Here are some questions to help you review for the quiz. Please note, I gave you some more specific guidelines in class to let you know what material we'll cover. Answer as many questions as you can. If you can't answer a whole question or part of a question, be prepared to ask for help on Friday.
What is a volcano?
A volcano is a weak place in the crust where magma erupts onto Earth’s surface.
What is magma?
Magma is an underground molten mixture of rock-forming substances, gases and water vapor that originates from the hot mantle.
What is lava?
Lava is magma that has reached the surface of earth.
How many active volcanoes are there?
Currently there are about 600 active volcanoes on land.
What is the “Ring of Fire?”
The ring of fire is a major volcanic belt that rims the Pacific Ocean.
Where do volcanoes occur most often?
Volcanoes exist on weak spots in the Earth’s lithosphere. Thus, volcanoes occur most often at plate boundaries. Most volcanoes occur along diverging plate boundaries (such as the mid-ocean ridge) and at converging plate boundaries where one plate subducts beneath another plate (such as the edges of continents). Volcanoes can also occur over hot spots.
What is a volcanic belt?
A volcanic belt is a string of volcanoes that occur along a plate boundary.
What is subduction?
Subduction is when one plate slides under another plate. Subduction occurs when continental crust and oceanic crust are moving towards one another; the more dense oceanic crust subducts under the continental crust.
What are volcanoes that occur along diverging plate boundaries like?
They form along mid-oceanic ridges, which occur when two oceanic plates are moving away from one another. These ridges make up long, underwater rift valleys that wind through the oceans. Lava pours out as the crust splits in the middle of the rift valleys.
What are volcanoes like at converging plate boundaries?
(Continental-Oceanic) Subduction cuases slabs of oceanic crust to sink through a deep ocean trench into the mantle. The crust melts and forms magma, which then rises back toward the surface. When the magma from the melted crusts erupts as lava, volcanoes are formed. The Cascade Mountain Range, which includes volcanoes like Mt. Hood and Mount St. Helens ,was caused by the melting of the Juan de Fuca plate as it subducted beneath the North American plate.
(oceanic-oceanic) When two oceanic plates collide the resulting volcanic activity often forms island arcs. The older, denser oceanic plate dives under the younger oceanic plate creating a deep ocean trench. The lower plate sinks beneath the deep ocean trench into the asthenosphere. There it begins to melt, forming magma. This magma is less dense than the surrounding rock, so it rises to the surface, creating volcanoes. Japan, New Zealand, Indonesia, the Caribbean islands, the Philippines, and the Aleutians (all major island chains) were created this way.
What are hot spot volcanoes?
A hot spot is an area where magma from deep within the mantle melts through the crust like a blow torch. Hot spots often lie in the middle of continental or oceanic plates far from any plate boundaries. Thus, they are not the result of subduction or diverging plates. The Hawaiian Islands formed as the Pacific plate drifted over a hot spot. A major hot spot in the middle of continental plate is Yellowstone National Park in Wyoming.
How does magma reach Earth’s surface?
Lava begins as magma in the mantle. There, magma forms in the asthenosphere, which lies beneath the lithosphere. Magma rises through the lithosphere because liquid magma is less dense than the surrounding solid material. Magma flows up through any cracks in the rock above, until it reaches the surface or is trapped beneath rock layers. Because the plates are weakest and have the most cracks at plate boundaries, this is why magma reaches the surface most often around the edges of plates.
What is a volcanic eruption?
Volcanic eruptions occur in different sizes and types. Generally, any time lava, tephra, and gases are expelled from a volcanic vent onto Earth’s surface it is called a volcanic eruption.
What is tephra?
Tephra is the general name for rocks that are shot out during a volcanic eruption. Depending on the size of the rock, tephra is classified with specific names:
· Ash – particles smaller than 2mm (.08 inches) in diameter
· Lapilli – between 2mm and 64mm (.08 inches and 2.5 inches)
· Volcanic bombs or Volcanic Blocks – larger than 64mm (2.5 inches)
What different types of Magma are there?
There are three main types of Magma
1. Basaltic magma – low viscosity (low silica content)
2. Andesitic magma – intermediate viscosity (medium silica content)
3. Ryolitic magma – high viscosity (high silica content)
SEE THE CHART BELOW
| Summary Table |
| Magma Type | Solidified Rock | Chemical Composition | Temperature | Viscosity | Gas Content |
| Mafic | Basalt | 45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na | 1000 - 1200 oC | Low | Low |
| Intermediate | Andesite | 55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K | 800 - 1000 oC | Intermediate | Intermediate |
| Felsic | Rhyolite | 65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na. | 650 - 800 oC | High | High |
Describe volcanic Eruptions:
During a volcanic eruption, the gases dissolved in magma rush out, carrying the magma with them. Depending on the viscosity and gas content of the magma, the eruption may either be explosive or non-explosive. Generally, they follow the pattern below:
· Explosive: Felsic magma (higher viscosity, higher gas content)
o Thick magma blocks up the vent allowing extreme pressure to build up, which finally erupts in an explosive eruption.
· Non-explosive: Mafic magma (lower viscosity, lower gas content)
o Thinner magma flows out more easily from the vent, sometimes jetting into the air, but mostly flowing downhill in rivers of lava.
