13.3.1 Are all processes natural? Natural processes such as tectonic uplift, erosion , deposition and weathering continuously shape the Earth’s surface and what lies below. People change landscapes when they clear the land for agriculture or to build cities and road networks. 13.3.2 The role of tectonic forces The Earth’s crust consists of a series of plates that fit together like a jigsaw puzzle. These plates sit on a layer of semi-molten material in the Earth’s mantle . Heat from the Earth’s core creates convection currents , causing the plates to move. When two plates collide, one will slide under the other in a process known as subduction andbecome part of the mantle. Other rocks will bend and buckle as they are forced upwards. Great mountain ranges such as the Himalayas in Asia and the Rocky Mountains in North America were formed this way. You will learn more about the formation of mountains in Topic 14. 13.3.3 Changing landscapes — how is the landscape worn away? Erosion is the wearing away of the Earth’s surface by wind, water, ice and human activity. Further erosion occurs as the eroded material is transported and deposited in a new location. Over time, this material can build up, creating new landforms. The Grand Canyon in Arizona FIGURES1b and 3 are examples of these elements at work. Stunning landforms have been created as the river cut deep channels into softer rocks.
FIGURE2 After tectonic forces cause a section of the Earth to be raised (uplifted), other processes take over and resculpt the landscape.
1
2
3
4
Erosion is the process whereby soil and rocks are worn away and moved to a new location by agents such as wind, water or ice.
Deposition—materials moved
Transportation is the
Weathering is the breakdown of rocks due to the action of rainwater, temperature change and biological action. The material is not transported (removed).
by wind and water eventually come to a halt. Over time new landforms are built. Sand dunes and beaches are common landforms associated with deposition.
process that moves eroded material to a new location—examples include soil carried by the wind, sediment or pebbles in a stream.
It can be physical, chemical or biological.
Transportation
2
Physical —occurs where water is continually freezing and thawing. The water penetrates cracks and holes in the rocks. As water freezes it expands, making the cracks larger. Over time the rock breaks apart.
3
Weathering
Deposition
Erosion
1
4
Igneous rock
Sedimentation
Uplift
Intrusive igneous rock
Metamorphic rock
Chemical —some rocks, such as limestone, contain chemicals that react with water, causing the limestone to dissolve.
Sedimentary rock
Magma
Melting
Biological —living organisms such as algae produce chemicals that break down rocks. They can also be forced apart by plant roots.
288 Jacaranda Humanities Alive 8 Victorian Curriculum Third Edition
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