Rocks, Soils and Fossils

A pebble, a brick and a crystal can all feel solid. To tell them apart, ask what they are made of. Ask whether nature or people made them.

Rock and mineral

A rock is a solid, natural part of Earth. Cliffs, mountains and the ground beneath soil contain rock. A boulder and a pebble can be pieces of the same rock.

A mineral is a natural solid that was never alive. It is one material all the way through. Its tiny parts repeat in an orderly pattern. Sometimes this makes crystals with flat sides. Quartz, feldspar, mica, calcite and halite are minerals. Halite is rock salt.

Most rocks contain one or more minerals. Think of a cookie with chips and nuts inside. The whole cookie is like a rock. The different pieces inside are like its minerals. Granite contains quartz, feldspar and mica packed together.

Sort by where it came from

A brick's clay came from the ground. But making clay into a brick changes it. Concrete, pottery and glass are human-made materials too.

Coal is a rock with a different story. It formed from ancient plant remains. It is not a mineral, because those remains were once alive.

Look beyond gray stones

Rocks have many colors and textures. Chalk is white, soft and crumbly. Pumice has many holes and can be light enough to float. A rock does not have to be gray or especially heavy.

You can find real rock in a town. A stone wall, statue or countertop may be rock. A nearby brick wall is made from a human-made material.

Two white rocks may look alike at first. A closer look can show quite different materials. Use several clues together when you describe a rock.

Look closely

Begin with a clean, dry rock. A wet surface looks darker and can hide useful clues. Check color, shine, layers, holes, grains and crystals. Feel whether the surface is rough, smooth or powdery.

A hand lens makes small details look bigger. Hold it near your eye. Move the rock toward it until the details look sharp.

Grains are little pieces stuck together. You can see sandy grains in sandstone. Crystals have flat sides and may sparkle. In granite, crystals fit together like puzzle pieces. Some crystals are too small to see, even with a lens.

Compare familiar rocks

Color alone will not name a rock. White marble and white chalk have different textures. A small granite pebble is still granite, just like a large block.

Turn a look into an observation

Suppose your rock has pink, white and black speckles. You see shiny pieces with flat sides. There are no holes or layers. Together, those clues fit granite.

Write an observation someone else can check. "I can see shiny crystals" describes a visible detail. "This rock is my favorite" tells someone what you like. "It looks old" gives no measured age.

A rock's shape can show what happened to it. A pebble rolling in a river bumps other stones. Its sharp edges wear down, leaving a rounded shape. Letters on an old gravestone can wear away too. Shape and surface wear give clues about water and weather.

A heavy rock is not always hard to scratch. Hardness is about scratching, not how much a piece weighs. You can compare it with a simple test.

Find a real scratch

Hardness means how well a material resists scratching. A harder material scratches a softer one.

1. Choose a clean, flat place on the rock.
2. Draw your fingernail across that place with a firm push.
3. If no groove forms, try a steel nail with adult help.
4. Rub away loose powder and look closely with a hand lens.

Wear goggles, use adult help with the steel nail, and scratch away from your body. A line of powder can rub off. A real scratch leaves a groove in the rock.

Use these groups for your results:

Chalk's loosely joined grains can scrape away under a fingernail. Marble resists a fingernail but a steel nail scratches it. Granite's hard crystals resist the steel nail. These samples go in the order chalk, marble, granite, from softest to hardest.

Test wearing away

A rub test checks how easily grains come loose. Hold a sample over dark paper. Rub it 20 times with sandpaper. Do the same with the next sample. Sandstone often leaves loose grains; granite often leaves very few.

Change only the rock. Use the same sandpaper, number of rubs and firm push. If you rub one piece much longer, the piles no longer give a fair comparison.

Keep the properties separate

A large chalk block can outweigh a small granite pebble. That does not make the chalk harder. A hard material can also crack: resisting a scratch differs from resisting a blow.

Scientists use the Mohs scale for minerals. Talc is 1, quartz is 7 and diamond is 10. A higher-numbered mineral scratches a lower-numbered one. A rock can contain several minerals, so different parts may scratch differently.

Rain can travel through a rock that looks solid. Tiny connected spaces make paths for water. Tests help you find out what happens inside.

