Volcanoes and earthquakes Caribbean Studies
Objectives • To explain what are earthquakes and volcanoes • To identify major area of earthquake and volcanic activity. • To assess the positive and negative effects of earthquakes and volcanoes.
What are Earthquakes? • The shaking or trembling caused by the sudden release of energy • Usually associated with faulting or breaking of rocks • Continuing adjustment of position results in aftershocks
Anatomy of an Earthquake • Focus (or hypocentre): the center of energy release. • Epicentre: the point on the ground surface immediately above the focus (closest point on the surface to the focus).
Two kinds of waves from earthquakes • P waves (compressional) 6–8 km/s. Parallel to direction of movement (slinky), also called primary waves. Similar to sound waves. • S waves (shear) 4–5 km/s. Perpendicular to direction of movement (rope); also called secondary waves. Result from the shear Strength of materials. Does not pass through liquids.
The amount of damage created by an earthquake depends on several factors. • The earthquake’s strength • The kind of rock and soil that underlie an area • The population of the area • The kind of buildings in the area • The time at which the earthquake occurs
Seismologists • Seismologists study earthquakes. They can determine the strength of an earthquake by the height of the wavy line recorded on the paper. • The seismograph record of waves is called a seismogram. • The Richter scale is used to calculate the strength of an earthquake.
Intensity and Magnitude of Earthquakes Magnitude • Often measured using the Richter scale • Based on the amplitude of the largest seismic wave • Each unit of Richter magnitude equates to roughly a 32-fold energy increase
Intensity and Magnitude of Earthquakes Intensity • A measure of the degree of earthquake shaking at a given locale based on the amount of damage • Most often measured by the Modified Mercalli Intensity Scale
What is a Volcano? • A volcano is a weak spot in the crust where molten material comes to the surface • Magma reaching the surface is called lava. • A volcanic eruption is the process wherein volcanic materials such as lava, fragmented rocks or gases are emitted or ejected through a crater, vent or fissure on to the earth's surface to form new deposits.
Volcanic Terms • A volcano not known to have erupted within modern history is classified as an extinct volcano. • A volcano that has been known to erupt within modern times but is now inactive is classified as a dormant volcano. • An active volcano is one that erupts wither continually or periodically
Where do volcanoes occur? • Volcanoes occur most frequently at plate boundaries. • Some volcanoes, like those that form the Hawaiian Islands, occur in the interior of plates at areas called hot spots . • The greatest number of volcanoes occur on the ocean floor along spreading ridges. • Over 80% of those on land occur at edges of continents, or subductionzones, where one plate dives, or subducts, under another plate.
Why do volcanoes occur? Temperatures in the mantle are hot enough to melt rock into magma magma. •Less dense than the solid rock around it, magma rises and some of it collects in magma chambers magma chambers. •As the magma rises, pressure decreases allowing trapped gasses to expand and propel the magma through openings in the Earth’s surface causing an eruption.
What causes volcanoes? Hot Spots
Types of Volcanoes Shield volcano Cinder cone volcano Composite volcano
What Erupts from a Volcano? • Pyroclastic material • Rock fragments created by eruptions • magma explodes from volcano and solidifies in the air • existing rock is shattered by powerful eruptions EXPLOSIVE Lapilli Volcanic bombs Volcanic blocks Volcanic ash