Stone icicles

Many a cave is filled with what look like giant stone icicles. Some of these "icicles" hang down from the cave's roof. Others stick up from the floor. They are often as thick as tree trunks. Sometimes, the ones on the ceiling and the ones on the floor meet to form thick pillars. These strange-looking stone "icicles" are usually found in limestone caves.

The "icicles" that hang from the ceiling are called stalactites. They are made by water trickling through cracks in a cave's limestone roof. The water carries tiny bits of the mineral called calcite with it. As some of the water dries, it leaves bits of calcite stuck to the ceiling. Each drop of water adds more calcite. Slowly, as more bits of calcite are added, the stalactite grows longer and longer.

Some of the water drips to the cave floor. This water also has bits of calcite in it. As the water dries up, bits of calcite are left on the floor. Slowly, the calcite piles grow higher and higher. These "icicles" sticking up from the floor of a cave are called stalagmites.

Often, water drips off a stalactite onto the top of a stalagmite below it. Slowly, the two grow toward one another until they finally join.

Many people get stalactites and stalagmite mixed up. They can't remember which hangs from the ceiling and which sticks up from the ground. But there is an easy way to remember. Just remember that ceiling begin with c, and stalactite has a c in it. And ground begins with g, and stalagmite has a g in it. Then you'll always know that stalactites hang from the ceiling and stalagmites stick up from the ground.

Holes in rock

A den for bears and a bedroom for bats. A home for prehistoric people. A dark place of gloom and mystery. A sparkling wonderland of strangely shaped rocks. That's a cave !

A cave is simply a hole in the earth. It may be a small hole, not much bigger than a room in a house. It may be a place of long, twisting tunnels and huge caverns. It may be high up on a mountainside or it may be in the ground.

Most caves are in the kind of rock called limestone. These limestone caves are made by water. Water seeps through cracks in the rock. It trickles downward, carrying dissolved limestone with it. Slowly, the cracks grow wider as more and more limestone is carried away. After many thousands of years a cave may have a great many winding passages and broad rooms, all made by the slow trickle of water.

Layer cakes of rock

The upper part of the earth's rocky crust is like a layer cake. It is made up of many layers of different kinds of rock, one on top of the other. These layers were built up slowly, one after the other. Here is how this might have happened in one place.

At first, there was only the top of the bare rock crust. Volcanoes erupted, pouring out ash and lava that hardened into a layer of black rock called basalt.

Earthquakes caused this part of the land to sink. Ocean water flowed in to form a smaller body of water called a sea. Rivers dumped tons of sand into the sea for thousands of years. The sand sank down and formed a thick layer on top of the basalt.

Snails, clams, and other little shelled animals moved in from the ocean to live in the sea. When they died, their bodies sank down onto the sand. Their soft parts rotted away and only the shells were left. For millions of years, these shells piled up until they formed a thick layer.

The weight of the shells and the water squeezed the sand together until it became a layer of the rock called sandstone. Squeezed together by their own weight and the weight of the water, the shells were crushed into powder. Minerals in the water glued the powder together. After millions of years, the powdered shells were a layer of the kind of rock called limestone. The layer of limestone was on top of the layer of sandstone, which was on top of the layer of basalt.

This kind of building up of layers of rock is still going on. The layer of mud that now lies at the bottom of a sea will some day be a layer of rock !

Pictures in stone

Tucked away in the earth's rocks there are often "pictures" from the past.

Many times, in ages past, animals walked through mud and left footprints. Slowly, over thousands of years, the mud hardened into rock. The footprints were preserved forever. We have found rocks in which there are footprints of dinosaurs and other animals. We have even found rocks with the footprints of prehistoric people.

Leaves of ancient plants and feathers from the first kinds of birds also fell into the mud. They, too, left prints that were preserved when the mud hardened into rock.

Ancient snails, clams, and other shelled sea creatures died and were covered up with mud. The mud hardened around them. When their soft parts rotted away, and their shells dissolved, a hole was left in the rock - a hole that exact shape of the dead animal. This hole was like a mold. Slowly, it filled up with minerals that hardened into stone. The stone was exact shape the dead animal had been.

