WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE A Q A Geography revision. 00:00:04.125 --> 00:00:06.617 Topic G 2: Tectonic hazards. 00:00:06.767 --> 00:00:11.620 Use your school’s selected case studies if they differ from these revision examples. 00:00:14.300 --> 00:00:18.714 Earth has a solid inner core, a liquid outer core, a mantle and a thin crust. 00:00:18.864 --> 00:00:24.750 The rigid lithosphere includes the crust and the uppermost mantle and is broken into tectonic plates. 00:00:27.400 --> 00:00:30.131 Plates move slowly over the mantle beneath them. 00:00:30.281 --> 00:00:36.829 Slab pull at subduction zones and ridge push help drive movement; mantle circulation is part of this system. 00:00:36.979 --> 00:00:42.493 The mantle is mainly solid rock that can flow very slowly, not a global liquid magma layer. 00:00:45.167 --> 00:00:49.437 Oceanic crust is generally thinner and denser than continental crust. 00:00:49.587 --> 00:00:55.577 These differences help explain why oceanic plates can sink beneath other plates at destructive margins. 00:00:58.233 --> 00:01:04.324 Most earthquakes and volcanoes occur in belts along plate margins, including around the Pacific Ocean. 00:01:04.474 --> 00:01:10.371 Earthquakes also occur away from margins, and some volcanoes form above hotspots such as Hawaii. 00:01:13.033 --> 00:01:17.642 An earthquake happens when stress builds up and rocks suddenly slip along a fault. 00:01:17.792 --> 00:01:20.669 The released energy travels as seismic waves. 00:01:20.819 --> 00:01:27.281 The focus is the point inside Earth where rupture starts; the epicentre is directly above it at the surface. 00:01:29.933 --> 00:01:32.863 At a constructive margin, plates move apart. 00:01:33.013 --> 00:01:36.561 Mantle rock rises and partially melts as pressure falls. 00:01:36.711 --> 00:01:43.013 Magma rises into the gap and cools to form new crust, producing volcanoes and usually shallow earthquakes. 00:01:43.163 --> 00:01:44.699 Plates move apart. 00:01:44.849 --> 00:01:49.694 Arrows show divergent plate movement and upward magma movement; not to scale. 00:01:54.367 --> 00:01:57.382 The Mid-Atlantic Ridge is a constructive margin. 00:01:57.532 --> 00:02:03.675 Iceland lies on the ridge between the North American and Eurasian plates and also has a hotspot beneath it. 00:02:06.333 --> 00:02:14.279 At a destructive margin involving oceanic crust, the denser oceanic plate sinks into the mantle in a process called subduction. 00:02:14.429 --> 00:02:19.441 Friction and sudden slip cause earthquakes, and an ocean trench forms near the margin. 00:02:22.100 --> 00:02:26.335 Water released from the descending plate helps the mantle above it melt. 00:02:26.485 --> 00:02:28.894 Magma may rise to form volcanoes. 00:02:29.044 --> 00:02:35.783 Thick, gas-rich magma can produce explosive eruptions; subduction is not simply a plate melting through friction. 00:02:35.933 --> 00:02:37.875 Oceanic plate subduction. 00:02:38.025 --> 00:02:41.943 The denser oceanic plate sinks beneath the overriding plate. 00:02:42.093 --> 00:02:44.531 Arrows show movement; not to scale. 00:02:49.200 --> 00:02:54.533 The Nazca Plate subducts beneath the South American Plate along western South America. 00:02:54.683 --> 00:02:57.678 This creates earthquakes and the volcanic Andes. 00:03:00.333 --> 00:03:04.079 When two continental plates collide, neither sinks easily. 00:03:04.229 --> 00:03:08.900 Compression folds and thickens the crust into mountains, as in the Himalayas. 00:03:09.050 --> 00:03:12.980 Collision causes earthquakes but usually no chain of volcanoes. 00:03:15.633 --> 00:03:19.117 At a conservative margin, plates slide past each other. 00:03:19.267 --> 00:03:23.717 Friction can lock them until stress produces sudden slip and an earthquake. 00:03:23.867 --> 00:03:29.615 Crust is neither created nor destroyed, and this movement does not normally generate volcanoes. 00:03:29.765 --> 00:03:31.643 Sliding plate boundary. 00:03:31.793 --> 00:03:35.003 Opposite arrows show relative movement along the fault. 00:03:35.153 --> 00:03:38.623 This movement causes earthquakes, not a volcanic chain. 00:03:43.300 --> 00:03:50.115 The San Andreas Fault in California is a conservative boundary between the Pacific and North American plates. 00:03:52.767 --> 00:03:59.475 Primary effects happen directly because of the event: shaking can collapse buildings, break roads and injure people. 00:03:59.625 --> 00:04:03.661 Lava, ash and hot flows can damage settlements during an eruption. 00:04:06.333 --> 00:04:16.263 Secondary effects follow from primary effects: damaged pipes can interrupt drinking water, fires can start from broken gas lines, and blocked roads can delay aid. 00:04:16.413 --> 00:04:20.654 Earthquakes may trigger landslides or tsunamis under suitable conditions. 00:04:23.333 --> 00:04:29.760 Immediate responses include rescue, emergency medical care, temporary shelters, food and clean water. 