Spaces and paths

A porous rock has spaces inside. A permeable rock lets water flow through it. The spaces must join up to make a path. A rock with separate, sealed bubbles may hold air without letting water through.

Sandstone has spaces between its grains. Water can move along these spaces. Granite's crystals fit closely together. Uncracked granite, slate and marble let almost no water through. Chalk and sandstone let water through.

Test water going in

For a drop test, put dry samples on a tray. Add 5 drops of water to each. Time how long the drops take to soak in. In one test, chalk took 8 seconds and sandstone took 25 seconds. The drops remained on slate after 5 minutes. Chalk took in water fastest in this test.

You can also estimate water uptake by weighing:

1. Weigh a dry rock.
2. Leave it in water for 10 minutes.
3. Pat the outside dry and weigh it again right away.
4. Subtract the dry mass from the wet mass.

A sandstone sample changes from 200 grams to 212 grams.

It took in about 12 grams of water. A chalk sample changing from 150 to 168 grams took in about 18 grams. Granite staying at 200 grams took in too little to measure.

Test water going through

Taking in water and letting it flow through are different. To check flow, put a modeling-clay ring on a thin, flat rock sample. Rest the sample over a cup. Add water inside the ring, keeping it away from the edges. Watch underneath. A wet underside or falling drops show water crossed the rock.

For a fair comparison, use equally dry samples with similar sizes and thicknesses. Keep the amount of water and waiting time the same.

Hardness and permeability need separate tests. A rock can be hard and still have connected spaces. In nature, water can collect above impermeable rock. It can later flow out of the ground at a spring.

The same tray of rocks can make several sensible groups. Your rule decides which rocks belong together. A clear rule lets another person check your work.

Pick one property

A property is something you can observe or test. You might sort by visible crystals, layers, hardness or permeability. Test every rock in the same way before you group them.

Here are results from five samples:

To sort by permeability, put chalk and sandstone together. Put marble, slate and granite together. Name the groups "Permeable" and "Lets almost no water through."

Now change the rule to hardness. Chalk has its own soft group. Marble, slate and sandstone share the medium group. Granite has its own hard group. Sandstone and slate are together now, although they were apart before.

Make a sorting key

A sorting key uses questions to find a name. Each answer sends you to the next question. Here is a key for these five samples:

1. Does water soak in? If yes, try your fingernail. A groove points to chalk; no groove points to sandstone.
2. If water does not soak in, try a steel nail. No groove points to granite.
3. If the steel nail makes a groove, check for thin, flat sheets. Sheets point to slate; no sheets point to marble.

The key combines clues about known samples. A drop soaking in still needs a flow test to check permeability.

Explain the group

Say which rule you used and what happened. For example: "Sandstone belongs in my permeable group. Water crossed the thin sample and dripped below it."

You can also compare: "Sandstone and chalk both let water through. Sandstone resisted my fingernail, but chalk did not."

"Nice" and "not nice" are opinions, so people may disagree. Sorting by the size of today's pieces tells you little about their rock materials. A big piece and a small piece can be the same rock.

A footprint can tell you an animal passed by. If that mark survives from long ago, it can be a fossil. Fossils include much more than dinosaur bones.

Remains and marks

A fossil is a preserved remain or mark of a living thing from long ago. Most fossils are thousands or millions of years old. Most are found in rock.

Fossil droppings can give clues about food. A trackway can show where an animal walked. You do not need the animal's body to have evidence of its life.

Many common fossils came from small sea animals. Ammonites had coiled shells. Trilobites had hard, jointed bodies. Plants, fish and insects left fossils too.

Decide what counts

Imagine a table holding an ammonite shape in stone, a fresh shell, a quartz crystal and a heart-shaped pebble.

The ammonite is a fossil: it preserves a long-ago animal's shape. The fresh shell is a recent remain. Quartz was never alive. The pebble's heart shape came from wear, not from a living thing. A fossil needs a connection to past life.

A fossil bone can still contain original bone. Water brings minerals into its tiny spaces. Minerals may also replace some or all of the bone. This can make it much heavier than a fresh bone.