These pictures and shapes in stone are called fossils. They tell us about the plants and animals of millions of years ago

Bones in rock

A giant dinosaur, with jaws shaped like the bill of a duck, plodded along the shore of a lake in search of food. Sighting a large bunch of cattails growing in shallow water, it waded out toward them.

Suddenly, the big dinosaur stepped into a wide, deep hole filled with soft, watery mud. At once, the animal began to sink.

It thrashed about wildly, trying to find solid ground under its feet. The weight of its own body dragged it down. The mud rose to its chest, then to its neck. Slowly, its head went under. Mud filled its mouth and nostrils and it died. This was eighty million years ago.

The dinosaur's body sank to the bottom of the hole. Slowly, the soft parts of the dinosaur rotted away. Only its bones were left.

Over many years, the mud around the bones was packed tightly together. In time, it became clay. Over many more years, the clay turned to rock. And, as still more years passed, the lake dried up. The rock with the dinosaur bones in it was now the side of a cliff.

During all those years, water filled with dissolved minerals often got into the bones. Slowly, all the hollow places in the bones were filled with dissolved minerals that then hardened.

As millions of years passed, wind and rain slowly wore away the side of the cliff. Finally, so much rock was worn away that some of the dinosaur bones, preserved in the rock all those years, were sticking out.

One day, eighty million years after the dinosaur had died, a group of scientists saw the bones sticking out of the cliff. With pickaxes and crowbars, the scientists removed the bones from the rock.

The scientists packed the bones carefully and sent them to a museum. At the museum, the bones were put together. People could then see the skeleton of the giant creature that had lived so long ago.

Most of the skeletons of prehistoric animals that you see in museums were preserved in rock in this way. Such preserved bones are called fossils. Sometimes, all of the original material is replaced with minerals. Then the fossil is said to be petrified, or turned to stone.

The magic metal

Have you ever watched someone wrap a sandwich or cover a bowl of food with what looks like a sheet of shining, silver paper ? It isn't paper at all - it's a thin sheet of the wonderful metal called aluminum.

Aluminum is a light, silvery metal. It is never found all by itself, as a pure metal. It is always mixed with other minerals. Most aluminum comes from a rock that looks like a bunch of brown pebbles mixed into a piece of gray concrete. This rock is called bauxite.

Aluminum is often called the "magic" metal. It is light, very strong, and won't rust. It can be stretched and rolled into almost any shape. So aluminum is used to make everything from airplanes to pots and pans to chewing-gum wrappers.

This metal has two names. People in the United States call it aluminum (uh Loo muh nuhm). But people in Great Britain, Canada, Australia, and many other countries call it aluminium (al yuh MIHN ee uhm).

A rock we eat !

If someone at the dinner table said, "Pass the rocks, please," what would you give him ?

Salt, of course !

The salt we use to make food taste better is a mineral called halite. Big lumps of halite are found in the earth's crust. The halite is dug out in chunks and crushed up small enough to fit through the hole in a saltshaker. Halite forms in square crystals. No matter how small it crushed, it nearly always breaks into the shape of a cube !

Sometimes, a hole is dug down to salt that is in the earth. Water is forced down into the salt, then pumped back up with salt in it. The water is then heated. When it dries up, crystals of pure salt are left. Salt is removed from seawater in much the same way.

We need salt to keep healthy, and, of course, salt makes our food taste better. So salt has always been very important to people. In ancient times, salt was so precious and hard to get that it was used as money ! The soldiers of ancient Rome were given salt as part of their pay. This part of their pay was called the salarium. Our word salary, which is another word for pay, comes from the Latin word salarium. A man who was not a very good soldier was "not worth his salt." We still say this today about people who don't do a good job for the money they are paid.

Black gold

The oil that keep your bicycle from squeaking and the gasoline that keeps your parents' car running both come from deep in the earth's crust. Both began as a thick black liquid found in big pools between layers of rock.

Oil is not a mineral. Like coal, it was made from things that were once alive. Oil is found in places where there were seas and oceans millions of years ago. Tiny plants and animals lived along the shores of these seas, just as they do today. When these creatures died, their bodies drifted down to the sea bottom. Over millions of years, billions of these tiny dead creatures formed deep piles.

