TOPIC 1:TECTONIC PROCESSES AND HAZARDS (Lesson 13)

1. Why are some locations more at risk from tectonic hazards? ENQUIRY QUESTION 1: 1. Why are some locations more at risk from tectonic hazards?

Previous lesson objectives To describe and explain the global distribution of volcanic eruptions To be able to explain the causes of volcanic eruptions To be able to recognise which physical processes impact on the magnitude and type of volcanic eruption

HOT SPOT VOLCANOES!

Today’s Lesson Objective Lesson Aims To know what hot spots are and the conditions in the mantle that cause them. To know some examples of volcanic hot spot activity Learning objectives To be able to describe and explain one named example of the formation a hot spot e.g. Hawaii

Pacific Ring of Fire Volcanism is mostly focused at plate margins but ………….. This map shows the margins of the Pacific tectonic plate and surrounding region. The red dots show the location of active volcanism. Notice how the majority of the volcanism is focused in lines along the plate boundaries? For this region this area is known as the “Pacific Ring of Fire”. But why are all of the volcanoes located at the plate margins?

Volcanoes are formed by: - Subduction - Rifting - Hotspots Volcanoes can be formed in three ways: Via subduction. The subducting slab dehydrates to form new melt that will rise through the crust to be erupted at the surface. Via rifting. When two plates pull apart magma rises, producing volcanic eruptions at the surface. At “Hotspots”….hotspot do not necessarily occur along a plate boundary. So hotspot volcanoes can form in the middle of tectonic plates….Click for example.

The Hawaiian islands are a chain of volcanoes in the Pacific Ocean. Look at their location on the map below. Why is this an unusual place for them to be located?

Global distribution of Hot Spots

What are Hotspot Volcanoes? Hot mantle plumes breaching the surface in the middle of a tectonic plate Firstly, what are hotspot volcanoes and how do they form? A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time. The source of this volcanism is a mantle plume of hot mantle material rising up from near the core-mantle boundary through the crust to the surface (see left diagram). A mantle plume may rise at any location in the mantle, and this is why hotspot volcanoes are independent from tectonic plate boundaries. The Hawaiian island chain are an example of hotspot volcanoes (see right photograph). The Hawaiian island chain are examples of hotspot volcanoes. Photo: Tom Pfeiffer / www.volcanodiscovery.com

The volcanoes get younger from one end to the other. The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. Hotspot’s commonly form volcanic island chains (like the Hawaiian islands). These result from the slow movement of a tectonic plate over a FIXED hotspot. Persistent volcanic activity at a hotspot will create new islands as the plate moves the position of the “old” volcanic island from over the hotspot. Therefore at one end of the island chain you see the youngest, most active volcanic islands (directly over the hotspot) and along the island chain the extinct volcanoes become older and more eroded (see diagram). This way geologists can use hotspot volcano chains to track the movement of the tectonic plate through time. The volcanoes get younger from one end to the other.

Web geology animations http://ansatte.uit.no/webgeology/webgeology_files/english/plate%20tectonics.html ( see volcanoes and vulcanism- the occurrence of volcanoes)

See BBC 2 clip- Mount Kilauea from ‘10 things you didn’t know about volcanoes’- Ian Stewart ( 2.33mins)from @ 32.50 secs in.. https://www.youtube.com/watch?v=cQMB7o3SXOw

Notes – see booklet p. The vast majority of volcanic eruptions occur on or near to plate boundaries, but there are some exceptions when activity occurs on the interior of a plate e g Hawaiian islands are entirely of volcanic origin, but have formed in the middle of the Pacific Ocean more than 3200km from the nearest plate boundary The Hot Spot Theory It is believed that small, localised exceptionally hot regions exist below the plates in the mantle- hot spots. These hot spots provide high heat energy or thermal plumes in the mantle, above which lava is poured out onto the earth’s surface. This plume is the magma source. The reason for these hot spots is unclear but may be due to an intense concentration of radioactive elements within the mantle.

The heat from the hotspot produces a persistent source of magma by partially melting the overriding plate, which rises through the mantle to erupt on the sea floor, forming an active seamount. Countless eruptions cause the seamount to grow, until it emerges as an island volcano.( N.B Don’t confuse with island arcs at destructive plate margins!) The hot spots are stationary and so active volcanoes only occur above the hot spot. As the plate moves, the island is carried beyond the hot spot and its magma source and volcanism ceases. As that volcano becomes extinct, another develops over the hot spot and the cycle is repeated. Thus a line of volcanoes is created over millions of yrs ( but only the plume above the hot spot is active, the rest are extinct and may even subside and sink below the ocean)

The Hawaiian islands in the Pacific form such a linear chain, resulting from the Pacific Plate moving slowly northward over a hot spot located beneath the present island of Hawaii. Evidence: The volcanoes in the Hawaiian chain should get progressively older and become more eroded the farther they travel beyond the hot spot. The oldest rocks are on Kauia, the north-western most inhabited island- 5.5.mill yrs old. On Hawaii, the southwestern most in the chain are less than 0.7 mill yrs old and new rock is continually being formed. Soil formation and vegetation will also give clues.

Other Hot spots? Others are thought to exist beneath the continents and oceans, at diverging boundaries and on plate interiors or on ocean ridges e.g. Galapagos Islands, the Azores. Some are thought to exist beneath the North American plate. A hot spot is presumed to exist under the continental crust in the region of the Yellowstone National Park, Wyoming, USA. Here, there are several large calderas ( large crater formed by ground collapse accompanying explosive volcanism)produced by three huge eruptions in the past 2 mill yrs. The thermal energy of the presumed hot spot fuels more than 10000 hot pools and springs, geysers and bubbling mudpools( see later notes on this)

TASKS 1. Read Geoactive and make brief notes on yellow stone national park and hot spot hazards 2. Draw an annotated diagram to explain the formation of hot spots away from plate boundaries 3. Complete AQA past paper 8 marker exam question Jan 2011 and/ or 7 marker June2016