People who find the clues

A paleontologist studies fossils. Exposed rock can reveal clues that were once buried. Scientists carefully record where a fossil came from as well as its shape.

Mary Anning lived from 1799 to 1847 in Lyme Regis, England. As a girl, she and her brother found an ichthyosaur, a large sea reptile. Later she found a nearly complete plesiosaur. Her finds helped people understand animals that no longer live on Earth.

Most dead plants and animals disappear. Animals eat them, and their remains rot. A fossil can form when some evidence survives those changes.

Follow a shell

Sediment is loose material such as sand, mud and shell pieces. Water or wind carries it, then it settles.

1. An ammonite dies and sinks to the seabed.
2. Its soft body rots or is eaten. Its hard shell remains.
3. Sediment covers the shell quickly.
4. More sediment builds up above it.
5. The weight squeezes the layers. Minerals in water glue grains together, making rock.
6. Minerals can fill tiny spaces in the shell or replace it.
7. Much later, the rock is exposed and worn away. The fossil becomes visible.

The shell and surrounding rock do not appear all at once. Burial, hardening and later uncovering happen at different stages.

Why burial helps

Quick burial covers remains before they are eaten or scattered. A hard shell, bone or tooth usually lasts longer than skin or muscle. That gives it more time to be buried. Shell fossils are common; jellyfish fossils are rare.

Water often carries the mud that does the burying. Seas, lakes, swamps and riverbanks give more chances for burial. A flood can cover an animal with mud near a river. Minerals can later fill spaces in its buried bones.

Age and forming time

A fossil that is 100 million years old did not need all that time to form. Its age tells you when the living thing lived. Some small fossils form quickly. Big bones can take thousands of years to fossilize, then stay buried for millions more.

Even buried remains can be destroyed later. The fossils we find are only a small part of past life.

Where to look for evidence

Fossils usually occur in rocks built from sediment layers. Limestone, sandstone, shale and chalk can hold fossils. Granite formed from very hot melted rock. That heat destroys remains, so you do not expect ordinary fossils inside it.

A fossil can be a hollow, a solid copy or real remains. Look closely at what survived. Its shape and material help you tell its story.

Hollow and filling

A mold is a hollow shape left in rock. A buried shell can dissolve after sediment hardens around it. The space left behind keeps the shell's outside shape.

A cast forms if sediment or minerals fill that hollow. The filling becomes a solid copy. Think of a cake pan and its cake. The hollow pan gives a shape; the cake fills it.

You can model the difference. Press a shell into modeling clay, then lift it away. You made a mold. Fill the hollow with plaster and let it set. The solid copy you lift out is a cast. A classroom model can set in about an hour. Natural fossil formation takes much longer, often thousands of years.

Other ways to keep a shape

An impression is a print left in sediment that later hardens. A leaf may leave a flat print. A footprint can survive when new sediment gently covers it. An impression does not need to preserve the whole original object.

Petrified wood has minerals filling and replacing its wood. It looks like a log but is stone. You may still see tree rings. Fossil trees in Arizona's Petrified Forest National Park are more than 200 million years old.

Remains that are not stone

Amber is hardened tree resin. An insect can stick in fresh resin and become sealed inside it. Tiny wings and hairs may survive. These are real remains inside amber, not a stone copy.

Other remains survive in icy ground. Frozen mammoths sometimes still have skin and hair. At the La Brea Tar Pits in Los Angeles, sticky natural asphalt trapped animals and preserved bones.

To separate a mold from a cast, ask, "Am I seeing the hollow or its filling?" A hollow spiral is a mold. Its solid spiral filling is a cast.

You cannot watch an extinct animal eat. You can look at its fossil teeth. Body parts give clues about how a living thing used them.

Read a body clue

Scientists compare fossils with living things today. Similar parts often do similar jobs. Long, sharp teeth can bite and tear meat. Wide, ridged teeth can grind plants.

A mammoth's broad teeth point to plant food. The sharp teeth of Tyrannosaurus rex point to meat. Fossil bones also show body size. T. rex could grow to about 12 meters long, around the length of a school bus.

A bone does not record every detail. Bones help show shape and size. Skin color is rarely preserved. Pictures fill some gaps with careful ideas that may change after new finds.

