Toward a theory of formation of the Earth’s crust A chance to consider evidence
Lesson objectives Recap basic Earth’s structure Take part in decision making / selection of evidence to support explanation of formation of crust end of slide
Learning outcomes Understand the development of theories of the formation of the Earth’s crust Appreciate that scientific evidence can be subject to bias Appreciate societal role in scientific developments Select and present evidence end of slide
Ideas Creationism Contracting Earth Isostatic Uplift Expanding Earth Plate Tectonics The Earth formed such a long time ago that it is hard for people to work out how the features of the crust have formed. Scientists formulate ideas based on evidence end of slide
Creationism Earth and all the living things were created as described in the book of Genesis in the Bible Earth’s creation happened quickly – during the six ‘days’ quoted in Genesis. One ‘day’ in Genesis may refer to an ‘age’ in the Earth’s creation and could have lasted many thousands of years. 1 st day : God created light 2 nd day : firmament of heaven 3 rd day : separation between water and land and the creation of plant life 4 th day : the sun, moon, and stars 5 th day : marine life and birds 6 th day : land animals, man and woman 7 th day : God rested. The surface of the Earth is not changing that much now end of slide
Contracting Earth Proposed by James Dwight Dana (19th century) Earth began as ball of hot matter that subsequently cooled and contracted Surface cooled first and contracted producing great cracks (valleys) Interior began to cool and shrink All geological features were caused by this contraction, and even the great mountain ranges were seen to be equivalent to the wrinkles in the skin of a drying apple end of slide
Isostatic uplift About 11,000 years ago, much of northern Europe and North America was covered by ice sheets up to 3 km thick The enormous weight of this ice caused the crust to sink into the fluid mantle After the ice age, the ice melted and this removal of mass from the crust made it ’float up’ in the mantle. Total localised uplift could be in excess of 400m by the end of deglaciation. This could account for the appearance of small mountains, hills and other geological features end of slide
Expanding Earth The Earth started life as a smaller planet with a diameter of around 7500km Addition of material from space and the expansion of the Earth’s core caused the diameter to increase to the present 12700km. The early Earth had no oceans – just a simple, flat land crust The molten core of the Earth is extruded from cracks in the expanding crust forming new crust under the oceans. As the Earth expands, less dense granite rises up in the mantle rock forming mountains As the Earth has expanded, the crust broke up and formed great cracks which filled with water from below forming the oceans end of slide
Plate tectonics The Earth's surface is covered by a series of separate sheets or ‘lithospheric plates’ Convection currents beneath the plates move the plates in different directions. The Earth’s core is very hot and this causes convection currents in the mantle rock The plates collide with each other and buckle forming mountain ranges or can sink, remelting into the mantle (subduction). Where plates move apart the mantle can melt and flow upwards to form new crust end of slide The continents on some plates were once joined in one huge land mass. This allowed animals to travel easily between continents.
Things to do... You will be assigned a ‘theory’ in your group You will be provided with a set of ‘evidence cards’. You will need to sift through the cards to find evidence that supports your theory. Also, try to look for evidence which contradicts other theories – always good to add some weight to your argument! Prepare a short (5 minute) presentation on your theory and the evidence which supports it Present your ideas and take 5 questions from the audience. end of slide
Plenary The currently accepted theory is ‘Plate tectonics’. Review your evidence cards now and choose the ones which relate to this theory. Do you have any doubts over the evidence for this? end of presentation
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