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Goemorphology

 

 

Structure and changing landforms of the Earth

 

 

Structure of the Earth

and rock types

Internal forces

External forces

 

· The three main rock types that form the crust are:

a.       igneous rock

b.       sedimentary rock

c.       metamorphic rock

· The Earth is composed of three main layers

a.       core

b.       mantle

c.       crust

 

· Internal forces occur beneath the Earth’s surface. These forces cause landforms such as mountains and valleys:

-          isostasy

-          plate tectonics

-          folding

-          faulting

-          earthquakes

-          volcanism

· Some of these forces create opportunities for people, but can also be obstacles and hazards that people try to manage.

 

· External forces operate above the surface of the Earth. These forces level out the landforms created by internal forces:

-          weathering

-          erosion

· Weathering and erosion influence human activities

· Activities of people also influence the processes of weathering and erosion.

 

Composition and Structure of the Earth

The Earth has three main layers:

1.    The core consists of:

·         The inner core, which is solid and is composed of nickel and iron

·         The outer core, which is more liquid

 

2.    The mantle is composed of molten rock or magma, which is very hot and under great pressurecan bend and flow

 

3.    The crust is very thin, but also quite solid in comparison to the core and the mantle. Two main types of rock make up the crust

·         The lighter rocks, composed mainly of silica and aluminium, are known as the sial. The sial is composed mainly of granite. These lighter rocks make up the continents.

·         The heavier rocks, composed mainly of silica and magnesium, are known as sima. The sima is found beneath the water of the oceans and beneath the continents and consists of basalt.

·         The state of balance between the lighter and heavier sima is known as isostacy.

 

4.    The Moho Discontinuity (plane) is the boundary between the more liquid mantle and the solid rocks that form the crust.

·         The plane that separates the molten mantle from the solid crust

 

What are the characteristics of rocks?

1.         Rocks are composed of different minerals.

 

2.         Rocks can be described as crystalline, massive or stratified according to their structure:

·      Crystalline rocks contain crystals of different sizes

·      Massive rocks form in one big mass that does not have different layers or separate areas

·      Stratified rocks form in horizontal layers composed of different types of minerals, referred to as strata. The lines that separate strata are called bedding planes.

 

3.         The extent to which rocks weather (is worn away) depends upon its structure and on the minerals that make up the rock. Resistant rocks do not weather easily.

 

4.         Permeable rocks are structured so that water is able to pass through them. There are two different forms of permeable rocks:

·      Porous rocks have air spaces through which water can pass. These are rocks composed of fairly large particles

·      Pervious rocks have cracks or joints in them that allow water to pass through them. Those are mostly rocks that have crystal structure that creates cracks and joints

 

 Types of rocks

There are three main groups of rock making up the earth’s crust:

·      Igneous rocks are formed from magma and lava that has cooled. Igneous rocks can be described as massive because the minerals cooled in a ‘mass’ rather than in layers

·      Sedimentary rocks consist of layers of deposited material. These are stratified rocks.

·      Metamorphic rocks have been altered by pressure and/or heat.

 

 

Igneous

Sedimentary

Metamorphic

Formation

Magma and lava cools and becomes solid

·       Extrusive – magma pours out onto Earth’s surface as lava. Cooling is rapid and crystals are so small that they can only be seen under a microscope

Particles are deposited and compacted to become solid rock

·       Organic- decaying plant or animal matter is laid down and compressed to form solid rock

·       Chemical- the water component of solutions evaporates, leaving the chemicals behind, which form solid rock

Igneous or sedimentary rock is heated or is under pressure and changes form

·       Dynamic –rocks are altered due to great pressure

·       Regional –rocks change form as a result of temperature and pressure

Examples

·       Granite is formed deep in the crust

·       Dolerite forms closer to the Earth’s surface

·       Basalt forms from lava above the Earth’s surface

·       Conglomerate –composed of large particles

·       Sandstone forms from sand-sized particles

·       Shale forms from smaller particles

·       Limestone forms from pieces of shells

·       Coal forms from vegetable matter

·       Quartzite forms from sandstone

·       Slate forms from shale

·       Gneiss forms from granite

·       Anthracite is metamorphosed coal

·       Marble is formed from limestone

Characteristic features

·       This rock type is composed of crystals of different sizes

·       Granite is described as a massive rock as it does not contain bedding planes

·       Granite does not weather easily

·       Granite can crack and have joints in it

·       This rock type occurs as layers or strata

·       There are bedding planes between each layer

·       Shale weathers easily

·       There may be fossils in sedimentary rock

·       Sandstone is porous (it can hold water)

·       This rock type does not weather easily

·       There are usually attractive colours

 

Uses

·       Igneous rock may contain precious metals and ores

·       Basalt weathers to form fertile soils

·       Granite used for building roads and kitchen counter tops

·       Coal is a fossil fuel

·       Limestone is used in the building industry

·       Fossils in the rock can give clues to the geological history of an area

·       Marble is used for monuments

·       Slate is used for floors and roofs

Landforms

Granite domes, tors, dolerite sills, ridges, poorts

·       Cuestas, logsbacks

·       Caves can develop in areas that are composed of limestone

 

 

Last Edited by: Ms Xaba
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