What extinct means

A kind of living thing is extinct when none is alive anywhere. Ammonites, trilobites and woolly mammoths are extinct. Their fossils are evidence that they once lived.

Extinct does not mean absent from just one pond or forest. A kind still living somewhere else is not extinct. Scientists study fossils and living populations to tell the difference.

Living things may die out when their world changes too quickly. About 66 million years ago, a huge space rock hit Earth. It changed the climate. The large dinosaurs that were not birds and the ammonites died out around that time. Birds are living dinosaurs.

Compare past and present

Horseshoe crabs and ginkgo trees resemble fossil relatives. A nautilus has a coiled shell and is a distant relative of ammonites. Similarity helps scientists compare bodies, but a living nautilus is not an ammonite.

People like us appeared about 300,000 years ago. That is far later than the large dinosaurs died out. The gap is about 65,700,000 years. A fossil scene showing a person beside a living T. rex mixes two very different times.

A dry hill can hold fossils of sea animals. The fossils are clues to a place that looked different long ago. Compare them with living things whose homes you know.

Link the fossil to its home

An environment includes land or water, weather and other living things. A coral needs a different home from a tree. That helps you use fossils as evidence.

1. Name the kind of fossil.
2. Think where similar living things live today.
3. Describe the likely past environment.
4. Give the fossil evidence for your idea.

Suppose a hill holds many fossil sea shells and corals. These animals lived in the sea. The rock formed in a sea environment before it became part of the hill. The animals did not climb there; the land and sea changed.

Sea-animal remains even occur in limestone near Mount Everest's summit. They are about 450 million years old. Rock now high above the sea was once part of the seafloor.

Use several clues

Imagine two fossil collections:

Site A has 52 sea-animal fossils. Site B has 36 plant fossils. Sea is a strong explanation for A. B suggests a forest or swamp. A rooted stump is especially helpful: it shows where a tree grew. A loose leaf can wash away from its home.

When similar sea fossils appear across a region, they give evidence of a widespread sea. Several matching clues support a story better than one isolated find.

Fossils can show climate

Fossils of warmth-loving plants occur in the Arctic. Fossil forests also grew in Antarctica more than 250 million years ago. Those places had much warmer conditions then. Today's environment does not tell you the whole history of a place.

New sediment settles above older sediment. When layers harden, their order keeps a record. You can read that order even without knowing exact dates.

Begin at the bottom

In layers that have not been turned over, the oldest is at the bottom. The youngest is on top. Think of papers piling onto a desk. The first sheet goes down before later sheets cover it.

The fossils in these layers follow the same order. A fossil buried in a lower layer is older than one buried above it.

Here is a cliff, shown from top to bottom:

Start at the bottom limestone. Next comes the shale. Last comes the sandstone. The trilobite fossils are older than the fish fossils.

This is relative age: age compared with something else. It tells older and younger, not a number of years. Knowing a cousin is older than you does not tell you their birthday.

Match separated layers

A valley can cut a once-joined set of layers apart. Suppose both walls show the same sequence of sandstone, shale with ammonites, then limestone. Matching their order and fossils supports the idea that the layers were once joined.

Some kinds of fossils help compare distant layers. A kind that lived for only a short part of Earth's history narrows the possible age. Finding that kind in two layers suggests similar ages. A kind that survived for a very long time gives a much wider age range.

Check which way is up

Earth movements can tilt, fold or turn layers over. The bottom-is-oldest rule needs layers still the right way up.

Thickness alone does not tell age. A flood may drop a thick layer quickly. A thin layer may build slowly. Read the order and fossil clues, not simply the thickness or how close a layer is to your eyes.

A stack of rock layers can tell a place's story. Each layer holds clues from one part of that story. Read from the bottom and link the clues in order.

Give each layer a setting

A landscape is the shape of the land and what covers it. Both rocks and fossils can reveal changes.

Limestone with sea shells often points to a sea. Fine mud that became shale points to quiet water. Sandstone can form at a beach, river or dune, so you need more clues. Coal formed from plants in swamps.