Sand and mud settled over the piles of dead creatures and formed thick layers. Slowly, these layers were squeezed together by their own weight and the weight of the water pressing down on them. They were squeezed so hard they became layers of rock. Scientists think that the heat and weight of the rock, pressing on the piles of dead, rotting plants and animals, turned them into drops of oil.

Today, people drill down through many layers of rock to find pools of oil. From oil we get gasoline to run the engines of cars, trucks, and tractors. Many kinds of plastics and other things are made from oil. And it is used to heat houses, factories, and schools.

Because oil was first found seeping up between rocks, it was called petroleum - from two Latin words meaning "rock oil". Today, petroleum is usually just called oil. But it is so important, and worth so much, it has been nicknamed "black gold".

Buried sunshine

Nearly all rocks are made of minerals - things that are not alive and never have been. But there is one kind of rock that is not made of minerals. It is coal, a black shiny rock made of - green plants !

Millions of years ago much of the world was hot and swampy. Strange-looking trees and giant ferns covered most of the land. When these big plants died, they toppled over and sank into the mud. Then they were buried under other plants that fell on top of them. In time, there was a thick layer of dead plants. As the plants rotted, they formed a thick, lumpy brown stuff called peat.

Parts of the land began to sink. Water poured in, bringing mud and sand that covered the peat. Tons of water, mud, and sand pressed down on the peat, squeezing it tightly together. All this weight and squeezing turned the peat into coal.

After millions of years, the earth changed. Places that had been covered with water became dry land. Mud and sand had turned to rock. The great masses of coal were buried deep in the rock.

Today, miners dig coal out of the earth. Coal is burned to heat houses, schools, and other buildings. Burning coal is used to make steam that turns the big machines that make electricity for heat, light, and power. When we burn coal, we are really burning plants that grew millions of years ago. This is why coal is sometimes called "buried sunshine".

Minerals we make into cloth

Long ago, after a dinner party, a French king surprised his friends by throwing the dirty tablecloth into the fire. They were even more surprised when the tablecloth did not burn. Instead of burning, it was cleaned !

It seemed like magic, but it wasn't. The tablecloth was made of asbestos. And asbestos is a mineral that will not burn.

Asbestos isn't just one mineral. It comes from several different kinds of minerals that can be split up into threads. Most asbestos is found in layers between rock. Its silky threads look like white hair growing on the rock.

Cloth made from asbestor thread is used in many ways that help keep fires from starting. Pipes that carry hot steam, the insides of stoves and furnaces, and other things that get dangerously hot are often covered with asbestos. It is also used to protect things from fire. Firemen can go right into burning buildings when wearing suits made of asbestos cloth.

The money minerals

Gold is a bright-yellow metal that looks like sunlight turned to stone. Silver is a pale-gray metal that looks like frozen moonlight. These two minerals are so beautiful and hard to find that people have used them for jewelry and for money for thousands of years.

Gold is found in many places. Usually it is mixed in with other minerals. But big nuggets and tiny grains of gold have been found in rivers. They are brought there when streams wash them out of rocks in the mountains. Pieces of gold have also been found lying on top of the ground. But most gold is now dug out of mines.

Gold is heavy, but it so soft it can be hammered into sheets thinner than tissue paper. Several minerals look so much like gold that people are often fooled by them. But these other minerals are usually much harder than gold.

Silver is also found in the earth. Chunks of silver often look like bunches of twisted wire or feathers. Silver is not shiny like gold - it is dull gray or black until it is polished. It, too, is very heavy and soft. When gold or silver is made into jewelry, other metals are added to make the gold or silver hard.

Precious stones

They shine, they sparkle, they flash and shimmer. The crowns of kings and emperors were covered with them, and wealthy people have always worn necklaces, rings, and other jewelry made of them. They are only stones, but they are so beautiful and hard to find that they have been symbols of wealth and power for thousand of years. They are the "precious stones" - diamonds, rubies, emeralds, sapphires, and opals.

Diamonds come from deep in the earth. They are buried in rock that is in or near dead volcanoes. Most diamonds are shaped like two pyramids with their bottoms stuck together. When they are taken from the ground they are dull and grayish. It is only after they are cut dan polished that they sparkle and flash.

A diamond is the hardest of all things that come from the earth. It can even cut rocks. The only thing that will scratch a diamond is another diamond. Most diamonds are used to make cutting and grinding tools. Only the biggest, most perfect diamonds become jewels.

