Main Topics: Module 3

Main Topics: Module 3

Overview of the main topics you will encounter in Module 3 

Many students find that they need a bit more effort to master the new material in this module than in Module 2 and when those students put in the effort, they reported that they enjoyed it and learned a lot! 

Subduction

• We saw that the sea floor is made at spreading ridges such as the one through the Gulf of California that almost reaches Death Valley. 
• The sea floor is basalt—just what you’d get if you melted a little bit of mantle rock, and let the melt rise and then freeze. 
• Although the sea floor is generally less dense than the mantle, very old, cold sea floor can be dense enough to sink into the hot mantle. 
• As new sea floor is made at spreading ridges, Earth does not get bigger and bigger like a balloon being inflated, so the old sea floor must be lost somewhere. 
• The sea floor is lost where it sinks back into the mantle at Subduction Zones. All sorts of things happen there: 
  • Moving rocks stick, then slip, giving earthquakes; 
  • As the rocks go down, they are squeezed until the arrangement of atoms in the minerals changes to one that takes up less space; sometimes this rearrangement affects a lot of rock at once, giving an “implosion” earthquake (the deepest quakes may be of this type); 
  • Mud and rocks and even parts of islands are scraped off the downgoing slab, piling up, something like groceries at the end of a check-out conveyor belt (that’s what makes up Olympic National Park); 
  • Some mud and rock are carried down a bit and then squeezed back out, something like squeezing a watermelon seed between your fingers until it squirts out (you may find some of this squeezed-back-out material at Olympic, too); 
  • Some mud and water and rock are carried even farther down, where the water lowers the melting point of the rocks (just as adding water to flour speeds cooking in the oven); 
  • A little melted rock is generated from this subducted water and rock, and the melt rises and feeds volcanoes (such as Crater Lake and Mt. St. Helens) that form lines or arcs near continent edges or offshore; 
  • This melt is richer in silica and poorer in iron than the basalt it comes from, and forms a rock called andesite (the name was chosen because the volcanoes in the Andes were formed this way), often with granite forming under the volcanoes from melt that did not erupt. 

A Bit of Review

• The Earth includes a colder, harder upper part called the lithosphere, which includes the crust at the top and the upper mantle beneath, and which floats on the deeper, softer part of the mantle called the asthenosphere; convection occurs in the asthenosphere; 
• The lithosphere is broken into a few big plates and many little ones; most “action” is at plate edges; 
• Plates pull apart at spreading ridges, splitting continents to make space for volcanic activity that makes basaltic sea floor; 
• Plates come together at subduction zones, where the cold sea floor is dense enough to sink into a hot mantle, where scraped-off materials pile up to make edges of continents (Olympic National Park, etc.), and where water and sediment taken down lower the melting point of rock, feeding silica-rich (andesitic) volcanoes; 
• Plates also may slide past each other, moving horizontally at transform faults, such as the San Andreas; 
• Earthquakes are generated by stick-slip behavior where some rocks move past other rocks; earthquakes also may occur deep in subduction zones where minerals being taken down rearrange suddenly into denser forms. 

Introducing Volcanoes

• Towers of rising rock from very deep in the Earth (often from the core-mantle boundary) feed “hot spots;” 
• Plates drift over the tops of these hot spots, and hot spots occasionally punch through, making lines of volcanoes, which often are oceanic islands (seamounts); 
• Hotspots are from the mantle, and their basaltic composition is very similar to sea floor; 
• If we could look down into the mantle, when a new hotspot is first rising and before its top reaches the surface, it looks like a mushroom; when it reaches the surface, the head feeds huge (state-sized) lava flows called flood basalts, and after the flood basalts form, the stem continues to flow to make lines of volcanoes ;  
• Hawaii is the classic example of a line of hot-spot volcanoes (its flood basalt has already gone down a subduction zone); 
• Yellowstone also is a hot spot; the head of the Yellowstone hot spot covered eastern Washington and Oregon with basalt, but the recent eruptions at Yellowstone have come from lava that was modified coming through the crust so that more silica is erupted than for Hawaii. 

Volcano Characteristics

• In melted rock, silicon and oxygen make SiO₄ tetrahedra that try to polymerize (stick together in chains, sheets, etc.) to make lumps; 
• The lumpiness can be reduced by making the melted rock hotter or adding iron, both of which help basalt flow easily and not explode;  
• This helps Hawaii be a much-wider-than-it-is–high shield volcano, because the flows spread out easily;  
• The lumpiness of melted rock can be reduced by adding volatiles (water, CO₂ , etc.), but when these escape near the surface, the lava gets lumpy again and won’t flow easily and may plug the system; more volatiles may get trapped beneath the plug and then explode; 
• This helps build steep stratovolcanoes, composed of alternating layers of steep (“lumpy”) flows and pyroclastics (blown-up bits from explosions). 

Volcanic Hazards

• Volcanoes are hazardous in many ways, including:  
  • Pyroclastic flows, deadly hot, fast-moving rock-gas mixes; 
  • And pyroclastics, big rocks that can fall on your head if you are close to an eruption, or smaller ones that can plug up jet engines on a passing plane; 
  • Also, poison gases, that can kill many people very quickly; 
  • And landslides and mudflows, that can bury whole cities; 
  • Tsunamis, giant waves that can devastate coasts; 
  • Climate change, especially short-term cooling from particles blocking the sun and frosting crops, but over geologic history, there have been terrible extinctions from too much heat from long-term volcanic carbon dioxide (the recent changes in carbon dioxide in the air are from humans, not volcanoes, as we will see later); 
• These hazards are especially dangerous from subduction-zone volcanoes; the flows on Hawaii mostly block roads and burn houses, and usually, you could run away from one and stay safe.  

Predicting Volcanoes

• It is fairly easy for geologists to figure out where volcanic eruptions are likely; 
• Often, but not always, it is possible to figure out that an eruption will happen in the next days or hours; 
• Predictions are valuable and have saved many lives, but are imperfect and probably will remain so far into the future; 
• And people get mad if you tell them to leave and then nothing happens; 
• The United States Geological Survey especially handles predictions in the USA; they are highly valuable and greatly under-appreciated; 
• Lots of people continue to build and live where dangers await.