Week 25 · Theme 4, Unit 4 Shifting Surfaces, Concept 4.1 Breaking Down and Moving Rocks (C) and Concept 4.2 Changing Landscapes (A) · 3 lessons
1
Deposition: where the pieces settle
Discovery Education Science P4: Term 2, Unit 4, pp. 122–134
GoalExplain deposition, say when moving water and wind drop what they carry, and describe how layers of sediment form.
New words
deposit · to drop and leave pieces of rock, sand or mud in a placelayer · one flat band lying on top of anothersandbar · a long bank of sand built by a river or the sea
Explanation
Erosion carries pieces away, but water and wind cannot carry them for ever. When a river slows down, when waves lose their push or when the wind drops, the pieces fall and settle. This is deposition. The heaviest pieces settle first, so pebbles drop before sand, and sand drops before the finest mud. Pieces that settle are sediment, and they build new landforms: a sandbar along a river bank, a delta where a river meets the sea, sand dunes in the desert and small dunes on a beach. Sediment often settles in layers, one on top of another. In a very long time, the weight of the upper layers can press the lower ones into solid rock. So sand that we find in one place was eroded from somewhere else. Erosion and deposition are connected: if something is eroded, it must be deposited somewhere.
Cause
Effect
wind blows across a desert and slows
sand dunes form
a river enters the sea and slows
a delta forms
waves lose their push on a beach
sand settles on the shore
a glacier melts
a pile of rocks is left
Worked examples
Layer (top to bottom)
What it is
top
fine mud
middle
sand
bottom
pebbles
A jar test. Youssef shakes a jar of water with pebbles, sand and soil, then lets it stand. The pebbles drop at once and form the bottom layer. The sand settles next. The fine mud settles last and the water above stays cloudy for a long time.
sand eroded up the beach→carried by waves and wind→deposited against the wall
At a sea wall, Farida sees a pile of sand against the wall on one side. Waves and wind carried the grains along the beach and they dropped where the wall stopped them. The sand was eroded from somewhere else.
layer of mud→more layers on top→weight presses→rock
Layers on layers. A river deposits a new layer of mud every flood. After many floods there are many layers. In a very long time, the weight presses the layers into rock.
Class activity
Settling jar. Fill a clear plastic jar halfway with water. Add a spoon each of small pebbles, sand and soil, close the lid tightly and shake. Place it on a table and watch. Draw the jar after 1 minute, after 10 minutes and after 1 hour. Label the layers. Which settled first? Which stayed in the water longest? Pour the muddy water onto the garden, not down the sink.
Practice
1
What is deposition?
When moved pieces of rock settle and are left in a new place.
2
In a jar test, which settles first: sand, pebbles or fine mud?
Pebbles (the heaviest).
3
A jar has a 2 cm layer of pebbles, a 3 cm layer of sand and a 1 cm layer of mud. How tall is the sediment in all?
2 + 3 + 1 = 6 cm.
4
Where do you think the sand of a beach came from?
It was eroded from rocks or land somewhere else and carried here by water or wind.
