PowerPoint Lectures to accompany Physical Science, 8e Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 19 Building Earth’s Surface Start 18

Core Concept The surface of Earth is involved in plate tectonic processes that result in an ongoing building-up of the surface.

Interpreting Earth’s Surface Principle of uniformity –“The present is the key to the past.” –Rocks are changed today by the same processes that changed them in the past. –Replaced catastrophic models of previous thinkers –People didn’t want to accept that Earth is over 4 billion years old –Catastrophic events contribute nonetheless Volcanoes, earthquakes, meteorite impacts, …

Diastrophism The process of deformation that changes the Earth’s surface Produces structures such as plateaus, mountains and folds in the crust Related to volcanism (the movement of magma) and earthquakes Basic working theory is plate tectonics

Stress and Strain Stress Force tending to compress, pull apart or deform a rock Three stress forces 1.Compressive stress Plates moving together 2.Tensional stress Plates moving apart 3.Shear stress Plates sliding past each other Strain Adjustment to stress Three strain types 1.Elastic strain Returns to original shape 2.Plastic strain Molded or bent Do not return to original shape 3.Fracture strain Rock cracks or breaks

Stress and Deformation Possible material responses to stress 1.No change 2.Elastic change with recovery 3.Plastic change with no recovery 4.Breaking from the pressure Rock variables 1.Nature of the rock 2.Temperature of the rock (cold rocks tend to break) 3.Speed of stress application 4.Confining pressure 5.Temperature & confining pressure increase with depth 1.Rocks under these conditions undergo plastic strain 6.Can better withstand compressional than pulling apart

Folding Sedimentary rocks –Originate from flat sediment deposits –Layers usually horizontal Folds –Bends in layered bedrock –Result of stress produced plastic strain –Widespread horizontal stress can produce domes and basins –Anticline: arch-shaped structure –Syncline: trough-shaped

Folding Folds –Bends in layered bedrock –Result of stress produced plastic strain –Widespread horizontal stress can produce domes and basins –Anticline: arch- shaped structure –Syncline: trough-shaped

Faulting Fault –Produced by relative movement on opposite sides of a crack –Footwall: mass of rock below the fault –Hanging wall: mass of rock above the fault –Fault plane: surface between the footwall and hanging wall Joints are fractures with no appreciable displacement

Classes of Faults Normal fault –Hanging wall has moved down relative to the footwall –Caused by tension forces –Related features (B) Graben –Block surrounded by normal faults drops down (A) Horst –Block surrounded by normal faults is uplifted –Many fault block mountains of west US

Other Faults Reverse fault –Hanging wall moved upward relative to footwall –Result of horizontal compressive stress Thrust fault –Reverse fault with a low- angle fault plane Faults provide information on the stresses producing the formation

Earthquakes Quaking, shaking, vibrating or upheaval of the ground Result from sudden release of energy from stress on rocks Vibrations are seismic waves Most occur along fault planes when one side is displaced with respect to the other

Causes of Earthquakes Elastic rebound theory –Two plates press tightly together –Friction restricts motion –Stress builds until friction or rock rupture strength is overcome –Stressed rock snaps suddenly into new position

Locating and Measuring Earthquakes Focus –Actual origin of seismic waves Epicenter –Location on Earth’s surface directly above the focus Seismograph –Instrument used to detect and measure earthquakes –Detects three kinds of waves 1.P-wave (longitudinal) 2.S-wave (transverse) 3.Surface wave (up and down )

Seismic Data P-waves travel faster than S-waves Difference in arrival times correlates to distance from earthquake Triangulation used to pinpoint epicenter and focus

Classification of Earthquakes Based upon depth of focus 1.Shallow-focus earthquakes Down to 70 km deep (within the depth of the continental crust) 85% of all earthquakes (surface rocks more brittle; more plate friction near surface) 2.Intermediate-focus earthquakes 70 to 300 km deep Upper part of the mantle 3.Deep-focus earthquakes 350 to 700 km deep Lower part of upper mantle About 3% of all earthquakes

Measuring Earthquake Strength Effects: structural damage to buildings, fires, landslides, displacement of land surfaces, tsunami (tidal wave) Mercalli scale –Relative intensity –I (not felt) to XII (total destruction with visible ground waves)

Measuring Earthquake Strength Richter scale –Based on swings in seismograph recordings –Logarithmic scale –3 (not felt); 9 (largest measured so far)

Earthquake Safety During the Shaking Don’t panic If indoors, stay there. Stay away from glass. Do not use any other flames. If outside, move away from buildings. Stay in open. If in car, bring to a stop as soon as possible but stay in car. After the Shaking Check but do not turn on utilities. Turn on radio or TV. Stay off telephone unless to report emergency. Stay out of damaged buildings. Don’t go sightseeing.

Origin of Mountains Mountains –Elevated parts of Earth’s crust rising abruptly above the surrounding surface –Created by folding and faulting of crust –Three basic origins 1.Folding 2.Faulting 3.Volcanic activity

Folded and Faulted Mountains Domed mountains –Begin as a broad arching fold –Overlying sedimentary rocks weather away, leaving more resistant granite peaks –Black Hills, SD –Adirondacks, NY

Folded and Faulted Mountains Folded sedimentary rocks form mountains such as some areas of the Rockies

Folded and Faulted Mountains Fault block mountains –Rise sharply (upthrust) along steeply inclined fault planes –Weathering erodes sharp edges –The Tetons & Sierra Nevadas

Volcanic Mountains Volcano A hill or mountain formed by the extrusions of lava or rock fragments from magma below Structure: vent, crater, lava flow

Types of Volcanoes Shield volcano –Constructed of solidified lava flows –Broad, gently sloping cones –Hawaiian Islands Cinder cone volcano –Constructed of rock fragments (cinders) –Steeper and smaller than shield volcanoes –Sunset Crater, Flagstaff AZ Composite volcano –Alternating layers of cinders, ash and lava flows with volcanic mud –Cascades Mountains

Other Features Most magma remains underground Cools and solidifies to form intrusive rocks Batholith –Large amount of crystalized magma –Stock: small protrusion from a batholith –Batholith intrusions can cause hogbacks Related processes: dikes, sills, laccoliths,…

Overall Picture Mountain ranges are composites of many different processes, each uniquely structured –Folding –Faulting –Volcanic activity Especially apparent along converging plate boundaries