Corundum is a stone that's like a cube with two corners cut off, so that it has six flat sides plus a top and bottom, instead of four sides. It is a very common mineral that is usually found stuck in other kinds of rock. But sometimes a piece of corundum has a tiny bit of another mineral, either titanium or cobalt, mixed in it - and then it is clear, blue stone called a sapphire. And, sometimes a tiny bit of a mineral called chromium is mixed into a piece of corundum. That makes it a dark red ruby.

Beryl is a rock that has eight or more flat sides. It is usually found in large chunks of granite or, sometimes, limestone. When a tiny bit of chromium is mixed into a piece of beryl, it makes a beautiful, deep-green stone called an emerald.

One of the most beautiful of all precious stones is mostly just a mixture of sand, water, and the gas called axygen. This stone, the opal, is often found in large patches near the top of the earth's crust. Opals come in all colors of the rainbow, plus black and brown. When they are cut and polished, they sparkle with many colors and glow as if there is fire locked inside them !

Precious stones are called gemstones. Diamonds, emeralds, rubies, sapphires, and opals are the most precious gemstones, but there are others. The gemstone called a cat's eye is a beautiful, rich brown with a golden stripe running through it. Jade is a deep, glossy green or milky white. Zircon is a flashing, brilliant gemstone that may be pale red, brown, green, violet, or colorless, like a diamond. Turquoise is bright sky-blue or blue-green. These, and many other mixtures of minerals that are gemstones, are used in jewelry and beautiful works of art.

The stone that burns

"I smell brimstone ! The Devil must be near !"

Long ago, that's what someone might have said if they smelled sulfur burning. Sulfur was called brimstone, which means "the stone that burns". Because it burned, and came from deep inside the earth, people once thought it had something to do with the Devil.

Sulfur is one of the most common of all minerals. It is often found aboveground, near volcanoes that have thrown it out while erupting. But it is usually found deep underground, mixed with rock. Sulfur looks like little chunks of bright, yellow glass. It burns with a blue flame and give off a smell like rotten eggs.

Hundreds of years ago, people discovered how to make gunpowder by mixing sulfur with charcoal and another mineral called saltpeter. As time went on sulfur became used for more and more things. Today, it is used to make medicines, tires, and many other things people use every day.

The dry liquid

Imagine a dry liquid - a liquid you can stick your finger into without getting it wet ! There is such a liquid. It's a melted metal that stays melted even when it's cool ! No other metal does such a thing.

This strange, liquid metal is called mercury. It comes from a sparky red rock called cinnabar that is found near hot springs and volcanoes. If you ever had your temperature taken with a fever thermometer, you have seen one way that mercury is used. The silvery stuff inside the thermometer is mercury. Dentists also use mercury, mixed with silver, to fill cavities in teeth.

Mercury is full of surprises. When a little of it is poured onto a slanting surface, the mercury doesn't trickle down in a stream, as other liquids do. The puddle stays together and just slides downhill ! Given a push, the puddle breaks into hundreds of tiny globs that roll like little balls. And if the globs are all pushed together, they form a puddle again.

Mercury was named after the Roman god Mercury, who was the swift messenger of the gods. But because mercury moves and quivers as if it were alive, it is often called quicksilver, which means live silver.

Mercury is a dangerous poison. It's dangerous to handle because it might get into your mouth and cause sickness. Factories sometimes dump wastes containing mercury into lakes and streams. The mercury then gets into the bodies of fish. If people eat the fish, they may get very sick or even die.

The Atomic Age mineral

The prospector moved slowly across a rocky, desert hillside. He was searching for a very valuable mineral. But he had no pick or shovel. As he walked along, he held a metal tube over the rocks. A wire connected the tube to a metal box he carried in his other hand.

Suddenly, a popping noise came from the box. Then another. Then the pops came so rapidly they sounded like a machine gun. The prospector let out a whoop of joy. He had found the mineral - uranium.

Uranium is a mineral that is the main source of atomic energy. It is a metal that is radioactive. This means that, in a way, it is "exploding" - that it shoots out tiny bits of itself in all directions. These tiny bits are parts of the atoms of which uranium is made. They are so small they cannot be seen with even the most powerful microscope. But they can be discovered by a machine called a Geiger counter.

