1. Volcanoes Chapter 18

2. Magma  molten rock, suspended mineral grains, and dissolved gases that fuels all volcanoes. Magma temperatures must be high enough to melt the rocks (800°C - 1200°C). base of the lithosphere in the asthenosphere.

3. –Pressure increases with depth –Pressure affects melting point in a direct relationship. How Magma Forms Pressure Magma –Despite high temperatures most of the rocks in Earth’s lower crust and upper mantle do not melt to form magma. –WHY don’t they melt?

4. –water also influences melting point. How Magma Forms Water Magma –Looking at the graphic to the right, what can you conclude is the relationship between water composition and melting point?

5. –A number of factors determine the composition of magma. Magma Viscosity: resistance to flow the higher the viscosity  the thicker the magma.

6. Viscosity  The viscosity of magma and lava depends on both temperature and composition. Magma –The hotter the magma or lava, the lower the viscosity. –What type of relationship is this? Graph it? –Magmas and lavas high in silica have higher viscosities than magmas and lavas low in silica.

7. 3 Types of Magma Magma 1. Basaltic magma : –same composition as basalt –volcanoes that make up the Hawaiian Islands –typically forms when rocks in the upper mantle melt. –rises relatively rapidly to Earth’s surface –reacts very little with crustal rocks because of its low viscosity –volcanoes fueled by basaltic magma erupt relatively quietly.

8. 3 Types of Magma  2. Andesitic magma – same composition as andesite – fuels Mount St. Helens in Washington State and Tambora in Indonesia – found along continental margins – oceanic crust is subducted into Earth’s mantle – formed from oceanic crust or oceanic sediments. – 60 percent silica – intermediate viscosity. – intermediate eruptions.  2. Andesitic magma – same composition as andesite – fuels Mount St. Helens in Washington State and Tambora in Indonesia – found along continental margins – oceanic crust is subducted into Earth’s mantle – formed from oceanic crust or oceanic sediments. – 60 percent silica – intermediate viscosity. – intermediate eruptions.

9. 3 Types of Magma  3. Rhyolitic magma – Same composition as granite – fueled the dormant volcanoes in Yellowstone National Park. – molten material rises and mixes with the overlying silica- and water-rich continental crust. – high viscosity – fuels very explosive volcanoes.  3. Rhyolitic magma – Same composition as granite – fueled the dormant volcanoes in Yellowstone National Park. – molten material rises and mixes with the overlying silica- and water-rich continental crust. – high viscosity – fuels very explosive volcanoes.

10. Magma

11. Intrusive Activity  Magma, because it is molten, is less dense than surrounding rocks. Intrusive Activity This density difference forces magma to move upward contacting with, or intruding into, the overlying crust.

12. Effects of Magma Intrusive Activity  Intruding magma can affect the crust in several ways: Intrusive Activity A.force the overlying rock apart and enter the formed fissures. B.cause blocks of rock to break off and sink into the magma C.melt the rock into which it intrudes.

13. Plutons Intrusive Activity intrusive igneous rock bodies exposed at Earth’s surface (How does this happen?) classified based on their size, shape, and relationship to surrounding rocks.

14. Types of Plutons Intrusive Activity Batholiths –the largest plutons –irregularly shaped –coarse-grained igneous rocks –covering at least 100 km 2 –take millions of years to form. Stocks irregularly shaped plutons similar to batholiths but smaller in size. –Both batholiths and stocks cut across older rocks and generally form 10–30 km beneath Earth’s surface. Laccolith relatively small; they are, at most, up to 16 km wide. mushroom-shaped pluton (round top and flat bottom) Magma intrusion into parallel rock layers close to Earth’s surface.

15. Plutons Intrusive Activity sill few centimeters to hundreds of meters in thickness, magma intrudes parallel to layers of rock. dike few centimeters to several meters wide and up to tens of kilometers long that cuts across preexisting rocks. –textures of sills and dikes vary, many are coarse grained, indicating that they formed deep in Earth’s crust and cooled slowly.

16. Anatomy of a Volcano  When magma reaches Earth’s surface it is called lava. Volcanoes vent where lava erupts through an opening in the crust. Lava cools and solidifies around the ventaccumulating to form a mountain known as a volcano. crater bowl-shaped depression at the top of a volcano connected to the magma chamber by a vent.

17. Anatomy of a Volcano  Volcanic craters are usually less than 1 km in diameter. Volcanoes Calderas large depressions up to 50 km in diameter forms when the summit or the side of a volcano collapses

18. Types of Volcanoes  The appearance of a volcano depends on two factors: Volcanoes Based on these two criteria, three major types of volcanoes have been identified: –material that forms the volcano –type of eruptions that occur –Shield volcanoes –Cinder-cone volcanoes –Composite volcanoes

19. Volcanoes shield volcano mountain with broad, gently sloping sides nearly circular base layer upon layer of basaltic lava accumulates during nonexplosive eruptions.

20. Volcanoes cinder-cone volcano generally small steep-sided material ejected high into the air falls back to Earth and piles up around the vent. magma contains more water and silica than shield volcanoes more explosive in nature.

21. Volcanoes –Composite volcanoes large volcanoes form when layers of volcanic fragments alternate with lava. magma contains large amounts of silica, water, and gases  violently explosive.

22. Summary: Sizes and Shape Volcanoes –Shield volcanoes are by far the largest, and cinder-cone volcanoes are the smallest. –Cinder-cone volcanoes have the steepest slopes, while shield volcanoes have the gentlest slopes. –The slopes of cinder-cone and composite volcanoes are concave, and the slopes of shield volcanoes are straight.

23.  Tephra : rock fragments thrown into the air during a volcanic eruption. Volcanoes Dust and ash: less than 0.25 mm and0.25–2 mm. Lapilli, or “little stones” 2–64 mm in diameter angular volcanic blocks rounded or streamlined volcanic bombs

24. Volcanoes pyroclastic flow: cloud of volcanic gas, dust, and other tephra traveling at speeds of nearly 200 km/h. –temperature at the center of a pyroclastic flow can exceed 700°C.

25. Where do volcanoes occur?  Most volcanoes form at plate boundaries. Volcanoes –80 percent along convergent boundaries. –15 percent along divergent boundaries. –5 percent occurs far from plate boundaries.

26. Volcanoes –involving oceanic plates –creates subduction zones –magma generated is forced upward through the overlying plate –forms volcanoes when it reaches the surface. two major belts: Circum-Pacific Belt (aka Pacific Ring of Fire) Mediterranean Belt. Convergent volcanism

27. Where do volcanoes occur? Volcanoes

28. Divergent volcanism Volcanoes fractures and faults form as plates separate. Magma is forced upward filling fractures These are called rift zones Most rift volcanism occurs under water What are these underwater rift volcanoes also known as?

29. Hot Spots Volcanoes –Some volcanoes are located far from plate boundaries and form as the result of hot spots. –not move laterally –results in a trail of progressively older volcanoes that formed as a plate moved over a hot spot. unusually hot regions of Earth’s mantle high-temperature plumes of mantle material rise toward the surface.

30. Hot Spots Volcanoes –The Hawaiian Islands continue to rise above the ocean floor as the Pacific Plate moves slowly over a hot spot. –The rate and direction of plate motion can be calculated from the positions of volcanoes in a chain that has formed over a hot spot. EXAMPLE: