Types of Stress, Earthquake Waves, and Igneous Phenomena

Types of Stress

• Compression: pressure from both sides
• Tension: pressure from pulling apart
• Shear: opposite pressure, land grinding past other land

Strength of the Crust

The strength of the continental crust (quartz and feldspar) increases with depth then weakens.

• Brittle (surface/less depth, temp, and pressure): stress makes rock fracture, but rock remains intact
• Ductile (more depth, temp, and pressure): stress makes rock flow in solid state, rock resists deformation.

Joint

The crack where the rock was pulled apart

Stresses that form joint:

• Burial and tectonic forces: force from every side of the rock, straight cracks
• Cooling and contraction: ex-when mud dries it cracks
• Unloading: pressure in an uplift, mountain

Fault

Rocks have slipped passed one another

Movement on Fault

• Normal fault: pressure pulling apart, left side moves up, right side moves down
• Reverse fault or thrust fault: pressure pushing in, left moves down, right moves up
• Strike-slip fault: two sides move horizontally relative to one another.
• Left lateral: block on opposite side moves to the left
• Right lateral: block on opposite side moves to the right

Folds

Layers can be folded

• Anticline: peak of fold (dome)
• Syncline: bottom peak of fold (basin)

Earthquake

Movements within the Earth’s crust cause stress to build up at point of weakness and rocks to deform. Seismic waves radiate from the earthquake’s epicenter.

Elastic Rebound Theory

An explanation of how energy is released during an earthquake. As plates move in opposite directions, the rocks on opposing sides of the fault line a subjected to shear stress. They deform until they fracture and they return to almost their original shape except shifted slightly. The distance of the slight shift is called the slip.

Types of Earthquake Waves

• Body waves:
  • Primary Wave (P-wave): compresses the material in the direction the wave moves; fastest wave. Moves front and back.
  • Secondary Wave (S-wave): shears material side to side, perpendicular to the direction the wave moves. Up and down.
• Surface waves:
  • Vertical Surface wave (Rayleigh wave): compresses material up and down in elliptical motion. Circles.
  • Horizontal Surface wave (Love waves): shears material side to side.

Earthquakes are detected using seismometers that record seismic waves.

To locate the epicenter of an earthquake, you measure the distance between the first p-wave and the first s-wave

To determine magnitude of an earthquake, you find the amplitude of the biggest wave on the seismogram and see where it is on the richter scale.

Eastern Hemisphere Earthquakes

• Africa-Europe convergence
• East African Rift
• Mid-Ocean ridges
• India collision
• Subduction of oceanic plates
• Australia collision

Western Hemisphere Earthquakes

• Alaska subduction
• San Andreas fault
• Central America subduction
• Mid-ocean ridges
• South America Subduction
• India collision
• Mid-Atlantic ridge
• Caribbean subduction

Earthquake Damage

• Tsunami and other flooding
• Landslide
• Rupture
• Structural damage
• Liquefaction
• Fire
• Earth composition
  • Upper layer is crust: continental and oceanic
  • Mantle: thickest layer
  • Core: deepest layer, iron and nickel (molten outer core, solid inner core)

Lithosphere

Crust (continental + oceanic) + uppermost mantle. Uppermost 100 km of the mantle that is stronger and more rigid than the asthenosphere (lower, hot and weak).

Isostasy

Relationship between crustal thickness and elevation. Thick blocks higher than thin blocks. Denser materials are lower.

Continental Drift

First suggested by glaciologist Alfred Wegener, 1915.

• Shapes fit together
• Fossils of the same land creatures found on different continents
• Glacial features indicate glaciers coming from directions that are now oceans

Earth’s Magnetism

Caused by convection currents in the liquid outer core of the Earth. Normal: North is up. Reversed: South is up. Lava flows record Earth’s magnetic field.

Sea Floor Spreading

• High elevation of ridges: more magma comes up through the ridges then cools at that height making the ridges taller. Thin lithosphere.
• Magnetic stripping: as magma comes out the normal magnetic stripes move away from the ridge and also create new stripes. The stripes are recorded in the floor over time.
• Gradually increasing age of basalt of ocean floor and overlying sediments from the ridge to continental margin.

Three Types of Plate Boundary

• Divergent (spreading, move apart)- earthquakes, mild volcanism
• Convergent (subduction, move toward each other)- earthquakes, explosive volcanism
• Transform (strike-slip, move horizontally past one another) -earthquakes, no volcanism

Closing of an Ocean-Continental Collision

Subduction of oceanic part of plate, subduction bring continents closer, continents then collide (wide deformation, thick crust-high elevation)

Formation of Linear Islands

Ex. - Hawaii

• Plate moves over hotspot
• Volcano forms over the hot spot
• The volcano then becomes inactive as that area of the plate moves away from the hot spot, leaving behind an island.
• The plate subsides as it cools, so islands become seamounts; underwater mountains.

Oceanic Plateaus

• Rising mantle plume at hot spot
• Submarine flood basalts pour onto seafloor
• Plateau forms over several million years.

Chapter 6 Igneous Phenomena - Volcanoes and Intrusions

Factors causing melting in mantle

• Decompression melting: when two plates move apart, they create a space that can be filled by hot rock that rises buoyantly from below. As this hot rock rises, the pressure on it decreases and the rock can melt.
• Hydrous melting: water being added to aid melting.

Melting at Different Kinds of Plate Boundaries

• Partial melting: occurs when only a portion of a solid is melted, different minerals might have different melting points.
• Melting in continental rifts: crust pulls apart, solid asthenosphere rises to fill the gap and melts.
• Melting in continental collision: burial and heating of subducted crustal rocks, melting during subduction

Magma Rising

Partial melting of source, accumulates into rising magma body, forms magma chambers (solidifies or rises), eruption as lava or ash.

Lava Properties

• High viscosity: lower temp, and abundant silica chains, lava piles up.
• Low viscosity: higher temp, fewer silica chains, lava spreads out.

Types of Volcanic Edifice

• Scoria cone: tall, hole at top
• Shield volcano: tall, flatter than scoria, magma fed by fissures
• Composite Volcano: tall, fat source of magma
• Volcanic dome: looks like a bulge
• Flood basalts: fissures in the ground release magma, flat

Volcanic Eruption Products

• Gas
• Ash: large pillars
• Lava: viscous
• Pyroclastic flows: ash flowing down volcano
• Lahars: mudflows

Volcanic Explosivity Index (VEI)

Measure size of plume, and age.

• Scale: 0-8, Hawaiian > Strombolian > Volcanian > Plinial
• Montserrat = VEI 3
• Mt. Pelee = VEI 4
• Krakatau = VEI 6
• Santorini = VEI 7
• Yellowstone Caldera = VEI 8

Atlantis: says it was sunk by earthquake

Mosaic legend: river turned to blood, hail and fire, three days of darkness, cloud by day and pillar of fire by night, collapse of the Red Sea. (earthquake? tsunami?)

Igneous Intrusions

• Dike: newer layer cuts across other layers
• Sill: newer layer cuts across but stays under newest layers
• Batholith: very large igneous intrusion extending deep in Earth crust