What are Plates The Earths crust and upper

What are Plates? • The Earth’s crust and upper mantle (Lithosphere) are broken into sections called plates Plates move around on top of the mantle like rafts A section of the lithosphere that slowly moves over the asthenosphere, carrying pieces of continental and oceanic crust.

What are Plate Tectonics? • Earth’s surface is made up of many plates that float on the soft rock of the mantle. As the mantle moves, the plates also move. • Plates move only a few centimeters each year. • When one plate moves, it affects the other plates. • As plates move around, they cause great changes in the Earth’s surface, such as mountains, valleys, volcanoes, and earthquakes!

What is the Theory of Plate Tectonics? The theory that pieces of Earth’s lithosphere are in constant motion, driven by convection currents in the mantle. • Plates move slowly in different directions q Cause different geologic events (like earthquake, volcano, etc. )

What makes the plates move? Convection Currents in the mantle move the plates as the core heats the slowly-flowing asthenosphere (the elastic/plastic-like part of the mantle).

Plate Boundaries The edges of Earth’s plates meet at plate boundaries. v. Extended deep into the lithosphere

Types of Boundaries Wegener’s Theory of Continental Drift

What are three types of boundaries? • Divergent Boundaries • Convergent Boundaries • Transform Boundaries

Divergent Boundaries A plate boundary where two plates move away from each other. RIFTING causes SEAFLOOR SPREADING

What happens next at Divergent Boundaries? • A geologic feature or event… May form RIFT VALLEYS on continents SEA-FLOOR SPREADING in the ocean

How is the rock pushed at convergent boundaries? A plate boundary where two plates move towards each other. Boundaries between two plates that are colliding This stress is called COMPRESSION

Convergent Boundaries • Places where plates crash (or crunch) together or subduct (one sinks under)

There are 3 types of Convergent Boundaries… Type 1 Ocean plate colliding with a less dense continental plate Subduction Zone: The process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary.

Type 3 • A continental plate colliding with another continental plate • Have Collision Zones: – A place where folded and thrust faulted mountains form.

What else happens at Convergent Boundaries? VOLCANOES occur at subduction zones

• May form Mountain Ranges. These are Folded Mountains, like the Himalayas or the Rockies.

Transform Boundaries A plate boundary where two plates move past each other in opposite direction.

Transform Boundaries • Where plates slide past each other Above: View of the San Andreas transform fault

What happens next at Transform Boundaries? • May cause Earthquakes when the rock snaps from the pressure. • A famous fault @ a Transform Boundary is the San Andreas Fault in California.

How is the rock broken at Transform Boundaries? • Rock is pushed in two opposite directions (or sideways, but no rock is lost) • This stress is called SHEARING

Faults • FAULT – Breaks in Earth’s crust where rocks have slipped past each other.

What happens when the rock is squeezed from the Stress of Compression? • A REVERSE FAULT • Rock is forced upward as it is squeezed.

What happens when the rock SNAPS from the Stress of Tension? A Normal Fault (fault is a break in Earth’s crust) Rock drops down as it breaks

What happens when the rock is sheared (or “cut”) from the Stress of Shearing? • A STRIKE-SLIP FAULT • Rocks on each side of the fault slip past each other as they break.

What Happens When Plates Move? Plate Tectonics Convergent Plate Boundaries plates crunch together and release energy Divergent Plate Boundaries plates pull apart and new crust is formed mountains develop, volcanoes erupt, and earthquakes can happen Transform Boundaries plates slide past each other valleys and volcanoes develop, earthquakes can occur Lots of earthquakes!

What are Earthquakes? • The shaking or trembling caused by the sudden release of energy • Usually associated with faulting or breaking of rocks • Continuing adjustment of position results in aftershocks

What is the Elastic Rebound Theory? • Explains how energy is stored in rocks – Rocks bend until the strength of the rock is exceeded – Rupture occurs and the rocks quickly rebound to an undeformed shape – Energy is released in waves that radiate outward from the fault

Seismic Wave • Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs. • There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves and surface waves.

Body Waves: P and S waves • Body waves – P or primary waves • fastest waves • travel through solids, liquids, or gases • compressional wave, material movement is in the same direction as wave movement – S or secondary waves • slower than P waves • travel through solids only • shear waves - move material perpendicular to wave movement

Types of Waves • Compression wave • Transverse Wave • Seismic Wave – Body Waves • Primary or p-wave – Compression wave • Secondary or s-wave – Transverse wave – Surface • Love wave • Rayleigh wave

Body Waves • P Waves (compression wave) • The first kind of body wave is the P wave or primary wave. This is the fastest kind of seismic wave. The P wave can move through solid rock and fluids, like water or the liquid layers of the earth. It pushes and pulls the rock it moves through just like sound waves push and pull the air.

