Continental drift. Plate tectonics. Plate boundaries. What Causes Geohazards Published on LEARNZ (

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1 Listen: Listen to this page [1] Listen to this page [2] Geohazards such as earthquakes and volcanoes would not happen if the Earth's crust was solid and did not move. The theory of continental drift explains how the continents have moved over time due to plate tectonics. We would not have geohazards if the Earth's crust was solid and did not move. The theory of continental drift explains how the land has moved over time due to plate tectonics. Continental drift The German scientist Alfred Wegener, proposed in 1912 a theory of continental drift to explain how the Earth's land masses came to be where they are now. The theory was based on the Earth s continents [3]once fitting together like a giant jigsaw puzzle. Continental drift explains how similar animals and plants could have lived at the same time on continents that are now widely separated by ocean, and how identical mountain ranges in different continents were once joined. 250 million years ago the Earth's continents were once joined together in one giant supercontinent called Pangaea. Slowly, over millions of years Pangaea [4]broke into Laurasia [5]and Gondwana [6]. These supercontinents have drifted apart forming the smaller continents of today. Plate tectonics Since the 1960s, detailed geological studies of the Earth s crust have greatly increased our understanding of how the continents move. The theory of continental drift has been replaced with the term plate tectonics. On the Earth today there are seven large plates and many smaller ones. They are made of blocks of continental and oceanic lithosphere (crust and upper mantle). Oceanic crust is usually about 10km thick while continental crust is about 30 50km thick. With crust and upper mantle combined, the slabs of lithosphere are between 40 and 200km thick. The plates move in slow motion and are constantly changing shape. It is thought that convection currents in the mantle of the Earth provide the energy to move the tectonic plates from a few millimetres to a maximum of about 15cm per year. Plate boundaries Page 1 of 10

2 At the edge of the tectonic [7]plates one of three processes can occur: Divergent boundary Also known as a spreading boundary, where two plates move apart allowing magma [8], or molten rock, to rise from inside the Earth to fill in the gap. The two plates move away from each other like two conveyor belts moving in opposite directions. This can create rift valleys on land or ocean ridges on the seafloor (for example the Atlantic Ocean). Convergent boundary Where two plates are colliding. Different things will happen depending on what type of plates are colliding: 1. If the plates are continental plates they are of the same density so neither plate can over-ride the other and the landmasses buckle and fold, creating mountain ranges. 2. If the plates are both oceanic plates then island arcs or basins can form. 3. If an oceanic plate collides with a continental plate the denser oceanic plate will be subducted under the continental plate. Transform boundary A transform boundary occurs where two plates slide against each other in a shear movement. But rather than sliding smoothly, the plates build up tension then release the tension with a burst of movement. This movement is felt as an earthquake. Plate Tectonics in New Zealand Page 2 of 10

3 New Zealand is located on the edge of two tectonic plates, the Indo-Australian and the Pacific plates. This position makes New Zealand geologically active with frequent earthquakes, geothermal areas and volcanoes. This plate boundary has shaped New Zealand: to the east of the North Island the Pacific plate is being forced under the Indo-Australian plate (convergent boundary - subduction) in the South Island the two plates push past each other sideways (transform boundary) to the south of New Zealand the Indo-Australian plate is being forced under the Pacific plate (convergent boundary - subduction) The movement of these two tectonic plates forms the New Zealand landscape that we know and love. Watch the GNS Science animation [9] (519k), showing the future shape and deformation of New Zealand if the movement measured between were to continue unchanged. Ready for a quiz? [10] Page 3 of 10

4 [11] Plate tectonics Plate tectonics is a theory which explains how the Earth's surface is broken into large plates which move. 250 million years ago the Earth s continents [3] were joined together in one giant landmass called Pangaea [4]. Slowly, over millions of years, Pangaea broke in two and drifted apart forming the smaller continents of today. Plate tectonics explains how similar animals and plants live in countries which are now a long way from each other, and how mountain ranges in different continents were once joined. On the Earth today there are seven large plates and many smaller ones. The plates move in slow motion and are always changing shape. It is thought that heat inside the Earth creates convection currents and provides the energy to move the plates from a few millimetres up to about 15cm per year. Plate boundaries At the edge of the tectonic [7]plates one of three things can happen: Page 4 of 10

5 Types of Plate Boundary. Image: GNS Science. Spreading boundary (divergent) A spreading boundary is where two plates move away from each other. The plates move apart so magma, or molten rock, can rise from inside the Earth to fill in the gap. This can form rift valleys on land or ocean ridges on the seafloor (for example the Atlantic Ocean). Colliding boundary (convergent) This is where two plates push together. Different things will happen depending on what type of plates are colliding: If the plates are continental plates they are the same weight so the plates cannot over-ride each other and the land buckles and folds, creating mountain ranges. If the plates are both oceanic plates then island arcs or basins can form. If an oceanic plate collides with a continental plate the heavier oceanic plate will sink (subduct) under the continental plate, this can lead to volcanic activity. Sliding boundary (transform) This is when two plates slide against each other, but rather than sliding smoothly, the plates can get stuck and build up stress. This stress is then released as a burst of movement and is felt as an earthquake. Plate tectonics in New Zealand Page 5 of 10

6 New Zealand sits on the edge of two tectonic plates, the Indo-Australian and the Pacific plates. This makes New Zealand geologically active with many earthquakes, geothermal areas and volcanoes. The movement of these two tectonic plates shapes the New Zealand landscape that we know and love. Watch the GNS Science animation [9] (519k), showing the future shape of New Zealand if the current movement continues. Ready for a quiz? [10] Page 6 of 10

7 [11] Māori keywords: Audio Māori keywords: rū whenua - shaking of the land, earthquake [12] puia - volcano [13] whenua - land [14] kōwhatu - rock [15] rohe - boundary [16] Complete the 'Pangaea to the Present' activity [17] on continental drift. Complete the 'Pangaea to the Present' activity [17] on continental drift. Page 7 of 10

8 [18] Convection currents caused by heating in the Earth's mantle explain how the continents move. Who came up with this theory and what is this theory called? Image: Public Domain. [19] New Zealand is located on the edge of two tectonic plates; the Indo Australian Plate and the Pacific Plate. How do you think this affects New Zealand? Image: United States Geological Survey Page 8 of 10

9 [20] Two tectonic plates push past each other along the Alpine Fault. What land feature can be seen alongside the Alpine Fault? Image: Mike Norton. [21] The Indo Australian Plate is like a bulldozer pushing at the weaker Pacific Plate causing it to rise up into the mountain range known as the Southern Alps. What else does this plate movement cause? Image: LEARNZ. Source URL: Links [1] [2] ds_1.mp3?uuid=5bafa5a24845e [3] [4] [5] [6] Page 9 of 10

10 [7] [8] [9] [10] tml5.html [11] [12] a5a248ab4 [13] 8ba7 [14] a248c94 [15] a248d7c [16] [17] [18] [19] [20] [21] Page 10 of 10