Living Physical Geography, 1st Edition c. 2014 W.H. Freeman &Co.By Bruce Gervais
Chapter 14 Geohazards: Volcanoes and Earthquakes
Geohazard A hazard posed to people by the physical Earth
Examples of Geohazards • Volcanic eruptions • Earthquakes • Tsunamis • Floods • Landslides • Rockfalls
Tsunami A large ocean wave triggered by an earthquake or other natural disturbance
About Volcanoes • Active Volcanoes: Those that have erupted in the last 10,000 years and could erupt again • Dormant Volcanoes: Those that have not erupted for 10,000 years or more, but could awaken again • Extinct Volcanoes: Those that have not erupted for tens of thousands of years and can never erupt again
Three Types of Volcanoes • Stratovolcanoes (composite volcanoes) • Shield volcanoes • Cinder Cones
Stratovolcano (composite volcano): A large, potentially explosive cone-shaped volcano composed of alternating layers of lava and pyroclast Pyroclasts, or pyroclastic materials, encompass any fragmented solid material that is ejected from a volcano.
Shield Volcano • Abroad, domed volcano formed from many layers of fluid basaltic lava Shield volcanoes are much larger than stratovolcanoes, and can be difficult to identify as volcanoes from the ground.
Cinder Cone • Small, cone-shaped volcanoes consisting of pyroclasts that settle at the angle of repose The angle of repose is the steepest angle at which loose sediments can settle.
Three Types of Volcanic Products • Lavas • Pyroclasts • Gases
Lavas Mafic Intermediate Felsic • Temperature of about 1,000°C to 1,200°C (1,800°F to 2,200°F) • Silica content of 50% or less • Low viscosity and flows easily • Temperature of about 800°C to 1,000°C (1,500°F to 1,800°F) • Silica content between 50% and 70% • Medium viscosity • Temperature of about 650°C to 800°C (1,200°F to 1,500°F) • Silica content of 70% or more • High viscosity restricts its ability to flow
Mafic Lava Types • When mafic lava solidifies into smooth, billowy lobes over the surface, it is called Pāhoehoe. • When it takes on a blocky, rough surface, it is called ʻAʻā.
Types of Pyroclasts • Volcanic ash: Fine volcanic powder consisting of pulverized rock particles and solidified droplets of lava • Lapilli: Marble-to golf ball-sized cooled fragments of lava
Types of Pyroclasts • Pumice: A lightweight, porous rock with at least 50% air content
Types of Pyroclasts • Volcanic bombs: A streamlined fragment of lava ejected from a volcano that cooled and hardened as it moved through the air • Volcanic blocks: A fragment of rock from the volcano’s cone that is ejected during an explosive eruption
Volcanic Gases • Gas is not a pyroclastic material, but gas emissions produce pyroclasts. • As gas forcefully exits a volcano, it blasts lava and rock debris into the air, generating pyroclasts.
Volcanic Landforms • Volcanic landforms are typically very conspicuous on Earth’s surface. • Some common volcanic landforms are volcanic mountains, columnar jointing, large igneous provinces, and calderas.
Columnar Jointing A geometric pattern of angular columns that forms from joints in basaltic lava during cooling
Large Igneous Province An accumulation of flood basalts that covers an extensive geographic area
Caldera A large depression that forms when a volcano’s magma chamber empties and collapses after the volcano erupts
Two Kinds of Eruptions: • Effusive: Anonexplosive eruption that produces mostly lava, (e.g., shield volcanoes) • Explosive: An eruption that sends rock, ash, and volcanic gases high into the troposphere, or even into the stratosphere, (e.g., many stratovolcanoes)
Ranking Volcano Strength: The Volcanic Explosivity Index (VEI) • Ranks volcanic eruption magnitude based on the amount of material a volcano ejects during an eruption
Volcanoes' Two Greatest Threats • Lahars • Pyroclastic Flows
Lahar A thick slurry of mud, ash, water, and other debris that flows rapidly down a snowcapped stratovolcano when it erupts
Pyroclastic Flow • Arapidly moving avalanche of searing hot gas and ash
Can Scientists Predict Volcanic Eruptions? • Scientists can sometimes predict an eruption within weeks or months if a volcano gives warning signs.
The Pacific Ring of Fire • More than 60% of the Pacific Ocean’s margins are subduction zones with active and dangerous stratovolcanoes. • Although shield volcanoes are found in the Pacific Ring of Fire, explosive stratovolcanoes are the most common type of volcano there.
Tectonic Hazards: Faults and Earthquakes • Although usually less noticeable than volcanic hazards, earthquakes are as dangerous as volcanoes, or even more so.
Three Types of Faults Normal Fault Reverse Fault Strike-Slip Fault • Occurs where one block moves horizontally in relation to another block as a result of shearing (lateral) force • A result of tensional force (extension) as two pieces of Earth’s crust, called fault blocks, are pulled apart • Results from compressional force, which pushes one block upward in relation to another block
Fault Indicators Reverse and normal faults create a fault scarp, or cliff face, that results from the vertical movement of the fault blocks.
Where strike-slip faults cross linear features, those features may be offset by fault movement, (left-lateral and right-lateral).
How Do Earthquakes Occur? • Earthquakes occur when geologic stress exceeds friction, and the crust breaks (either along a preexisting fault or along a new fault), and the blocks move. • As each block moves, the built-up stress energy is released and travels through the crust as seismic waves, resulting in an earthquake.
Earthquake Terminology • Focus: The location of initial movement along a fault during an earthquake • Epicenter: The location on the ground’s surface immediately above the focus of an earthquake, where earthquake intensity is usually greatest • Foreshock: A small earthquake that sometimes precedes a larger earthquake • Aftershock: A small earthquake that follows the main earthquake