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3/4/24, 9:46 PM VO Lesson 1. Why Do Volcanoes Have Different Characteristics?: EOSC 114 99C 2023W2 The Catastrophic Earth: Natural Disasters https://canvas.ubc.ca/courses/130808/pages/vo-lesson-1-why-do-volcanoes-have-different-characteristics?module_item_id=6430913 1/21 VO Lesson 1. Why Do Volcanoes Have Different Characteristics? Outline In this Lesson you will examine the physical and chemical characteristics of magma and lava, learn about a wide range of eruptive products, and study different types of volcanic landforms. With this information, you will go on to study volcanic hazards in the next Lesson. A. Formation of Magma/Lava B. Composition of Magma/Lava C. Volcanic Products D. Explosivity of Magmas E. Anatomy of an Explosive Eruption F. Types of Volcanoes G. Summary A. Formation of Magma/Lava What is magma made of? What is the range of products erupted from volcanoes? Which magmas are the most explosive? What sequence of events takes place during an explosive eruption? The answers to all of these questions depend on the chemical and physical properties of the melted source rock. Magma is formed by melting pre-existing rock in Earth's interior. The resulting material is comprised of melt, crystals of minerals, and gas bubbles. Magma is less dense than solid rock, so it will rise towards the surface of the Earth. Some magmas are trapped in the crust, whereas other magmas travel through fractures in the crust and eventually extrude onto Earth's surface. For magma to rise through the crust it must be: (1) Less dense than the crust; (2) Runny enough to flow (i.e., it must have low viscosity); and (3) Hot enough to stay liquid. All these properties depend on the magma's composition and temperature. Magma that has erupted and cools to form solid rock is called lava . The Table and Figure below list the common elements of the Earth's crust, which were all formed by magma rising to the Earth's surface. Table VO.1 Common Elements of the Earth's Crust ELEMENT COMMON FORM, SYMBOL WEIGHT % Oxygen oxide, O 45.20 Silicon silica, SiO 27.20 -2 2

3/4/24, 9:46 PM VO Lesson 1. Why Do Volcanoes Have Different Characteristics?: EOSC 114 99C 2023W2 The Catastrophic Earth: Natural Disasters https://canvas.ubc.ca/courses/130808/pages/vo-lesson-1-why-do-volcanoes-have-different-characteristics?module_item_id=6430913 2/21 Aluminum aluminum, Al 8.00 Iron iron, Fe and Fe 5.80 Calcium calcium, Ca 5.06 Magnesium magnesium, Mg 2.77 Sodium sodium, Na 2.32 Potassium potassium, K 1.68 Titanium titanium, Ti and Ti 0.86 Hydrogen hydrogen, H 0.14 Phosphorus phosphate, (PO ) 0.10 Manganese manganese, Mn 0.10 Figure VO.1 Elemental abundance of the Earth's crust by weight percent. CYU VO.1 Check Your Understanding For magma to rise through the crust it must be (choose all that apply) A) Cool enough to solidify B) Runny enough to flow C) Less dense than the crust D) Full of gas bubbles to float +3 +3 +2 +2 +2 +2 +1 +3 +4 +1 4 -3 +2

