Transcript Earth Science - Westmoreland Central School

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.
2
– 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
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
•2 Plate Tectonics
– Earth’s crust is divided into several sections
(plates) that move across Earth’s surface.
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.
1. The age of igneous rocks increases as the
distance from the mid-ocean ridges increases.
2. 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.