Imagine a rock wall with rooted tree stumps in old soil at the bottom. Above them is a layer of sea shells. The layers are still the right way up.

1. The rooted stumps show trees growing on land.
2. The shell layer is above, so it formed later.
3. The sea shells show that seawater covered the place later.

You can write: "Trees grew here first. Later, the sea covered this land. Rooted stumps below sea shells support that order."

Loose plant fossils alone need care. Leaves can wash into water. Roots still in their old soil give stronger evidence of land at that spot.

Follow a longer change

Another wall has coral fossils at the bottom. Above is sandstone with beach-like ripple marks. Coal with plant fossils is at the top. Read the evidence as sea, then sandy shore, then swamp. Over time, the place became shallower and land took over.

Read the gap too

A canyon can have matching layers on both walls. A river runs between them. Those matching layers once stretched across the gap. The river slowly cut down through them.

The layers came before the canyon. They did not grow separately on two ready-made walls. The Colorado River cut through rock layers to form the Grand Canyon.

Some changes, such as floods and landslides, happen quickly. Water and wind also make small changes over many years. A long series of small changes can leave a very different landscape.

A handful of soil holds more than crushed rock. It also holds water, air, plant remains and living things. These parts work together around roots.

Find the parts

Soil is the loose surface material where plants can grow. Its rock particles come in three main sizes. Sand grains are largest and visible. Silt particles are smaller and feel floury. Clay particles are tiniest.

Organic matter comes from living things. It includes dead leaves, roots and droppings. Well-rotted organic matter becomes dark humus. It helps supply nutrients, the materials plants need to grow.

Gaps between the particles hold water and air. Roots and soil animals need both. Rain fills more gaps with water. As water drains or dries, air enters those spaces.

A typical garden-soil picture shows 100 parts:

The solid parts add to 50. Air and water fill the other 50 parts. Real soils differ, and their air and water amounts change with the weather.

How soil begins

Rain, ice, wind and roots break rock into smaller pieces. This breaking down is called weathering. Dead plants and animals decay and mix with the pieces. Worms and other animals help stir the mixture.

Making about 2.5 centimeters, or 1 inch, of topsoil can take hundreds of years or more. A patch of soil holds a long history of rock breaking and living things adding material.

Look for life

Spread a little soil on a tray. Use a hand lens to look for stones, roots, leaf bits and tiny animals. Put living things back gently, and wash your hands afterward.

Earthworms, ants, beetles, fungi and bacteria live in soil. Many are too tiny to see. A small spoonful of healthy soil can hold billions of tiny living things. Sand by itself leaves out many parts of this living mixture.

Below fallen leaves, the ground can change color and texture. A side view reveals layers with different materials. This view is called a soil profile.

Travel downward

Leaf litter is mostly organic matter. Topsoil mixes organic matter with rock particles. It usually has more organic matter than the layers underneath. That helps make it dark and useful for plant growth.

Topsoil is less than 30 centimeters deep in many places. Deep roots can reach the subsoil. Water carries some minerals downward from the topsoil into that layer.

Farther down, fewer roots and living things appear. The rock pieces get larger until you reach solid bedrock.

Add layer depths

Suppose leaf litter is 3 centimeters thick. Topsoil is 20 centimeters thick, and subsoil is 50 centimeters thick. Where does the broken-rock layer begin?

Add the layers above it:

The broken rock starts 73 centimeters below the surface. The 50 centimeters measures just the subsoil, not the whole depth above broken rock.

Profiles can differ

A forest can have a thick cover of leaves. A rocky hill can have thin soil over bedrock. Some places have no clear leaf-litter layer. Not every profile shows every layer.

Soil layers develop as rock breaks down below and organic matter arrives above. They do not build like a stack of settled sediment layers. You cannot label the bottom soil layer oldest just by borrowing the rock-layer rule.

Some soil slips through your fingers. Some makes a sticky ribbon. Different mixes of particles and plant remains give soils different properties.

Compare soils

Sandy soil can dry quickly and holds few nutrients. Clay holds plenty of nutrients but can become waterlogged. Loam contains sand, silt, clay and organic matter. Many garden plants grow well in it.

Peaty soil builds in wet places such as bogs. It differs from a soil that is mainly rock particles: much of it is partly decayed plant material.

