Answers to common test

1	(a)
		With the aid of Fig. 1, explain the movement of plates.                                -        Magma in the mantle is heated by the core -        which causes it to expand, rise and spread out, therefore plates are dragged and move away from each other -        When the magma cools and sinks, it pulls the plates along with it, hence plates move towards each other -        The sinking magma heats up again near the core and the process repeats. -        This repeated heating and cooling of magma generates convection currents which causes plates to move. Note: Students forgot about 'sinking magma heats up again'/ 'convection currents' Good answers: Wai Teng, Nadia		[5]
	(b)	Name the 2 types of crust and compare their characteristics.   1)     continental crust is located beneath land masses and under shallow seas close to continents whereas oceanic crust is located beneath the deep ocean. [2] 2)     Oceanic crust is very thick (between 35 to 70 km) whereas the continental crust is very thin (between 5 to 8 km) only. [2] 3)      Oceanic crust is made up of lighter rock such as granite but continental crust comprise of denser and heavy rock such as basalt. [2] Choose from any of the 2 points above Perfect answers : Nadia , Darren, Enzo, Larry, Jocelyn, Tasmin Note: Only awards half mark if write 'the oceanic crust is denser than continental crust'		[4]
	(c)	The Himalayas, which are a chain of mountains, were formed millions of years ago and can grow to impressive heights. Explain the formation of the Himalayas. -        Himalayas are formed along convergent plate boundaries -        When plates move towards each other, they collide -        The resultant compressional force creates immense pressure which causes layers of rocks to buck/fracture -        The rocks fold into different layers and forms a fold mountain Note: Most students didnt include key words 'collide','compressional forces/buckle' Good Ans: Xuan Chan, Anna		[4]
	(d)	Study Fig. 2 which shows the global distribution of earthquakes and volcanoes. Fig. 2 Source : http://earth.rice.edu/mtpe/geo/geosphere/hot/earthquakes
		With reference to Fig. 2, describe the global distribution of earthquakes and volcanoes.                                                                                                                           -        The distribution of earthquakes and volcanoes are mostly found along plate boundaries -        For instance in the middle of the Atlantic Ocean stretching in a North – South direction. -        The Western coast of America /along the Philippines Plate / Japan and Indonesia -        Southern portion of Eurasian Plate/Northern part of African Plate -        These tectonic activities form the Ring of Fire surrounding the boundaries of the Pacific plate Choose from any of the 4 points above. Note: Badly answered!! Many of you made up stories! and talk about convergent and divergent movement. Even convection currents! Most students did not mentioned the volcanoes in the Atlantic Ocean		[4]
	(e)	Study Fig. 3 which shows tectonic movements at plate boundaries. Identify each of the features labelled above.                                                                           [3]
		(i)	A Island arc / volcanic island
		(ii)	B -  Oceanic ridge
		(iii)	C - Trench
	(f)	With the aid of well-labelled diagrams, explain how rift valleys and block mountains are formed. Diagram (0.5m each) -           Tensional forces, -           Fault lines -           Drawing of middle section subsiding -           Labeling of Block mountain Explanation (1m each) -           When two plates move away from each other, -           Tensional forces causes fault lines to be formed -           The central block sinks due to gravity to form rift valley, and the remaining             adjacent block stays as block mountain		[5]