Why is there volcanic activity in the middle of plates?

Hot Spots Learning Objectives To be able to describe and explain the features of hot spots To describe a case study of hot spot volcanoes

Key Ideas Not all volcanic activity can be related to present day active plate margins The Pacific Ocean shows a number of sub-parallel chains of volcanic islands which run diagonally across the Pacific plate These islands are all shield volcanoes built up of basaltic lavas

The basalt that makes up these islands contain significantly more potassium and sodium than the basalts which make up the ocean crust. This suggests that while the mantle forms the source for both these rocks suites, there must be a different mechanism involved in their formation The formation of these volcanic islands is related to the occurrence of long-lived, stationary hot spots within the mantle

How do they form? Hot spots occur when radioactive decay in the core is concentrated. It heats the lower mantle above it. These hot spots heat the lower mantle creating localised thermal currents where plumes of magma rise vertically. Hot spots provide localised intensive heat sources called mantle plumes - which partly melt the overlying plate, creating a continuous magma source. Magma is lighter than the solid lithosphere and rises up, erupting at the surface. Where crust is thin above a hot spot, this provides further opportunity for magma to escape as volcanic eruption. Lava builds up over time, eventually creating an island, above the present sea-level.

Sort the cards into the correct order ACTIVITY 1 Sort the cards into the correct order Hot spots provide localised intensive heat sources called mantle plumes - which partly melt the overlying plate, creating a continuous magma source. These hot spots heat the lower mantle creating localised thermal currents where plumes of magma rise vertically. Hot spots occur when radioactive decay in the core is concentrated. It heats the lower mantle above it. Lava builds up over time, eventually creating an island, above the present sea-level. Where crust is thin above a hot spot, this provides further opportunity for magma to escape as volcanic eruption. Magma is lighter than the solid lithosphere and rises up, erupting at the surface.

The formation of hotspots

http://www.youtube.com/watch?v=jZdeavURhk8

Examples of Hot Spots HAWAIIAN ISLANDS (series of volcanic islands) - in middle of Pacific Plate   YELLOWSTONE NATIONAL PARK - USA - hotspot located in interior of a continental plate (North American Plate)

1. HAWAIIAN ISLANDS What direction is the Pacific Plate moving? Here the bigger Island of Hawaii is currently over the hot spot

Formation of the HAWAIIAN ISLANDS The current hotspot is near the southern end of the island of Hawaii. Pacific plate moves northwest at a rate of 5-10cm a year. Lo’ihi Seamount (YOUNGEST) is forming on the sea floor just south of Hawaii North of Hawaii there are extinct volcanoes, being battered by erosion Bends in the island chains show changes in plate movement.

Formation of the HAWAIIAN ISLANDS New shield volcanoes appear above the hotspot. Lava is runny Pahoehoe – ropy lava and AA – blocky lava Hawaii is home to the world’s largest volcano – Mauna Loa As the volcano moves away from the hotspot the lava becomes more alkaline spatter cones may appear. More violent eruptions e.g. Pyroclastic flows More cone like volcanoes Eg OLDEST - Haleakala on the island of Maui

World’s largest volcano! Mauna Loa - Hawaii World’s largest volcano!

2. YELLOWSTONE NATIONAL PARK Yellowstone appears to be over a continental hot spot that has produced a chain of volcanoes as the North American Plate moves southwestward over the hot spot

YELLOWSTONE NATIONAL PARK

Key Terms Hotspot Magma plume Basalt Volcanic islands Lithosphere