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VOLCANOES. CHAPTER 7. 7.1 VOLCANOES AND PLATE TECTONICS. Section Objectives: Describe the formation and movement of magma Define volcanism List three locations where volcanism occurs. I. VOLCANISM.
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VOLCANOES CHAPTER 7
7.1 VOLCANOES AND PLATE TECTONICS • Section Objectives: • Describe the formation and movement of magma • Define volcanism • List three locations where volcanism occurs
I. VOLCANISM • Volcanism is any activity that includes the movement of magma toward or onto the surface of the earth. • Asthenosphere is very hot but stays mostly solid because of the great pressure of the surrounding rock. • Magma forms where decreased pressure allows the rock to melt • Lower density of magma compared to the surrounding rock and allows it to rise toward the surface.
I. VOLCANISM • Lava: Magma that reaches Earth’s surface • Vent: The opening through which the molten rock flows onto the surface • Volcano: The structure that is formed by the vent and volcanic material that builds up on Earth’s surface around the vent
volcano lava vent cone
II. MAJOR VOLCANIC ZONES Three major locations where volcanism occurs: • Subduction Zone • Mid-ocean Ridge 3. Hot Spot
VOLCANIC ERUPTIONS Section Objectives: • Summarize the relationship between lava types and the force of volcanic eruptions. • Describe the major types of pyroclastic material. • Identify the three main types of volcanic cones. • Summarize the events that may signal a volcanic eruption.
VOLCANIC ERUPTIONS Mafic lava textures: pahoehoe: “ropy”; forms as lava first forms a crust aa: jagged chunks; forms when crust is too thick pillow lava: lava that cools very quickly, usually underwater.
“Quiet” eruptions: Mafic lavas flow more easily gases escape easily lava flows out onto the surface Explosive eruptions: Felsic lava is thicker gasses cannot escape easily gas pressure builds up and creates a more violent eruption pyroclastic material is thrown into the air. KINDS OF ERUPTIONS FOR EACH TYPE OF LAVA
VOLCANIC ROCK FRAGMENTS Pyroclastic Material • Associated mainly with felsic eruptions because they are explosive. • Dust: less than .25 mm in diameter; can travel completely around the Earth in the upper atmosphere • Ash: less than 2 mm diameter • Lapilli: less than 64 mm diameter • Blocks: larger than 64 mm; some as big as a house • Bombs: formed when lava is thrown into the air and solidifies while in the air
Shield Volcano Here are 4 of the volcanoes that make up the big island of Hawai'i. They are Mauna Kea (MK), Mauna Loa (ML), Hualalai (H), and Kohala (K). The photo was taken from near the summit of East Maui volcano (EM). These are the largest volcanoes on Earth
Composite Cone Popocatepetl
Fujiyama, Japan Mt. Merapi, Indonesia Mt. Merapi, Indonesia Mt. St. Helens, Washington Mt. St Helens, Washington Mt. Shasta, California Mt. Shasta, California
VOLCANIC FEATURES craters and calderas • crater = the funnel shaped pit at the top of a volcanic vents • caldera = when the magma chamber below a volcano is emptied, the volcanic cone may collapse leaving, a large basin shaped depression
crater VOLCANIC FEATURES
caldera VOLCANIC FEATURES
What controls eruptions? The amount of trapped gas • When more gas is trapped, pressure builds up • MORE PRESSURE MAKES A MORE VIOLENT ERUPTION The amount of silica in the magma • Silica makes the magma thicker • Thicker magma does not flow easily so can get stuck underground, allowing pressure to build up • THICKER MAGMA MAKES A MORE VIOLENT ERUPTION
Predicting Volcanic Eruptions • Increase in the number and strength of small earthquakes (magma working its way upward) • Slight bulging of the surface of a volcano (upward movement of magma just below the surface) • Information from previous eruptions
7.3 EXTRATERRESTRIAL VOLCANISM • Mars • volcanic cones on Mars • most famous is Olympus Mons • Mars seems to be seismically active • Io • one of Jupiter’s moons • has active volcanoes
FAMOUS ERUPTIONS • ELDFELL • MOUNT ST. HELENS • KILAUEA • VESUVIUS • MOUNT PINATUBO • CRATER LAKE