Earth, Rocks and Soil
FoundationThe Earth has a crust, mantle and core; the crust is made of three kinds of rock, which weather into soil — and loam grows plants best.
- The Earth has three layers: crust, mantle and core; the crust is thin like an eggshell.
- Rocks come in three kinds: igneous, sedimentary and metamorphic.
- Rock weathers into sand and mud; with humus, water and air it becomes soil.
文體小知識：說明文把知識講得清清楚楚。這一篇帶你鑽入地底，認識地球的三層結構、三種岩石和三種土壤。
§Text Passage
1The Earth under our feet is like a boiled egg. An egg has a shell, a white and a yolk; the Earth has three layers too: a thin outer layer called the crust, a thick middle layer called the mantle, and the very centre called the core. The crust is thin and hard like the shell, and it is where we live; the mantle and the core lie deep underground and are extremely hot.
2The crust is made of rocks. By the way they form, rocks come in three main kinds. Igneous rock forms when magma cools and hardens, such as granite. Sedimentary rock forms when sand settles layer upon layer and is pressed for a very long time, such as sandstone. Metamorphic rock forms when rock is changed by great heat and pressure, such as marble.
3Rocks are battered by wind and rain day and night and slowly break into grains of sand and mud; this is called weathering. The sand and mud mix with rotted remains of plants and animals (humus), water and air, and over a long time they become soil. In this way soil comes from rock, and plant roots hold on to it to grow.
4By the mix of sand and mud particles, soil is sorted into three kinds. Sandy soil has many sand grains; water drains fast but it holds little water or nutrients. Clay soil has fine, closely packed particles; it holds much water but is so dense that roots spread with difficulty. Loam sits between the two: it drains well and keeps nutrients, so most plants grow best in it.
5Before growing flowers or vegetables, learn about the soil first and the plants will thrive. Next time you pass a flower bed in the park or a hillside in the countryside, crouch down and look at the colour and feel of the soil, and think which kind it is. You will find that science has been hiding right under our feet all along.
1Summary 重點摘要
The Earth has a crust, mantle and core; the crust is made of three kinds of rock, which weather into soil — and loam grows plants best.
2Vocabulary cards Vocabulary
The thin, hard outermost layer of the Earth, like the shell of an egg.
The thick layer beneath the crust, made of very hot, slowly flowing rock.
The very centre of the Earth, extremely hot — the heart of our planet.
Rock formed when magma cools and hardens, such as granite.
Rock formed from layers of sand and mud pressed together over long years, such as sandstone.
Rock changed by great heat and pressure, such as marble.
The slow breaking up of rock by wind, rain, sun and frost.
Rotted leaves and remains of living things that enrich the soil.
Soil with fine, tightly packed particles; it holds much water but roots spread with difficulty.
Soil with a balanced mix of sand and clay; it holds water and nutrients and is best for planting.
3Step by step Steps
- Step 1: Know the three layers of the EarthImagine the Earth as a boiled egg and its structure is easy to remember. The thin, hard outer layer is the crust, like the eggshell; our tall buildings and mountains all stand on it. Beneath the crust is the mantle, far thicker, filled with hot, slowly flowing rock, like the egg white. The very centre is the core, extremely hot, like the yolk. Although we cannot see deep underground, scientists study earthquake waves to work out what the inside of the Earth is like.
- Step 2: The crust is made of rocksThe crust is not one solid block but countless rocks, big and small. Boulders on mountains, reefs by the sea and the stone slabs in parks are all rocks. Look closely at a rock and you will see it is made of grains of different minerals, with different colours and patterns. Building granite and carving marble are common rocks. Rocks record the Earth's long history; by studying them, scientists learn what happened on Earth long, long ago.
- Step 3: How the three kinds of rock formRocks are sorted into three kinds by how they form. First, igneous rock: magma cools and hardens underground or after pouring out at the surface, such as the hard granite that makes up many hills in Hong Kong. Second, sedimentary rock: sand, mud and remains of living things sink layer upon layer on the seabed and harden over millions of years, such as sandstone, which often looks layered like a thousand-layer cake. Third, metamorphic rock: rock buried deep is changed by great heat and pressure, such as limestone turning into marble.
- Step 4: How rock turns into soilMountain rocks are blown by wind and beaten by rain day and night; expanding in heat and shrinking in cold, they slowly crack into small stones, then into grains of sand and mud — this is weathering. The sand and mud mix with rotted leaves and remains of animals, called humus, plus water and air, and over a very long time they form soil. So the soil in flower pots, fields and hillsides all came from rock changed over millions of years. Soil is not easy to form, so we must treasure it.
- Step 5: Three soils, three charactersSoil is sorted into three kinds by the mix of sand and mud particles. Sandy soil has many grains with big gaps between them; water runs straight through — it drains fast but cannot hold water or nutrients, and it feels rough. Clay soil has fine particles with small gaps; it holds much water but drains slowly, feeling sticky when wet and hard when dry. Loam is in between, a balanced mix that drains and breathes yet keeps just enough water and nutrients. Rub a little in your fingers: sandy soil falls apart, clay rolls into a strip, loam feels slightly sticky — then you can tell them apart.
- Step 6: Choose the right soil for plantingBefore planting, learn what each plant likes. Sandy soil drains fast and suits drought-loving plants such as cacti. Clay soil holds water well and suits water-loving plants such as rice. Most common vegetables and flowers, such as choy sum, tomatoes and sunflowers, grow best in loam, because it breathes well, holds water and is rich in nutrients, so roots neither soak nor dry out. The potting mix sold in flower shops is mostly adjusted loam — now you know why.
5Worked example Example
The school gardening club wants to grow beans. There are three pots of soil: Pot A feels rough and grainy, and water runs through it very fast; Pot B is wet and sticky and can be rolled into a ball; Pot C is loose and slightly sticky, neither balling nor crumbling when wet. Which pot should they use to grow beans? Why?
- Tell the soils apart by feel: Pot A is rough and drains very fast, so it is sandy soil; Pot B is fine and sticky and can be rolled into a strip, so it is clay; Pot C has a balanced mix, so it is loam.
- Sandy soil cannot hold water or nutrients, so bean sprouts soon go short of water and food; clay is too dense, so roots cannot breathe or spread.
- For beans to sprout and grow they need water, air and nutrients all at once — missing any one will not do.
- Loam lets air through, holds water and keeps nutrients, meeting all three needs, so Pot C is the best for beans.
- Answer: They should use the loam in Pot C. Loam has a balanced mix of sand and mud: it holds water and nutrients yet drains and breathes, so bean roots can breathe and spread. Pot A's sandy soil drains too fast and Pot B's clay is too dense, so neither suits beans.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the three layers of the Earth, from outside in?
Q2Which kind of rock is granite?
Q3What does sedimentary rock often look like?
Q4Which soil suits most plants best?
Q5What is the process of rock breaking down into soil called?
8Recap Recap
- The Earth has three layers: crust, mantle and core; the crust is thin like an eggshell.
- Rocks come in three kinds: igneous, sedimentary and metamorphic.
- Rock weathers into sand and mud; with humus, water and air it becomes soil.
- Sandy soil drains fast and clay holds water; loam does both.
- Loam suits most plants best — treasure our soil.