Geologic Activity Tectonics Volcanism Earthquakes Our Changing Earth

Geologic Activity Tectonics Volcanism Earthquakes

Our Changing Earth:

What is Plate Tectonics? Earths crust is made of pieces = plates ► These plates move slowly over time ► § Plates can move away from each other § Plates can move toward each other § Plates can slide sideways against each other Tectonics is the movement of Earth’s plates ► Tectonics only affects the Lithosphere ► § The plates are about 100 km thick under the ocean § The continents are about 200 km thick ► Plate Movement: § § § The plates “float” on the molten rock (Mantle) under the crust Large, slow moving convection currents push and pull the plates Convection Currents are cause by hotter magma rising and cooler magma sinking

Interior of Earth: Notice how thin the crust actually is Asthenosphere (Upper Mantle) Area of convection currents

Plate Movement ► Due to opposite directions of convection currents the plates move together or apart

Evidence for Tectonics ► Africa and South America: § Fit like puzzle pieces; Fossil beds match up ► Earthquakes and Volcanoes: § Only occur in specific areas (called belts) § The belts mark the plate boundaries ► Magnetic Shifts: § Over time the magnetic field of the Earth reverses § Bands of rock on the ocean floor show this reversal ► Rock Age: § Ocean ridges are much younger than the rocks aside of them ► Spreading Centers: Mid-Atlantic Ridge § New rock material pushes older rock away from the ridge IR scan of Mid-Atlantic Ridge

Types of Plate Boundaries ► Diverging Boundaries: § Called Spreading Centers- Plates are moving apart § Most found along mid-ocean ridges (Ex: Mid-Atlantic Ridge) ► Sliding Boundaries: § Plates move parallel to each other in opposite directions § Fault lines form: Faults are breaks or cracks in the lithosphere § San Andreas Fault in California is an example ► Converging Boundaries: § Plates move toward each other § Continental plates converging = Collision boundary ► Continents bunch up against each other = mountain building ► Himalayan Mountains (Indian and Eurasian Plates colliding) § Ocean plates converging = Subduction boundary ► One plate slides under another = deep sea trenches

Boundary Types: Subduction

Shield cone Volcanism: Magma is molten rock underground ► Magma that reaches the surface is lava ► Volcano: a build up of cooled lava that could erupt ► Types of Eruptions ► § Rift eruptions: occur along narrow crust fractures ► Normally “gently” flowing ► Form shield cones, and pillow lava § Explosive eruptions: occur along subduction zones and hot spots ► Normally very violent, Form cinder cones ► Hot Spots: Areas within a plate of magma rising to the surface ► The Hawaiian Islands were formed by a hot spot

Volcanoes & Earth’s Moving Plates Most volcanoes are dormant (non-active) ► There are over 600 active volcanoes ► § Active volcanoes spew smoke, steam, ash, etc. § Active does not mean erupting

Active Eruptions: ► 1980: Mt. Saint Helens (Washington) § § § ► 63 people died Meters of ash deposited over a wide area Heat from eruption melted snow = flooding 1991: Mt. Pinatubo (Philippines) § 900 people died § 27 metric tons of sulfur dioxide & ash thrown into atmosphere ► Decreased global temperatures (WHY ? ) ► Caused record loss of ozone layer Mt. St. Helens Eruption

Kilauea: Has been erupting for centuries ► World’s most actively erupting volcano ► Located above a hot spot, not a fault line ► § Hot Spot: Abnormally hot area under Earth’s crust § Reason for Hawaiian Island chain

Hot Spots: ► Hawaiian Islands: § § Geologists believe some area of the mantle are hotter than others Melt rock, which is forced upward as lava Volcanoes form a line as the crust moves Kauai, Oahu, Molokai, Maui, Hawaii ► All are extinct volcanoes except Hawaii ► Eventually Hawaii will go dormant, then extinct ► A new Island will eventually form in front

What causes Volcanoes? ► Magma forms under the Earth’s surface § Heat & pressure melt bedrock ► Magma is less dense than rock § Slowly forces upward (Like air bubble in syrup) § At weak spots magma forces it’s way to the surface ► Vent: opening from which magma flows out ► Lava: magma reaching the surface ► Lava cools forming igneous rock ► As the lava builds up a volcano forms ► Steep walled central depression = crater

Eruptions & Forms of Volcanoes ► Types of Eruptions: § Volcanoes erupt in different ways ► Some explosive & violent (explosive) ► Some quietly (rift) § 2 major factors ► Amount of water vapor & other gases in magma ► Type of magma (Basaltic or Granitic) Pillow lava

Trapped Gases: ► Trapped gases under pressure are volatile § Once pressure is removed, gasses escape quickly § Depending on magma type – results vary ► Gases easily escape = gentile eruption ► Gases slowly escape = explosive eruption § Due to rapid expansion at the surface

Magma Composition: ► Basaltic (Mafic) Magma: § § § ► Contains less silica = very fluid (low viscosity) Produces quiet, non-explosive eruptions “Pours out like water” Gases escape easily Normally found in rift valleys & over hot spots Granitic (Felsic) Magma: § § § High silica content = very thick (high viscosity) Produces violent, explosive eruptions “Oozes out like cold syrup” Gases don’t escape easily Normally found along subduction zones

Magma Water Content Water content = steam ► Steam pressure = violent eruptions ►

Forms of Volcanoes: ► Volcano forms depend on type of eruption § Magma type also plays a roll § Volcanoes have 3 basic forms ► Shield ► Cinder cones ► Composite Shield Cinder Cone

Shield Volcanoes ► Quiet, flowing eruptions allow basaltic lava to flow over large areas § Build up is minimal § Slowly building in height over long periods

Cinder Cones: Explosive eruptions throw lava high in the air ► The material hardens into tephra ► § § § Tephra varies in size from ash to cinders Tephra forms steep-sided, loosely consolidated mounds Cinder cones can grow quickly ► Mexican farmer had a “sink hole” in one of his fields ► On night the “sink hole” began spewing hot ash ► In a few days, volcano several hundred meters high grew ► Now: Paricutin volcano covers the field

Composite Volcanoes: ► Some volcanoes switch eruption types § Erupt gently, then explosively, then gently again § Forms alternating layers § Lava – tephra – lava, and so on

Igneous Rock Features: ► Intrusive Features: § Most volcanic activity is under ground § Batholith: Large intrusive igneous rock formation ► Forms when magma cools before reaching the surface ► Erosion expose batholiths as large Granitic domes § Dikes: Vertical magma intrusions § Sills: Horizontal magma intrusions ► Sills can form overlying rocks to bulge = Laccoliths § Volcanic Neck: hardened lava inside a volcanic vent ► Erosion wears away the volcano ► Exposes the igneous rock in the vent § Caldera: volcanic crater implodes into magma chamber ► Caldera normally fill with rain water = crater lake

10. Volcanic Neck

Earthquakes: ► Shaking or tearing of the Earth’s crust releasing energy § Most are unnoticeable- weaker than thunder shaking a house § Result from pressure build up along plate boundaries ► Where Earthquakes start § Focus: exact point under the surface where the crust breaks § Epicenter: point on the surface where earthquake began

Forces Inside Earth ► Causes of Earthquakes § All materials can stretch or bend & return to original shape § If a material is forced, it will not return to it’s original shape ► Elastic deformation: object returning to original shape after stretched ► Plastic deformation: object will not return to original shape ► Elastic Limit: the amount of force required to cause Plastic deformation § Passing Elastic Limit Causes Faulting ► Rocks act much like a rubber band ► Rocks will bend and stretch over time, up to a point ► Rocks will eventually crack & break = Fault ► Rocks on either side of the fault move in different directions

Forces at work: ► Faults are created by specific forces § § § Compression: squeezing (compaction) of materials together Tension: stretching (elongation) of materials Shear: Slipping in opposite directions Map of recorded earthquakes over the past 50 years

Types of Faults: ► Normal Faults: § Tensional forces pull rocks apart § Result in rocks behind the fault dropping down ► Reverse Faults: § Compressional forces push rocks together § Results in rocks behind the fault rising up ► Strike-Slip (transform) Faults: § Shearing forces push sideways along a fault § Results in surface moving sideways in opposite directions ► San Andreas Fault (CA): Rocks build up pressure as they try to move ► Rocks get twisted & deformed, jagged edges catch on each other ► Force continues to build until Earthquakes occur

Base Height

Seismic Waves: ► Earthquakes produce 3 types of waves § P waves: Primary or compressional waves ► Fastest moving waves; Travel through any material (rock, water, air) ► Move material side to side (stretching and squeezing) § S waves: Secondary or shear waves ► Move slower than P waves; Only travel through rock ► Move material front to back (shaking back and forth) § L waves: Lateral or surface waves ► Can only affects the surface; Rolls the surface causing ripples

Locating Earthquakes: ► Seismograph: an instrument that detects and records earthquake waves ► Seismic data is used to triangulate position ► Where the 3 intersect Epicenter

Destruction by Earthquakes ► Measuring Earthquakes § How should Earthquakes be measured? § Deaths, Cost of Destruction, Energy? § This is an age old question ► Seismologists: scientists who study earthquakes § § They use seismographs to record vibrations The size of the vibrations = magnitude Not all seismologists use the same scale Most use the Richter scale

Measuring Earthquakes: ► Quakes Most widely used: Richter Scale § Measures the amount of energy released § Each number is 32 times more powerful than the one before

Predicting Earthquakes: Many have tried: Many have failed Did you Know? ? 500, 000 Earthquakes yearly 100, 000 are felt 2, 000 cause destruction Less than 100 are devastating

Alaska: 1964 California: 1989

Earthquake Destruction: Tsunami: Giant tidal wave cause by under ocean earthquake Wave can be over 30 m (90 ft) tall

Indonesian Tsunami

Earthquake Safety: First: know the area’s earthquake history ► Second: Quake-safe your home ► § Place heavy objects on the floor or lowest shelves § Secure all gas & electric appliances ► During an earthquake § § Stay away from windows Avoid places where objects could fall on you Avoid open or fallen power lines Get to a secure area ► Cellars are not always the best…the rest of the house could fall on you

Living on a Fault ► Seismic-Safe structures: § Resistant to vibrations produced by Earthquakes § Use several different technologies ► Rubber cushions under the foundation ► Shock absorbing struts ► Materials that will bend but not break (high elasticity) ► People Should be prepared § § § Areas on fault lines are prone to Earthquakes California (San Andreas Fault) has strict building codes Seismic Safe structures are expensive