Uranium is never found by itself. It is always mixed with other minerals. Prospectors usually look for it in a mineral called pitchblende. Pitchblende is a lumpy black rock that shines like fresh tar. It is found near the top of the ground. Uranium is also found in a powdery, yellowish mineral called carnotite that forms in a thick coat on sandstone and other rocks.

When uranium is taken out of pitchblende or canotite, it is a heavy, silver-gray metal. Copper is the metal of the Bronze Age, and iron is the metal of the Iron Age and Steel Age, but uranium is the metal of the Atomic Age - which has just begun.

The Iron Age mineral

Safety pins and skyscrapers, scissors and steamships, automobiles and airplanes - none of these things could be made without the mineral called iron.

Iron is a metal, like copper and tin and bronze. But iron is much harder and tougher. It is the best kind of metal for making tools and things that need lots of strength.

There is iron almost everywhere in the earth's crust, but it almost always locked inside other minerals. Minerals that contain iron are called iron ores. When these ores are heated, the iron flows out as a thick liquid. When the iron cools, it is a hard, dull-looking black metal.

Iron has always been an important metal since people first began using it, about four thousand years ago. It made better, longer-lasting tools and weapons than either stone or bronze. The use of iron brought the Bronze Age to an end. When people began making things out of iron, it was the start of the Iron Age.

Many things, such as fire hydrants, fancy fences, and parts of automobile engines are still made from iron. But much of the iron now taken from the earth is mixed with other minerals to make an even harder, tougher metal called steel.

People have known for a long time how to meake steel, but it was always very expensive. It was only a little more than a hundred years ago that we learned how to make lots of steel cheaply. So many things are now made out of steel - such as safety pins, scissors, skyscrapers, and automobiles - that it is sometimes said
that we live in the Steel Age

The Bronze Age minerals

Do you have a brand-new, shiny penny ? If so, you know what the mineral called copper looks like. Pennies are made of almost pure copper - a gleaming, reddish-brown metal.

Copper was one of the first metals that people used. During the Stone Age, people had only stones and bones with which to make weapons and tools. But about ten thousand years ago, people found that they could pound and shape lumps of copper into knives, spearheads, and bracelets.

Those people of long ago found great lumps of pure copper in the earth. But people have taken copper out of the ground for thousands of years. Now there is very little pure copper left. Today, most of the copper we find is mixed with other minerals. Minerals that contain copper are called copper ores.

Copper is an important metal. Most of the wires that carry electricity into our house and factories are made o copper. This is because electricity moves better through copper than through any other common metal.

Tin is another metal that people have used for a long time. It is a bluish-white metal that is heavy and soft, like copper. It comes from a brownish, glassy, mineral called cassiterite. People of long ago often found pebbles of cassiterite in riverbeds. Today, most cassiterite has to be dug out of the earth.

About five thousand years ago, ancient people learned how to mix tin with copper. By melting tin and copper together, they could make a metal called bronze. Bronze is much stronger and tougher than either in or copper. From bronze, the ancient people made swords, helmets, armor, cups, bowls, and many other things. When people learned to make bronze, it was the end of the Stone Age and the beginning of the Bronze Age.

When we think of tin, we usually think of tin cans. But tin cans are really made of iron or steel covered with a thin coating of tin. The tin gives the cans a shiny look and keeps the iron or steel from rusting.

A crystal garden

You can see how crystals of a mineral grow by growing some crystals yourself. Here's what you need:

6 tablespoons of water
1 tablespoons of ammonia
6 tablesspoons of laundry bluing
6 teaspoons of salt
6 pieces of charcoal-the kind used for outdoor cooking

Mix the water, ammonia, bluing, and salt together in a bowl. Put the charcoal in a pie pan. Now pour the liquid over the pieces of charcoal. Put the pie pan in a warm place where the water will evaporate quickly. When the water has evaporated, you will find you have grown a fantastic "garden" of crystals !

If you can't get some of the things listed above, you can grow crystals with nothing more than salt and water. Common table salt is a mineral called halite that forms in cube-shaped crystals.