00:04:29.910 --> 00:04:35.955 Longer-term responses include repairing infrastructure, rebuilding homes and improving preparedness. 00:04:38.633 --> 00:04:46.249 To compare places of contrasting wealth, consider building quality, access to emergency care, transport and money for recovery. 00:04:46.399 --> 00:04:53.302 Also consider depth, magnitude, population exposure and time of day; wealth alone does not determine the outcome. 00:04:55.967 --> 00:05:02.227 People stay near volcanoes for fertile soils, geothermal energy, tourism and mineral resources. 00:05:02.377 --> 00:05:09.787 Jobs, family ties, confidence in protection and the cost of moving also explain why people live in earthquake zones. 00:05:12.467 --> 00:05:19.206 Monitoring volcanoes includes measuring small earthquakes, ground swelling, gas emissions and temperature changes. 00:05:19.356 --> 00:05:23.699 These may indicate rising magma, but forecasts still have uncertainty. 00:05:26.367 --> 00:05:35.410 Scientists can estimate earthquake probabilities over periods of time, but cannot reliably predict an earthquake's exact place, time and magnitude. 00:05:35.560 --> 00:05:39.180 Monitoring and prediction are different from guaranteeing a warning. 00:05:41.833 --> 00:05:48.904 Protection includes earthquake-resistant buildings with reinforced structures, flexible connections or base isolation. 00:05:49.054 --> 00:05:54.717 Buildings must be designed and maintained properly; no structure is completely earthquake-proof. 00:05:57.367 --> 00:06:02.223 Planning includes hazard maps, evacuation routes, drills and emergency supplies. 00:06:02.373 --> 00:06:07.427 Keeping important facilities away from the most hazardous areas can reduce losses. 00:06:10.100 --> 00:06:15.528 Tsunami warning systems use earthquake data and sea-level instruments to warn coasts. 00:06:15.678 --> 00:06:22.834 People close to the source may need to evacuate immediately after strong shaking because official warnings can arrive too late. 00:06:25.500 --> 00:06:31.772 Chile's Maule earthquake struck on 27 February 2010 with magnitude 8.8. 00:06:31.922 --> 00:06:35.973 It occurred where the Nazca Plate subducts beneath the South American Plate. 00:06:36.123 --> 00:06:40.513 Chile had substantially more resources for protection and recovery than Nepal. 00:06:43.167 --> 00:06:46.851 Shaking damaged buildings, roads and services in Chile. 00:06:47.001 --> 00:06:49.815 A tsunami then flooded coastal settlements. 00:06:49.965 --> 00:06:57.504 Around 500 people died in the earthquake and tsunami; strong building standards reduced collapse but did not prevent every loss. 00:07:00.167 --> 00:07:07.910 Chile's immediate responses included search and rescue, emergency shelter, medical care and restoring essential services. 00:07:08.060 --> 00:07:15.846 Military support helped with logistics and security; communication and tsunami-warning failures showed weaknesses in preparedness. 00:07:18.500 --> 00:07:25.687 Longer-term work in Chile included rebuilding housing and infrastructure and reviewing warning and emergency arrangements. 00:07:25.837 --> 00:07:31.628 Existing institutions, engineering expertise and financial resources supported recovery. 00:07:34.300 --> 00:07:41.169 Nepal's Gorkha earthquake struck on 25 April 2015 with magnitude 7.8. 00:07:41.319 --> 00:07:49.874 Movement on the collision zone between the Indian and Eurasian plates caused the event; a major magnitude 7.3 aftershock followed on 12 May. 00:07:52.533 --> 00:07:57.791 Buildings collapsed in Kathmandu and rural settlements, and historic sites were damaged. 00:07:57.941 --> 00:08:03.828 The earthquake sequence killed nearly 9,000 people and left many households without safe homes. 00:08:06.500 --> 00:08:10.070 Landslides and avalanches were secondary effects in Nepal. 00:08:10.220 --> 00:08:16.321 They damaged routes and isolated mountain communities, making rescue and delivery of supplies harder. 00:08:19.000 --> 00:08:24.789 Immediate help came from local people, Nepal's emergency services and international teams. 00:08:24.939 --> 00:08:32.521 Rescue, temporary shelter, medical support, food and water were priorities, with difficult access slowing work in remote areas. 00:08:35.200 --> 00:08:42.046 Nepal's longer-term housing reconstruction used grants, engineering support and training for safer construction. 00:08:42.196 --> 00:08:47.647 International financing helped households rebuild, but recovery required years of work. 00:08:50.300 --> 00:08:54.823 Chile's quake had a larger magnitude, yet Nepal suffered far more deaths. 00:08:54.973 --> 00:09:03.793 Building vulnerability, resources and access help explain this contrast; differences in physical conditions and population exposure also matter. 00:09:06.467 --> 00:09:08.621 That completes Tectonic hazards. 00:09:08.771 --> 00:09:12.117 Revisit the notes and test yourself on the revision website.