Geology of the Hawaiian Islands

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1 Geology of the Hawaiian Islands Class 4 22 January 2004 Turn in Homework #1 Any Questions? IMPORTANT Big Island Field Trip We need a $162 payment for airfare BEFORE January 29 th Description of logistics, costs, etc., to be handed out in class Plate Tectonics and the Origin of the Hawaiian Islands Hot spots are regions in the Asthenosphere that are hotter than their surroundings Molten magma rises to the surface to form volcanoes, similar to what happens at Spreading Centers Hot spot volcanoes are relatively small, isolated features. Hot spots are believed to be fixed relative to the mantle. BUT this is controversial! 1

2 Our Islands were formed by a Hot Spot When a plate moves over a fixed Hot Spot, a linear chain of volcanoes is formed. The trail of the Hawaiian Hot Spot goes all the way to the far northwest Pacific In addition to the Hawaiian Hot Spot track, there are several others in the Pacific Note the major bend in the Hawaii Emperor ridge The bend in the Hawaii-Emperor Ridge was caused by a change in the direction of motion of the Pacific Plate relative to the underlying mantle. The islands (and former islands) of the Hawaii-Emperor Ridge get progressively older to the North. The northern extension has been subducted in the Aleutian Trench. The Hawaiian Hot Spot is only one of many Hot Spots that are found on the Earth. 2

3 Homework #2: How fast does the Pacific Plate move over the Hawaiian Hot Spot? Change Due Date to 29 January Legend of Pele Hawaiian Volcanic Activity Each island is the summit of a large volcano that rises from the seafloor. How deep is the seafloor near the islands? ~ 5000 m (~ 16,000 ft) How high are the U.S. s highest mountains? Mt. Whitney is about 4400 m (~14,500 ft) high. Oahu stands 5000 m above the seafloor and is therefore a higher mountain than Mt. Whitney. How high are the Earth s highest mountains? The Himalayas are about 8000 m (~26,000 ft). How high are the Hawai`i s highest mountains? Mauna Loa and Mauna Kea on the Big Island are about 4000 m above sea level. This makes them about 9000 m above the sea floor. Mauna Loa and Mauna Kea are 9000 m high. The Himalayas are only 8000 m high. The Hawaiian Islands were built completely by volcanic activity Our study of the Geology of Hawaii necessarily deals mostly with volcanism. Other processes are responsible for modifying the islands, but volcanism is the most important process of formation of the islands. 3

4 Definitions: A volcano is a place where molten rock (magma( magma) ) and/or gas comes to the earth's surface. Magma on the surface is often called lava. The hole through which the magma comes out is called a vent. The term volcano is often applied to the mountain built by magma. Craters are bowl- or funnel- shaped depressions They are often found at the summits of volcanoes, but They can also be found on volcano flanks 4

5 Styles of volcanic eruptions Some volcanoes may erupt only once Diamond Head is one of this kind of volcano. Other volcanoes erupt many times Kilauea is one of this kind. Styles of volcanic eruptions Some volcanoes are very gentle during eruptions Kilauea s s eruptions are very gentle Styles of volcanic eruptions Some are very violent and explosive Mt. St. Helens erupted Violently in the 1980 s 5

6 Mt. St. Helens ash cloud went up at least 50,000 ft into the upper atmosphere Some people didn t listen to warnings about the impending eruption Mt. Pinatubo in the Philippines also erupted explosively in the 1980 s Explosiveness of a volcano is a function of the fluidity or viscosity of the magma. The magmas that generate the explosive volcanoes are much more viscous than the non-explosive magmas. 6

7 Viscosity is controlled by: Chemical composition of the magma Magma temperature The amount of gas that it contains The higher the content of silica (SiO ), 2 the more viscous the magma. Hawaiian magmas: Not very explosive Low silica content Kilauea and Mauna Loa Very typical of the eruptive style of Hawaiian volcanoes Called Hawaiian-type eruptions The magmas are very fluid with small amounts of gas. Kilauea and Mauna Loa The eruptions are relatively gentle -- explosions are not common. Although there are often fire fountains, these are not explosions. Where does the magma come from? Where does the magma come from? From the Hot Spot in the Asthenosphere To magma chambers 2-5 km below the volcanoes Kilauea and Mauna Loa probably have separate magma chambers How does magma get to the surface? Melting at the Hot Spot Molten magma less dense, so rises What happens to the magma? Cools as it rises Minerals form by crystallization First to crystallize is Olivine Minerals grow larger and expand into each other to form a rock when all liquid is used up 7

8 Definitions Rocks that crystallize from magma are Igneous rocks Volcanic rocks are erupted at the surface Plutonic rocks form when magma chambers crystallize in place Differences Plutonic rocks have large crystals because they cool slowly Volcanic rocks have small crystals because they cool quickly Definitions Most volcanic rocks have dissolved gas Vesicles are the holes left when the gas bubbles out of the rock Rocks with lots of vesicles are called pumice and are light enough to float on water When a magma has lots of dissolved gas, lava shoots into the air carrying lots of bits and pieces of lava This is called a pyroclastic eruption pyro=fire clastic=fragments The rocks formed are pyroclastic rocks, also known as tephra Cinder cones and spatter cones are built by fragments falling around the vents. 8

9 Definitions Ash Cones and Tuff Cones form when hot magma erupts into cold sea water This is an eruption of the volcano Surtsey off the coast of Iceland Resulting tuff cone is asymmetric due to ash particles being blown by the wind. Compare Sursey to Diamond Head Questions? Homework #2 Due 27 January Don t forget to go to the web site and fill out the Student Information form! Next Time Please read Chapter 5 in the text 9