Put a little hot water into a small pie pan or a saucer. Than, put several spoonfuls of salt into the water. Stir the water until all the salt dissolves. Put the pan in a place where it won't be disturbed. Leave it there for a few days.

Slowly, the water will dry up. On the bottom of the pan there will be a lot of shiny, cube-shaped crystals of salt.

Flat sides and sharp corners

A chunk of mineral may seem to be just a jagged, lumpy, or twisty piece of hard stuff with no special shape. But each kind of mineral is actually made up of tiny shapes called crystals - shapes that have flat sides and sharp corners, such as cubes, squares, and pyramids.

Each kind of mineral is made up of crystals that have the same shape. The mineral halite, which is the salt we put on food, is made up of crystals shaped like pyramids. Graphite, which is the "lead" in pencils, is made up of square crystals.

It seems strange to think of hard, lifeless things growing. But crystals actually do grow. They don't grow from the inside, as living things do, however. They grow by joining together. For example, the walls of cave may be covered with a particular mineral. Water trickling down the walls washes crystals of this mineral onto the cave floor. The water, filled with many tiny crystals, forms a puddle on the floor. As the puddle slowly dries up, the crystals stick together. They form larger crystals. That's how crystals grow - when more and more of them join together.

Some minerals have the same shape as the crystals they are made of. Diamonds usually look like two pyramids with their bottoms stuck together. Graphite comes in flat sheets. Pyrite comes in clusters of shiny, golden cubes. Quartz is often found in clusters of six-sided, pointed shapes.

Solid stuff

It takes a lot of things to make cookies - flour, milk, eggs, butter, sugar. But when the cookies are baked, you can't see that there are a lot of things in them. Everything is all mixed together.

A rock is like a cooky. Mixed into most rocks are things that you can't see. Rocks are mixtures of materials called minerals.

Minerals are the solid stuff that the earth is made of. There are about two thousand different kinds of minerals. Some are shiny, some are sparkly, some are dull. Some are hard enough to scratch glass. Others are so soft you can scratch them with your fingernail. There are minerals that look like chunks of ice, minerals that look like clusters of metal cubes, and minerals that look like bunches of fuzzy string or shiny hair.

Minerals are scattered throughout the earth's rocky crust. Many of them are mixed together, but many are by themselves, in huge chunks, little lumps, and broad patches between layers of rock.

The rock factory

The earth is a rock factory. Scientists believe it has been making rocks for billion of years.

The earth makes three different kinds of rocks. One kind is made from hot, syrupy, liquids, deep inside the earth. Sometimes, some of this liquid rock pushes its way between two layers of solid rock - making a sort of rock sandwich. Than the liquid cools off and becomes solid too. Sometimes, when volcanoes erupt, some of the liquid rock is pushed up out of the earth. When it reaches the earth's surface, it cools and becomes solid.

Rock that was once a hot liquid is called igneous rock. Igneous means "of fire". Granite, the hard, light-colored sparkly rock used on the outside of many buildings, is an igneous rock. And so is the black glassy rock called obsidian that some prehistoric people made into knives and arrowheads.

Another kind of rock is made out of "rock powder". Wind and rain wear off tiny, powdery bits of rock from mountains. Rivers carry the powdered rock to the sea, where it sinks to the bottom. Over thousands of year, the bottom layers of powder are squeezed together by the weight of new layers. Slowly, the powdery bits on the bottom are turned into a layer of solid rock. Over millions of years, earthquakes and other forces may lift up the layers of new rock and they become dry land.

Rocks that are made this way are called sedimentary rocks. Sedimentary comes from the word sediment, which means "to settle". Limestone and sandstone are sedimentary rocks.

There is also rock that is changed deep in the earth. The heat and weight of the earth slowly change it into a different kind of rock. Rocks changed this way are called metamorphic rocks. Metamorphic means "changed".

Slate, a gray-black rock from which blackboards used to be made, is a metamorphic rock that was changed from clay. Marble is a metamorphic rock that was changed from limestone. Most metamorphic rocks are very old. They stay buried unless erosion, an earthquake, or the birth of a mountain lifts them to the earth's surface.

In fact, all the rocks we see were made long, long ago. The oldest rocks ever found on earth are more than three billion years old. But the earth hasn't stopped making rocks - it's making them right now. It takes a long, long time to make a rock.