الأسبوع 28 · المحور الرابع، الوحدة الرابعة، المفهوم 4.3 «البراكين» (أ): الثورات البركانية والقشرة الأرضية وتدفق الحمم · 3 دروس
2
Where volcanoes are
Discovery Education Science P4: Term 2, Unit 4, pp. 163–178
GoalDescribe Earth’s layers and tectonic plates, and explain why volcanoes are found in certain places.
New words
crust · the thin, rocky outer layer of the Earthtectonic plate · one of the huge pieces that the Earth’s crust is broken intomagma · melted rock under the ground
Explanation
The Earth is built in layers, like an apple: a core in the centre, a thick middle layer called the mantle, and a thin outer skin called the crust, where we live. The crust is not one solid piece. It is broken into huge slabs called tectonic plates that fit together like a giant jigsaw. The plates move very slowly, only a few centimetres a year, about as fast as fingernails grow. Where plates push together or pull apart, the crust cracks and bends, mountains are pushed up and melted rock can rise from below. Melted rock under the ground is called magma. When it reaches the surface it is called lava. Maps show that many volcanoes lie along mountain chains and around the edges of plates, on land and on the ocean floor. This is a pattern scientists use to find where volcanoes are likely.
Layer
What it is like
crust
thin, rocky outer skin; we live on it
mantle
thick middle layer of very hot rock
core
the centre of the Earth, the hottest part
Worked examples
Apple
Earth
skin
crust
flesh
mantle
core
core
Apple model. Cut an apple in half in your mind. The skin is thin, like the crust. The flesh is thick, like the mantle. The core in the middle stands for the Earth’s core.
Years
Distance moved
1
3 cm
10
30 cm
100
300 cm = 3 m
Plates move slowly. A plate moves 3 cm a year. In 10 years it moves 3 × 10 = 30 cm. In 100 years it moves 300 cm, which is 3 m. We cannot feel this, but over millions of years it moves continents.
Reading a pattern. On a pretend map, Seif counts 10 volcanoes. Eight lie along mountain chains or plate edges, and two lie elsewhere. He says: volcanoes are mostly found where the crust is moving and mountains are rising.
Class activity
Puzzle plates. Cut a piece of card into six irregular pieces that fit together, like plates. Draw a world-like shape across them and write Crust on the card. Slide two pieces slowly toward each other until they buckle: this shows mountains being pushed up. Slide two apart and see the gap where magma could rise. Draw both. A grown-up cuts the card.
Practice
1
What are the three main layers of the Earth, from the outside in?
Crust, mantle and core.
2
What are tectonic plates?
Huge pieces that the Earth’s crust is broken into.
