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Formation of the Hawaiian Islands Volcanoes, Magma, and Hot Spots Topics • Hawai’i • Ring of Fire • Hot Spots • Decompression Melting • The Hawai’i Hot Spot Topics • Hawai’i • Ring of Fire • Hot Spots • Decompression Melting • The Hawai’i Hot Spot Decompression Melting Source of Magma Question Why is magma generated in Earth’s interior? Question Why is magma generated in Earth’s interior? Question What is the source of heat inside Earth? Question What is the source of heat inside Earth? Earth’s Interior Heat Source Earth’s Interior Heat Source The primary heat source for the interior of Earth is radioactivity 238U 235U 87Rb 232Th 40K Earth’s Interior Heat Source The primary heat source for the interior of Earth is radioactivity 238U 235U 87Rb 232Th 40K Periodic Table Radioactive Decay Fission fragment n n n n Uranium-235 nucleus Unstable nucleus Fission fragment Energy Geothermal Gradient Geothermal Gradient Temperature increases with increasing depth Geothermal Gradient Temperature increases with increasing depth Called the geothermal gradient Geothermal Gradient Temperature increases with increasing depth Called the geothermal gradient The geothermal gradient in the crust averages 30oC/km Geothermal Gradient Temperature increases with increasing depth Called the geothermal gradient The geothermal gradient in the crust averages 30oC/km The geothermal gradient decreases in the mantle to an average of 0.1oC/km Geothermal Gradient Temperature increases with increasing depth Called the geothermal gradient The geothermal gradient in the crust averages 30oC/km The geothermal gradient decreases in the mantle to an average of 0.1oC/km Core temperature > 5000oC Average Geothermal Gradient of the Crust Variations in Geothermal Gradient Geothermal Gradient Most rocks melt at temperatures > 1000oC Most rocks melt at temperatures > 1000oC Most of Earth’s interior is hotter than 1000oC Most rocks melt at temperatures > 1000oC Most of Earth’s interior is hotter than 1000oC However Earth is almost entirely solid rock Most rocks melt at temperatures > 1000oC Most of Earth’s interior is hotter than 1000oC However Earth is almost entirely solid rock Only the outer core is mostly liquid Most rocks melt at temperatures > 1000oC Most of Earth’s interior is hotter than 1000oC However Earth is almost entirely solid rock Only the outer core is mostly liquid Why don’t the rocks in the mantle and core melt? Mostly Solid Earth Pressure Prevents Melting Most rocks inside Earth are hot enough to melt if not for the tremendous pressure of the overlying rocks Magma Generation Why is magma generated in the upper asthenosphere in a hot spot? The process is called decompression melting Decompression melting Decompression melting Rocks are poor conductors of heat Decompression melting Rocks are poor conductors of heat Ascending hot rocks retain heat Decompression melting Rocks are poor conductors of heat Ascending hot rocks retain heat Rise to upper asthenosphere Decompression melting Rocks are poor conductors of heat Ascending hot rocks retain heat Rise to upper asthenosphere The T of the ascending rocks is hot enough and P low enough for a small amount of melting Decompression melting Rocks are poor conductors of heat Ascending hot rocks retain heat Rise to upper asthenosphere The T of the ascending rocks is hot enough and P low enough for a small amount of melting Decompression melting Rocks are poor conductors of heat Ascending hot rocks retain heat Melting Rise to upper asthenosphere The T of the ascending rocks is hot enough and P low enough for a small amount of melting Partial Melting Partial Melting Only a small percentage of the rocks melt Partial Melting Only a small percentage of the rocks melt Generally 1-4% of rocks melt Partial Melting Only a small percentage of the rocks melt Generally 1-4% of rocks melt Magma is less dense than solid rock Partial Melting Only a small percentage of the rocks melt Generally 1-4% of rocks melt Magma is less dense than solid rock Magma rises to lithosphere The Hawaii Hot Spot Next The Hawaii Hot Spot Back
