Earthquakes, Volcanoes, and Plate Tectonics

• This study Guide reviews Earthquakes, Volcanoes, Continental Drift, Seafloor Spreading, and Plate Tectonics

Vocabulary

• Key terms include Continental Drift, Plate Tectonics, Tectonic Plates, Pangaea, Lithosphere, Divergent/Convergent/Transform Plate Boundaries
• Also includes: Convection Currents, Asthenosphere, Mid-Ocean Ridge, Subduction Zone, Abyssal Plain, Seafloor Spreading, Oceanic/Continental Crust
• Also includes: Deep Sea Trenches, Magnetic Field Reversal, Earthquake, Seismograph, Epicenter, Focus, Fault, Aftershock
• Also includes: Magnitude, Tsunami, Volcano, Lava, Magma, Caldera, Magma Chamber, Pyroclastic Flow, Active/Dormant/Extinct Volcano

Earthquakes and Volcanoes

• Both epicenter and focus indicate where earthquakes occur

• Focus: Underground location of the earthquake along the fault line

• Epicenter: Location on the Earth's surface directly above the focus

• Fault: A crack in the Earth’s crust where earthquakes happen

• Foreshock: Smaller earthquakes before the main one, caused by built-up stress

• Main shock: The actual shift of plates, causes the ground to move, and has the largest magnitude

• Aftershock: Smaller earthquakes following the main shock, lasting days or weeks

• Magnitude: Strength of the earthquake and damage caused

• Measured using a seismograph and reported on the Richter Scale

• Magma: Molten rock underground

• Lava: Molten rock above ground

• Pyroclastic Flow: Fast-moving cloud of superheated ash and poisonous gas from an explosive eruption

• Active Volcano: Currently erupting or has erupted recently

• Dormant Volcano: Has not erupted recently but could

• Extinct Volcano: Will not erupt again

• Ring of Fire: Area around the Pacific Ocean where the Pacific Plate subducts under other plates

• Characterized by dangerous convergent plate boundaries producing earthquakes, tsunamis, and volcanic eruptions

Continental Drift Hypothesis

• Alfred Wegener: German meteorologist

• Noticed continents look like they fit together

• Continental Drift: Continents were once joined in a supercontinent named Pangaea and have since drifted apart

• Fossil Evidence: Glossopteris

• Glossopteris Seed-bearing plant fossils found on separated continents

• Seeds cannot travel across oceans, supporting the idea of joined continents for seed dispersal

• Fossil Evidence: Lystrosaurus, Mesosaurus, Cyanognathus

• Reptile fossils found on continents separated by oceans

• These animals could not swim

• Rock Evidence: Appalachian and Caledonian Mountain Ranges

• Mountain ranges in North America and Northern Europe with similar rock composition and age

• The ranges line up when continents are placed next to each other

• Wegener's hypothesis was rejected because he could not explain how the continents moved

Sea Floor Spreading

• Harry Hess discovered seafloor spreading

• Sonar technology from World War 2 was used to measure ocean depth and map the ocean floor

• Seafloor spreading: Convection currents push magma to the surface at the mid-ocean ridge

• Lava cools into new oceanic crust

• New crust pushes older crust away from the ridge, moving continents

• Basalt is the rock created from cooled magma, denser than continental crust

• Mid-Ocean Ridge: Mountain chain on the ocean floor where seafloor spreading occurs

• Deep Sea Trench: Valley on the ocean floor where one plate sinks under another (subduction)

• Abyssal Plain: Flat area on the ocean floor

• Continental Shelf: Shallow part of the ocean extending from the continents

• Hess’s discovery explained how continents move, supporting Wegener's hypothesis

• Normal Polarity: Iron particles in rocks point north

• Reverse Polarity: Iron particles in rocks point south

• Symmetrical pole reversal patterns on either side of the ridge confirm seafloor spreading

• Youngest sediments are at the mid-ocean ridge

• Oldest sediments are far from the ridge, near continents

• Sediment age supports seafloor spreading: new crust at the ridge has younger sediments, older sediments are pushed towards continents

Theory of Plate Tectonics

• Earth's crust is divided into plates that move on a layer of molten rock (asthenosphere)

• Plate movement causes earthquakes and volcanic eruptions

• Tectonic Plate: A piece of Earth’s crust

• Convection Current: Magma heats, rises, cools, and sinks, creating a current

• Found in the asthenosphere

• Drives the movement of molten rock, allowing plates to move

• Divergent Plate Boundary: Plates divide or move apart

• Features: Rift valleys, mid-ocean ridges

• Activity: Volcanoes, earthquakes, seafloor spreading

• Convergent Plate Boundary: Plates collide or move together

• Features: Mountains, deep sea trenches, subduction zones

• Activity: Earthquakes and volcanoes, possible tsunamis

• Transform Plate Boundary: Plates slide past each other causing earthquakes

• Activity: Earthquakes

• Subduction: One plate sinks under another at a convergent boundary, occurring at deep sea trenches

• Denser plate sinks, causing earthquakes

• Earthquakes underwater displace water, leading to tsunami formation as the water moves towards shallow coastal areas