Body Waves • S wave (transverse wave) • The second type of body wave is the S wave or secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock. This wave moves rock up and down, or side-to-side.

Surface Waves: R and L waves • Surface Waves – Travel just below or along the ground’s surface – Slower than body waves; rolling and side-to-side movement – Especially damaging to buildings

How are Earthquakes Measured? • Earthquakes are measured by instruments called seismographs. It has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the heavy weight does not. The spring that it is hanging from absorbs all the movement. The seismograph records the difference in position between the shaking part and the motionless part. • The recording is called a seismogram. It is used to determine how large the earthquake was. A short wiggly line that wiggles very little means a small earthquake. A long wiggly line that wiggles a lot means a large earthquake.

Seismographs record earthquake events

What Happens When an Earthquake Starts? • The sudden release of energy from an earthquake sends out several different shaking movements, or seismic waves. • Surface waves are ripples of energy that spread outward when rocks slip past each other along a fault, just like throwing a stone into a calm pond. • Body waves are seismic waves that travel through material rather than over its surface. There are two types of body waves: P-waves and S-waves. • The P-wave is also known as the sound wave. It travels through the interior of the Earth rather than over the surface as a series of squeezes and stretches. P-waves reach everywhere around the Earth after about 20 minutes. • The S-wave, or shear wave, produces a shaking motion, like if you tied a rope to a poll and shook the other end side to side. S-waves can only travel in solid material.

How is an Earthquake’s Epicenter Located? Seismic wave behavior – P waves arrive first, then S waves, then L and R – Average speeds for all these waves is known – After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

The Richter Magnitude Scale Description Richter Magnitudes Micro Less than 2. 0 Very Minor 2. 0 -2. 9 Usually not felt, but recorded. Minor 3. 0 -3. 9 Often felt, but rarely causes damage. Light 4. 0 -4. 9 Noticeable shaking of indoor items, rattling noises. Significant damage unlikely. 5. 0 -5. 9 Can cause major damage to poorly constructed buildings over small regions. Slight damage to well-designed buildings. Strong 6. 0 -6. 9 Can be destructive in areas up to about 100 miles across in populated areas. Major 7. 0 -7. 9 Can cause serious damage over larger areas. Great 8. 0 -8. 9 Can cause serious damage in areas several hundred miles across. Rare Great 9. 0 or greater Moderate Earthquake Effects Microearthquakes, not felt. Devastating in areas several thousand miles across.

The Focus and Epicenter of an Earthquake • • The point within Earth where faulting begins is the focus, or hypocenter The point directly above the focus on the surface is the epicenter

How is an Earthquake’s Epicenter Located? • • • Three seismograph stations are needed to locate the epicenter of an earthquake A circle where the radius equals the distance to the epicenter is drawn The intersection of the circles locates the epicenter

How are the Size and Strength of an Earthquake Measured? • • Intensity – subjective measure of the kind of damage done and people’s reactions to it – isoseismal lines identify areas of equal intensity Modified Mercalli Intensity Map – 1994 Northridge, CA earthquake, magnitude 6. 7

Where Do Earthquakes Occur and How Often? ~80% of all earthquakes occur in the circum-Pacific belt – most of these result from convergent margin activity – ~15% occur in the Mediterranean-Asiatic belt – remaining 5% occur in the interiors of plates and on spreading ridge centers – more than 150, 000 quakes strong enough to be felt are recorded each year

How Do Scientists Know Where an Earthquake Has Happened? • The location below the Earth’s surface where the earthquake starts is called the hypocenter. • The location directly above the hypocenter on the Earth’s surface is called the epicenter. • It take three seismographs to locate an earthquake. Scientists draw a circle on a map around the three seismographs where the radius of each is the distance from the station to the earthquake, the intersection of those three circles is the epicenter.

How is an Earthquake’s Epicenter Located? Time-distance graph showing the average travel times for P- and Swaves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves

The Economics and Societal Impacts of EQs • • Building collapse Fire Tsunami Ground failure Damage in Oakland, CA, 1989

Take-Away Points 1. Earthquakes generate waves that travel through the earth 2. Earthquakes occur when rocks slip along faults 3. Faults are classified by the kinds of movement that occur along them 4. Earthquakes don’t kill people, buildings kill people 5. Magnitude and Intensity 6. Seismic waves are used to map the earth’s interior 7. Predicting earthquakes is not yet possible