# Chapter 4 The Dynamic Crust

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Chapter 4 The Dynamic Crust
Earth Science Chapter 4 The Dynamic Crust

Earth’s Crust and Interior
Earthquake waves help determine properties of Earth’s interior such as: thickness composition temperature density pressure

Earth’s Crust and Interior
The speed of earthquake waves change abruptly at the interfaces between layers of the Earth. Waves speed up due to increasing density. Example: speed increases at the interface between the crust and asthenosphere. This interface is the Mohorovicic discontinuity, or “Moho” interface.

Earthquakes An earthquake is the shaking of Earth’s crust caused by rapid moving of rocks. Point beneath Earth’s surface where an earthquake originates is the focus Point on Earth’s surface directly above the focus is called the epicenter 2

Earthquakes When an earthquake occurs, it generates energy waves, called seismic waves. Seismograph – instrument used to detect and record earthquake waves Seismogram –recording of earthquake waves

Earthquakes Main types of earthquake waves: P-waves (primary waves)
Particles vibrate in same direction as wave Travel fast through solids, liquids & gasses S-waves (secondary waves) Particles vibrate at right angles to wave Travel slower than p-waves Only travel through solids

Earthquakes Since the outer core is liquid, S-waves do not travel through it and P-waves refract. This results in a shadow zone where no seismic waves are received.

Earthquakes P-waves travel fast and arrive at a seismic station ahead of S-waves. The difference in travel times can be used to determine the distance from a seismograph station to the epicenter of an earthquake.

Earthquakes

Earthquakes To determine the exact epicenter data from three seismic stations must be used to construct three circles of possible locations. The point where all three circles intersect is the epicenter.

Earthquakes

Earthquakes The intensity and energy of an earthquake are measured on two different scales. Richter Scale – measure of energy released Scale from 1 to 10 Each step increases 10 times in magnitude Modified Mercalli Scale – measure of effect Scale uses Roman numerals from I to XII Higher numbers indicate greater damage

Evidence for Crustal Movement
Zones of frequent crustal activity can be located on the Earth’s surface. Most earthquakes, volcanoes, ridges, trenches, and mountains occur here.

Evidence for Crustal Movement
Continental Drift Continental land masses have been moving across the Earth’s surface for millions of years Evidence supporting continental drift Continents look like they fit together Similar rocks, minerals, and fossils are found on different continents in areas that look like they once fit together.

Evidence for Crustal Movement

Evidence for Crustal Movement

Evidence for Crustal Movement

Evidence for Crustal Movement
Plate Tectonics Earth’s crust is divided into several sections (plates) that move across Earth’s surface. 2

Evidence for Crustal Movement
6 min video Plates move due to convection cells produced by unequal heating of the fluids within the asthenosphere.

Evidence for Crustal Movement
There are three types of plate interactions Divergent boundaries Convergent boundaries Transform boundaries

Evidence for Crustal Movement
Divergent Boundaries Plates move apart from each other New crust is formed here Most commonly found at mid-ocean ridges and develop deep rift valleys.

Evidence for Crustal Movement
Convergent Boundaries Plates move toward each other When 2 continental plates meet mountains are formed. Himalayan Mountains formed like this

Evidence for Crustal Movement
When an ocean plate and a continental plate converge subduction occurs. Denser ocean plate slides under the less dense continental plate. Ocean trench develops at point of subduction. Volcanoes and mountains form along the edge of the continental plate.

Evidence for Crustal Movement
Transform Boundaries Plates slide horizontally past one another Tension builds as plates slide and when tension is released earthquakes occur. San Andreas Fault is an example

Evidence for Crustal Movement

Evidence for Crustal Movement
The ocean floor appears to be spreading based on two major pieces of evidence. The age of igneous rocks increases as the distance from the mid-ocean ridges increases. Parallel strips of igneous rocks on either side of the mid ocean ridge show periodic reversals of magnetic polarity.

Evidence for Crustal Movement

Evidence for Crustal Movement
Deformed rock strata Sedimentary rocks normally form in horizontal layers. Sedimentary layers that are not horizontal provides evidence of change.

Evidence for Crustal Movement
Syncline Anticline

Evidence for Crustal Movement
Displaced rock strata occurs along faults (cracks in rock formations). The 3 types of faults are: Normal Reverse Transform or strike-slip

Evidence for Crustal Movement
Normal Fault

Evidence for Crustal Movement
Reverse Fault

Evidence for Crustal Movement
Transform or Strike-slip Fault

Evidence for Crustal Movement
Displaced marine fossils are sometimes found high up in the mountains. Benchmarks labeled with date and elevation are set into ground and checked periodically.