Use a feel test

Wet a small sample and squeeze it. Gritty soil that falls apart is sandy. Soil that rolls into a long, sticky ribbon contains much clay. A ball that crumbles when poked may be loam. Combine feel with other clues when naming a soil.

Let particles settle

For a jar test, half fill a jar with soil. Add water, close the lid and shake. Leave it standing for a day. Sand settles first and forms the bottom layer. Silt settles above it. Clay settles last and may keep the water cloudy. Some plant bits float.

Suppose the settled layers measure 6 centimeters of sand, 3 of silt and 1 of clay. Together they make 10 centimeters. Sand is 6 of those 10 parts, making it the largest share. This sample is sandy.

Match soil to a plant

Carrots suit loose, sandy soil. Rice can grow in water-holding clay in flooded fields. Blueberries suit acidic, organic-rich soil, such as suitable peaty soil. A label saying "well-drained soil" asks for soil that lets extra water escape.

Dark color can show organic matter. It does not make a soil best for every plant. Check the plant's water and soil needs.

After rain, some ground dries quickly and some stays soggy. Drainage means water moving down through soil. You can compare drainage with funnels and measuring tubes.

Set up a fair test

Put filter paper inside three matching funnels. Stand each funnel over a graduated cylinder. This is a tall tube marked for measuring liquid.

Add the same amount of sandy soil, loam and clay soil. Start with samples equally dry and packed in the same way. Pour 100 milliliters of water into each. After 5 minutes, measure the water collected below.

Change just the soil type. Keep soil amount, water amount, equipment and waiting time alike. Repeat the test to check whether a result happens again.

Read the water collected

Here are one test's results:

More water came through sandy soil in the same time. It drained fastest. Clay let the least water through.

Work out how much water had not come through yet. Subtract the collected water from the 100 milliliters poured in.

For sand:

For loam:

For clay:

About 20, 50 and 85 milliliters stayed behind in the three setups at that time. That includes water held in the soil and filter paper.

Explain the pattern

Sand has larger gaps, so water moves through easily. Clay has tiny gaps, so water moves slowly. Draw a bar chart with a bar for each collected amount. Your conclusion can say: "Sandy soil drained fastest in our test."

Roots need both air and water. Very fast drainage can leave too little water. Waterlogged soil fills spaces that could hold air. Loam often keeps a helpful balance: some water stays while extra water drains.

Soil helps grow your food and gives roots a place to hold. It stores water and shelters living things. Looking after it helps all these jobs continue.

Keep the mixture healthy

Healthy soil needs organic matter, nutrients, water and air spaces. Earthworms make tunnels that let air and water move. They pull leaves into soil, mix it and add nutrient-rich droppings. Fungi and bacteria break down dead material, returning nutrients.

Compost is rotted plant waste. Leaves, grass and fruit peels can become compost. Adding it supplies organic matter and nutrients. It helps sandy soil hold water and helps clay soil drain.

Packing soil down squeezes out air spaces. Repeated walking or driving over it can harm root growth. Too much salt or pollution can harm roots and the small living things around them.

Read a growing test

Two pots hold the same kind of bean plant. One has added compost; the other does not. Both receive the same light and water. After 3 weeks, their heights are 18 and 11 centimeters.

The plant with compost is 7 centimeters taller. This suggests compost helped in this test. Repeating with more plants helps check the result.

Keep topsoil in place

Erosion happens when moving water or wind carries soil away. Topsoil is easily lost because it is at the surface. It is also valuable for roots and soil life.

Compare two trays tilted to the same slope. One has grass; one is bare. Pour 200 milliliters of water over each in the same way. Muddy water from the bare tray carries soil away. Clearer water from the grassy tray shows less soil loss. Roots help hold the particles together.

People keep soil covered with plants or mulch. Rows of trees slow wind. Plowing across a slope slows water running downhill.

Lost soil takes a long time to replace. At a place where 1 inch forms in about 500 years, 3 inches would take about 1,500 years. A storm can remove soil much faster than it forms.

Look around at a window, a pencil and a metal can. Their materials began in nature. Many everyday objects can be traced back to rocks or soil.

