in most igneous rocks like granite and gneiss) are silicates of aluminum with potassium, sodium, and calcium.

20. Are some minerals harder than others?
Yes. A German mineralogist, Friedrick Mohs (1773-1839) developed a very practical mineral scale of relative hardness. He listed ten minerals ranging from 1. talc (the softest) to 10, diamond (the hardest).
On Mohs' scale of hardness a mineral with a value up to 2.5 can be scratched with a fingernail, up to 4 can be scratched with a coin, and up to 6 by a knife.
The minerals used by Mohs to establish his scale were: 1) talc; 2) gypsum; 3) calcite; 4) fluorite; 5) apatite; 6) orthoclase; 7) quartz; 8) topaz; 9) corundum; 10) diamond.

21. What about ruby and emerald. Are these minerals?
Yes. Emerald (beryl - beryllium aluminum silicate), ruby (corundum - aluminum oxide), sapphire (corundum), opal (silica) and diamond (carbon) are all minerals and have been highly valued by people over the ages because of their rarity and their beauty. Diamond is highly valued by modern societies but historically has been valued less than the more colorful minerals. Most natural diamonds come from Africa but they are found all over the world. Small, synthetic diamonds can now be made and are used in industry where hard cutting edges are needed.
True emeralds are a deep green and a large, flawless emerald is more valuable than a diamond. The finest rubies come from Burma, and most opals come from Australia.

22. How do scientists classify/organize minerals?
Minerals are grouped/organized/classified by scientists in several different ways depending upon the properties the scientists want to emphasize in different contexts. For example, minerals whose molecules contain the metal element iron (such as hematite, magnetite, siderite) can be grouped together as can be the zinc minerals such as sphalerite, smithsonite, willemite.
However, a mineralogist might prefer a classification based on the type of compound and its properties and so would group minerals together that have similar chemical composition even though they contain different elements. For example a carbonates group would include both siderite (iron carbonate) and smithsonite (zinc carbonate) which would not be grouped together if they were being classified according to the metal element in the molecule (iron and zinc are both metal elements.)

23. What are Native Elements?
These are chemical elements that are found in nature in their native (uncombined) state. Most elements do not exist in nature as elements but instead are always found as compounds - each molecule being a combination of several elements. There are three groups of elements that are found in nature in their native state: 1) a few metals such as gold, silver, copper, platinum and iron; 2) a few semi-metals such as arsenic, antimony, and bismuth; and 3) a few non metals such as carbon, and sulphur.

24. What is ore?
An ore is a mineral or rock from which a metal and certain other substances can be extracted for commercial purposes. Sometimes a metal is present in its native form; gold and platinum are found this way. Almost always however the metals occur as oxides, sulphides, sulphates, silicates etc. and the metal has to be extracted from its ore (separated from the other chemical elements) by chemical means.
The branch of applied science that is concerned with the production of metals from their ores is metallurgy.

25. Which is the most abundant metallic element in Earth's crust?
Aluminum - called aluminium in some parts of the world.
While aluminum is the most abundant metallic element in the Earth's crust it is not the most abundant 'element'. Oxygen is the most abundant element followed by silicon and then aluminum.
Most of the aluminum is physically and chemically bound inside rocks that comprise Earth's crust. However in some areas (largely under tropical conditions in Australia, Guinea, Jamaica, Russia, Brazil, and Surinam) those rocks exposed at the surface have weathered and broken down into bauxite which is the chief ore of aluminum. Bauxite is a claylike amorphous substance from which aluminum can be extracted.
Aluminum is a very light, very strong metal and is used in circumstances where lightness is as important as strength.

26. Are minerals much in demand for modern technology?
Yes. There are at least 30 minerals that are particularly important in the U.S. for industrial/technological needs. The metals most in demand are: aluminum, copper, gold, silver, iron, tin, platinum, chromium, nicket, lead and zinc. The non-metals most in demand are: diamond, salt, limestone, sulphur (sulfur) and asbestos.
Most of these exist almost everywhere but can not always be used. This is sometimes because there are no appropriate methods of separating the desired mineral from the other minerals in the rock. In other cases the amounts present are too small.
When minerals are present in situations where they can be extracted at a profit, they are called ore deposits.

27. How are minerals extracted from their ore deposits?
In several different ways. For example, when tiny grains of gold are spread through a gravel deposit the gravel is poured onto a table that is coated with mercury. When the table is vibrated the gold grains work their way to the table surface and combine with the mercury. This method works because gold has a strong tendency to combine with mercury. The gold of course then has to be separated from the mercury.

A different method is used to separate sulphide (sulfide) minerals from others in order to obtain their sulphur. In this case the conglomeration of minerals might be finely ground up and then vogorously mixed in a tank of water through which air is being bubbled. In this process, which is called flotation, the sulphide minerals cling to the bubbles and are collected from the froth that spills over the top of the tank.

28. Do minerals in rocks ever pose a health hazard?
Sometimes. For example serpentine is a common rock in California where it occurs abundantly in coastal mountain ranges; and there is a commercially important belt of serpentine in Quebec, Canada. Serpentine (magnesium silicate) is a metamorphic rock which contains soft, silky fibres of chrysotile asbestos. Asbestos fibres can withstand fire and insulate against heat and they are used in automobile brake linings, heat resistant tapes, cloth and insulating materials.

A health hazard can occur during the asbestos extraction process and aslo when the serpentine rock is crushed for use in road construction. Asbestos fibres in the air are breathed into the lungs. This is a problem because asbestos fibres can work their way into tissues and cells. This can cause asbestosis which is a chronic lung disease. Certain types of asbestos are carcinogenic in that they cause lung and other cancers.

29. What is geologic time?
Geologic time is a time scale of units of relative (not absolute) time in chronological order from the time of Earth's early beginnings to the present day.

The time periods are called eras, periods and epochs. We call the earliest era the Paleozoic Era and it includes several periods, the earliest of which we call the Cambrian Period. The most recent era is called the Cenozoic Era which includes the time following the extinction of the dinosaurs and the much more recent time of the appearance of modern man.

Geologists interested in this aspect of Earth Science have the intriguing task of determining which rocks are older or younger than others - and by how much! - and determining approximate dates for geologic events such as, for example, the various Ice Ages.

30. When was modern geology born?
Many ancient scholars observed and developed theories about the development and structure of Earth, and over the centuries the interest and body of knowledge increased but it was not until the 18th century that geological science really matured.

The father of modern geology is generally agreed to have been James Hutton (1726-1797), a Scottish medical man and farmer who was very interested in the history of the Earth and in the processes that modify the Earth's surface.
Before Hutton's time most geologists believed Earth was only a few thousand years old. They believed that Earth's physical features were the result of catastrophic events.
Hutton's great contribution was the formulation of the theory that natural agents now at work on and within the Earth have operated with general uniformity throughout immensely long periods of time. This is an underlying tenet of modern geology.

On these pages, one billion is the US billion and equals 1,000 million.
(In the UK one billion is one million million - a much larger number.)
101 Things Everyone Should Know About Science by James Trefil; published 1992, Doubleday, NY.
Encyclopedia Britanicca
Physical Geology by Leet and Judson, publishd by Prentice Hall

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