3/4/24, 9:46 PM VO Lesson 1. Why Do Volcanoes Have Different Characteristics?: EOSC 114 99C 2023W2 The Catastrophic Earth: Natural Disasters https://canvas.ubc.ca/courses/130808/pages/vo-lesson-1-why-do-volcanoes-have-different-characteristics?module_item_id=6430913 3/21 E) More dense than the crust F) Hot enough to stay liquid B. Composition of Magma/Lava Chemical, mineralogical, and physical composition of magma. As listed in the Table above, most magmas are composed of varying proportions of 8 elements: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K), with O and Si being present in the greatest amounts. Lesser amounts of titanium (Ti), manganese (Mn), phosphorus (P), and hydrogen (H) are present. The amounts of the elements in rocks are reported as weight percentages of the oxides (e.g., SiO rather than Si and O) because oxygen bonds with all other common elements. The Table below compares the elemental composition of continental and oceanic crusts and shows that average continental crust is higher in SiO than oceanic crust, a fact that you will find influences the locations of explosive volcanoes on Earth. Table VO.2 Crustal Elements in Weight-Percent Oxides CONTINENTAL CRUST OCEANIC CRUST SiO 60.2 SiO 48.7 Al O 15.2 Al O 16.5 Fe O 2.5 Fe O 2.3 FeO 3.8 FeO 6.2 CaO 5.5 CaO 12.3 MgO 3.1 MgO 6.8 Na O 3.0 Na O 2.6 K O 2.9 K O 0.4 When magma cools and crystallizes, these chemical components bond together to form minerals. A mineral is a naturally occurring element or compound that has an ordered internal structure, a characteristic chemical composition, crystal form, and physical properties. Because Si and O are the most common elements in Earth's crust and upper mantle, most of the minerals that crystallize out of magma are silicate minerals . Because the O anion is very large, you can think of the crust of our planet as being a sea of O ions with all of the smaller cations in the interstices. Silicate minerals have interesting structures. The main building block of all silicate minerals is the (SiO ) tetrahedron, a geometric form in which one Si with a +4 charge is bonded to four O's, each with a -2 2 2 2 2 2 3 2 3 2 3 2 3 2 2 2 2 -2 4 -4

3/4/24, 9:46 PM VO Lesson 1. Why Do Volcanoes Have Different Characteristics?: EOSC 114 99C 2023W2 The Catastrophic Earth: Natural Disasters https://canvas.ubc.ca/courses/130808/pages/vo-lesson-1-why-do-volcanoes-have-different-characteristics?module_item_id=6430913 4/21 charge. The Si-O bond is strong. Common silicate minerals in volcanic rocks include feldspar, quartz, micas, hornblende, pyroxene, and olivine. A graphical representation of the silicate radical is shown below. Figure VO.2 The silicate radical is composed of a silicon atom (purple ball) linked to four oxygen atoms (red balls). The sizes of the crystals that form in a solidified magma are dependent on how long it takes the magma to cool. Magmas that cool slowly underground, in crustal magma chambers, form intrusive rocks . The minerals crystallize in an interlocking (interconnected) texture of grains that are big enough to be seen with the naked eye. It might take hundreds to thousands of years to grow crystals that large. In contrast, magma that extrudes at Earth's surface cools quite quickly in terms of geologic time, commonly solidifying over days to weeks. These erupted magmas, called lavas, typically have very small sizes of crystals, most of which are too small to be seen with your eye. The rocks that form are called extrusive rocks . The resulting texture is also called extrusive or fine-grained. C. Volcanic Products 1. Lava flows. When magma erupts at Earth's surface, it can form an array of eruptive products, depending on its chemical composition. Lava flows result when magma that is relatively low in gas content (less than a few percent) erupts effusively at Earth's surface. The surface of a lava flow may be ropy-textured, which is called pahoehoe (pronounced pah-hoy-hoy). This is a Hawaiian term that comes from the prevalence of these flows on Hawaii. A lava flow with a rubble-y flow top consisting of broken fragment of lava is called an ’a’a (pronounced ah-ah) flow. This too is a Hawaiian term, meaning "a painful surface to walk on"!

3/4/24, 9:46 PM VO Lesson 1. Why Do Volcanoes Have Different Characteristics?: EOSC 114 99C 2023W2 The Catastrophic Earth: Natural Disasters https://canvas.ubc.ca/courses/130808/pages/vo-lesson-1-why-do-volcanoes-have-different-characteristics?module_item_id=6430913 5/21 Figure VO.3 Lava flows from shield volcanoes in Hawaii: (left) pahoehoe and (right) ’a’a. Figure VO.4 (left) Lava flow in a channel showing fluid but fairly slow flow. (right) Photo of a lava flow stopped between buildings in Heimaey, Iceland in 1974. These flows rarely kill people because they move very slowly. View an animation of pahoehoe flow. Title: CSAV Hawaii: Kilauea Lava Cascades Duration: 2 minutes Source: https://youtu.be/IkVr1e5PI54 (https://youtu.be/IkVr1e5PI54)