Find Earth's resources

A natural resource is something from nature that people use. Stone, sand and gravel help make buildings and roads. Clay makes bricks and pots. Soil grows food, cotton and trees for wood.

Minerals supply other useful materials. Halite gives us salt. Quartz sand helps make glass. Gypsum is used in drywall. Graphite makes the gray mark in a pencil. Pencil "lead" is graphite, not the metal lead.

Follow the metals

An ore is rock containing enough useful metal to be worth digging up. People take iron from iron ore. Bauxite is an ore used to get aluminum. Copper is useful in wires, and gold is used in jewelry.

The starting material may look very different from the finished object. A shiny can does not look like the ore that supplied its metal.

Fuels from long-ago life

Coal, oil and natural gas are fossil fuels. They formed from ancient living things. Coal came mainly from swamp plants. Oil and natural gas came mainly from tiny sea organisms. Dinosaurs were not their main starting material.

Oil often fills tiny spaces in rock, like water held in a sponge. It is not usually a big underground lake. Connected spaces let the oil move through rock toward wells.

Look near home

Different places have different resources. Local stone in old buildings can give a clue about what was available nearby. A museum display or local history page may explain what people once quarried or mined. What material did they collect, and what did they make with it?

A roof needs to keep rain outside. A statue needs to be carved. Choosing a useful rock starts with what the job needs.

Link a need to a property

Use three steps. Name the job. Decide which properties matter. Compare observations and test results.

Chalk is soft and crumbly. It does not last well in most building jobs. Sandstone can be useful for a wall even though water can move through it. Different jobs need different combinations of properties.

Make a choice from results

Suppose you compare chalk, sandstone and slate for a roof. Chalk and sandstone let water through. Slate lets almost none through and splits into flat sheets. Choose slate: both properties help it cover a roof.

For busy steps, compare wear. In the same rub test, sandstone leaves a pile of grains. Granite leaves almost none. Granite better resists the wear from many feet.

The hardest material does not win every job. A sculptor needs to shape a statue. Carving and polishing matter as well as lasting outdoors.

Watch what weather does

Rain slowly dissolves marble and limestone. Letters on old marble gravestones can fade as the surface wears away. Slate gravestones often keep sharp letters longer. Looking at old objects can show how materials last.

Minerals have useful properties too. Diamond is very hard, so it helps tools cut rock. Gold resists rusting and stays shiny in jewelry. Salt dissolves in water, helping it spread through food.

Cost, weight and what is available nearby also matter. A local rock may be easier to obtain than one brought from far away. A good choice fits the job and the situation.

A clay lump cannot keep its shape like a finished brick. People change natural materials to make useful objects. You can follow the changes from ground to product.

Get the material

A quarry is an open pit where people dig stone, sand or gravel. A mine provides ore or coal from tunnels or open pits. Deep wells bring oil and natural gas upward. Stay out of mines and quarries; they are dangerous places.

Getting a material is only the first step. Processing means changing it for a use. Washing and sorting sand are simple kinds of processing. Extracting iron from ore needs much more change.

Follow clay into a brick

1. Dig clay from the ground.
2. Mix it with water so it can be shaped.
3. Form the brick and let it dry.
4. Bake it in a very hot oven called a kiln.

The heat makes the clay permanently hard. The clay was natural; the brick is human-made.

Glass has another path. People heat sand with other materials until they melt. They form the hot material into sheets and let it cool. A window starts with natural resources, but people make the glass.

Limestone and clay are heated to make cement. Cement mixed with sand, gravel and water makes concrete. Iron ore is processed in a hot furnace to obtain iron. Each finished material has a different job from its starting resources.

Use resources carefully

Rocks, ores, coal and oil form extremely slowly. They are non-renewable resources: supplies are not replaced on the timescale people use them. Digging more out reduces what remains.

Using less, reusing objects and recycling materials reduces the need for new resources. Recycling an aluminum can needs much less energy than making aluminum from new ore. Reusing a jar gives its glass another job.

Quarries and mines change the land. They can create dust and noise and remove wildlife homes. Repairing worked land can provide new habitats. Some old quarries become parks or lakes.