Unit 4 Movement of the Lithosphere Plate Tectonics, Volcanoes, Earthquakes Vocab.foreshock Divergentaftershock ConvergentP-waves TransformS-waves VolcanoL-waves Magma / lavaTsunami Intrusion / Extrusiongeologic hazard Seismology Epicenter Focus Faults strike-slip fault Reverse fault (dip-slip) normal fault Seismic waves

Theory of Plate Tectonics The theory of plate tectonics states that Earth’s crust and rigid upper mantle are broken into enormous moving slabs called plates. There are a dozen or so major plates and several smaller ones.

What are the tectonic plates? Lithosphere Is the ~100-km-thick surface of Earth; Contains crust and upper mantle; Is rigid and brittle; Fractures to produce earthquakes.

What is the asthenosphere? Asthenosphere: Is the hotter upper mantle below the lithospheric plate; Can flow like silly putty; and Is a viscoelastic solid, NOT liquid!! USGS Graphics

Convection is like a boiling pot. Plate tectonics Plates are driven by cooling of Earth. Gravity provides additional force to move plates. Modified from USGS Graphics

Convection in Earth’s interior is like a boiling pot. Plate tectonics The heated soup rises to the surface, spreads and begins to cool, and then sinks back to the bottom of the pot where it is reheated and rises again. Modified from USGS Graphics

Ridge Push and Slab Pull Ridge push – lithosphere is thicker the further away from the mid- ocean ridge, boundary slopes away from the ridge, weight creates a downward slope and older lithosphere is pushed down Slab (gravity) pull – as plates move away from ridge become cool and denser, after subduction sinks into mantle under own weight

Divergent Boundaries Divergent boundaries - places where two tectonic plates are moving apart. – Most divergent boundaries are found in rifts, or fault-bounded valleys – rift valley - narrow depression, is created when a divergent boundary forms on a continent.

New crust is generated as the plates pull apart. Occur at spreading ocean ridges and in continental rifts. Earthquakes are shallow and small. Example: East Pacific Rise (moving apart at about 15 cm/year) Examples: Atlantic mid-ocean ridge Basin and Range, USA African Rift Valley Northern Red Sea USGS sea-floor maps Divergent boundaries

Convergent Boundary Convergent boundaries - places where two tectonic plates are moving toward each other. three types of convergent boundaries: 1.Oceanic crust and oceanic crust 2.Oceanic crust continental crust 3.Continental crust and continental crust.

Subduction When an oceanic plate converges with a continental plate, the denser oceanic plate is subducted. – Oceanic-continental convergence produces a trench and a series of volcanoes along the edge of the continental plate.

Convergent Plate Boundaries Ocean /Ocean convergence (Marianas) Ocean /Continent convergence (Cascades) Continent/Continent Collision (Himalayas) Plates push together. A) The denser plate subducts, or B) two continental plates crunch together to form high mountains.

Transform Boundary transform boundary - where two plates slide horizontally past each other, deforming or fracturing the crust. – characterized by long faults and usually offset sections of ocean ridges. – The San Andreas Fault is an exception to the fact that transform boundaries rarely occur on continents.

Transform Boundaries Lithosphere is neither produced nor destroyed as the plates slide horizontally past each other. Example: San Andreas Fault, California Strike-slip fault Strike-slip fault between two spreading ridges allows the two plates to move apart.

Volcanoes

Magma mixture of molten rock, suspended mineral grains, and dissolved gases that fuels all volcanoes. forms when temperatures are high enough to melt the rocks, between 800°C and 1200°C. temperatures exist in the lithosphere and asthenosphere

Lava When magma reaches Earth’s surface it is called lava. A vent is where lava erupts through an opening in the crust. As lava flows out onto the surface, it cools and solidifies around the vent, eventually accumulating to form a mountain known as a volcano. A crater is a bowl-shaped depression at the top of a volcano that is connected to the magma chamber by a vent.

Shield Volcano A shield volcano is a mountain with broad, gently sloping sides and a nearly circular base that forms when layer upon layer of basaltic lava accumulates during non-explosive eruptions.

Cinder-Cone Volcano A cinder-cone volcano is a small, steep-sided volcano that forms when material ejected high into the air falls back to Earth and piles up around the vent.

Composite Volcanoes Composite volcanoes are large volcanoes that form when layers of volcanic fragments alternate with lava. The magma has large amounts of silica, water, and gases violently explosive.

Volcano Facts Most volcanoes form at plate boundaries. – About 80% of all volcanoes are along convergent boundaries. – About 15% are along divergent boundaries.

Earthquakes

natural vibrations of the ground caused by movement along fractures in Earth’s crust, or sometimes, by volcanic eruptions. In some instances more than 100,000 people have died and cities destroyed

Earthquakes Most occur when rocks break deep within Earth. Fractures form when stress exceeds the strength of the rocks involved.

Elastic Rebound Theory—Stick-slip Jerky motions on faults produce earthquakes Three Fs of earthquakes: Forces, Faults, and Friction.

Causes of Earthquakes Most earthquakes are caused by movements along faults. Irregular surfaces in rocks can snag and lock, causing stress to build in the rocks. When the rocks reach their elastic limit they break, and this produces an earthquake.

Stress There are three kinds of stress: Compression decreases the volume of a material. Tension pulls a material apart. Shear causes a material to twist.

Faults Three types of faults: Reverse faults form as a result of horizontal compression. Normal faults are caused by horizontal tension Strike-slip faults are caused by horizontal shear.

Normal Fractures caused by horizontal tension Movement is horizontal and vertical Horizontal movement causes crust to extend

Reverse Fractures that form as a result of horizontal compression Horizontal shortening of crust results

Strike-Slip Fractures caused by horizontal shear Movement is mainly horizontal San Andreas Fault is an example One of the reasons for the many earthquakes in the area

Seismic Wave – The vibrations of the ground during an earthquake are called seismic waves. Primary waves, or P-waves, squeeze and pull rocks in the same direction along which the waves are traveling. Secondary waves, or S-waves, cause rocks to move at right angles in relation to the direction of the waves.

Surface Waves Surface waves (called L – waves for long waves) travel along Earth’s surface, moving in two directions as they pass through rock. P-waves and S-waves, also called body waves, pass through Earth’s interior.

Seismic Waves focus is the point of failure of rocks at the depth where an earthquake originates. epicenter is the point on Earth’s surface directly above the focus.

Richter Scale – The Richter scale is a numerical scale based on the size of the largest seismic waves, used to describe magnitude. – Each successive number in the scale represents an increase in seismic-wave size, or amplitude, of a factor of 10. – Each increase in magnitude corresponds to about a 32-fold increase in seismic energy.

Modified Mercalli Scale The modified Mercalli scale, measures the amount of damage done to the structures involved, is used to determine the intensity of an earthquake. – This scale uses the Roman numerals I to XII to designate the degree of intensity. – Specific effects or damage correspond to specific numerals; the higher the numeral, the worse the damage.

Earthquake Locations The majority of the world’s earthquakes occur in relatively narrow seismic belts associated with tectonic plate boundaries. – Almost 80% of all earthquakes occur in the Circum-Pacific Belt. – About 15% take place across southern Europe and Asia. – A very small percentage of earthquakes happen far from tectonic plate boundaries