7th Grade Science Chapter 9 Answers
1. A volcano is a mountain that forms when molten rock, called magma, is forced to the Earth’s
surface.
2. Nonexplosive eruptions are characterized by a continuous stream of lava. P 222
3. While a nonexplosive eruption is characterized by lava, an explosive eruption shoots clouds of hot
debris and gas out from the volcano at supersonic speeds. Instead of lava the volcano shoots
molten rock into the air and tons of solid rock. P 222/223
4. All volcanoes share a magma chamber that form deep underground and vents. Magma rises up
from the magma chamber through holes in the Earth’s crust called the vents.
5. The two types of magma are made with either water or silica.
6. The make-up of the magma dictates what kind of eruption the volcano with have. Scientists have
found that magma that is high in water content usually leads to an explosive eruption. The water
acts like carbon dioxide does with a can of soda, the pressure builds up & leads to an explosive
volcanic eruption. For magma high in silica, this too is found in explosive eruptions. Silica-rich
magma has a thick & stiff consistency and flows slowly and hardens in the vents. This plugging up
of the vents causes the pressure to build up & can eventually lead to an explosive eruption. P 224
7. Magma erupts during an eruption in 2 different types of materials, lava or pyroclastic material.
8. Pyroclastic material consists of the rock fragments created by explosive volcanic eruptions.
9. Lava is generally produced during a nonexplosive eruption.
10. The four types of lava are blocky lava, pahoehoe, aa, and pillow lava. Make sure to look at photos
on page 225.
Blocky lava is cool stiff lava, that oozes from a volcano & forms jumbled heaps of sharp-edged
chunks.
Pahoehoe lava flows slowly and forms a glassy surface with rounded wrinkles. Pahoehoe lava
looks like wax drippings from a candle or ropes laid out.
Aa lava has a jagged surface because this stiffer lava pours out quickly to form a brittle crust with
jagged pieces as the molten lava underneath still moves.
Pillow Lava forms when lava erupts underwater forming rounded lumps that look like pillows. P 225
11. Pyroclastic Material is produced when magma explodes and solidifies in the air or when existing
rock is shattered by powerful eruptions.
12. The four kinds of pyroclastic material are:
Volcanic Blocks – The largest type of pyroclastic material, these are the solid rocks that are blasted
out of the volcano.
Volcanic Bombs – These are the large blobs of magma that harden in the air.
Lapilli - “Little Stones” – These are the pebble-like bits of magma that become solid before they hit
the ground.
Volcanic Ash – Volcanic ash forms when the gases in the stiff magma expand rapidly and the walls
of the gas bubbles explode into tiny glasslike slivers. This is the smallest pyroclastic material.
13. A volcano that contains magma with small proportions of water and silica would lead to a
nonexplosive eruption. This is because the lack of water and silica within the magma will not cause
the pressure within the volcano to build up, therefore there is no force to cause an explosive
eruption.
14. Mt. St. Helen’s is an example of an explosive eruption. The people of Seattle witnessed ash and
pyroclastic material shooting out of the volcano’s top. P 223
15. Nonexplosive eruptions cause the greatest threat to property because of the lava they produce.
The lava can flow for many kilometers before cooling and hardening. P 222
16. Ash that is ejected during an explosive eruption can cause buildings to collapse, dam up rivers,
smoother crops, block out the sun, and lower the global surface temperature. P 227
17. The three types of volcanoes are shield, cinder cone, and composite.
Shield Volcano – these are large wide volcanoes built out of layers of lava as a result of repeated
nonexplosive eruptions.
Cinder Cone Volcano – these volcanoes are made entirely of pyroclastic material from moderately
explosive eruptions. The volcano forms a steep slope with a narrow base. These volcanoes erode
quickly because the pyroclastic material particles are not cemented by lava.
Composite Volcano - Also known as Stratovolcano, are the most common type of volcano.
Composite volcanoes are formed by explosive eruptions of pyroclastic material followed by quieter
outpourings of lava.
18. Mauna Kea is the largest mountain on Earth and an example of a shield volcano.
19. Paricutin is a cinder cone volcano and Mount Fuji is a composite volcano.
20. A volcano’s crater is a funnel-shaped pit that forms at the top of the central vent in most volcanoes.
The funnel shape is a result of numerous explosions that shoot material out of the vent as well as a
collapse of the material from the rim around the crater back down into the vents.
21. A caldera forms when a magma chamber that supplies material to a volcano empties and its’ roof
collapses. This causes the ground to sink and leaves a large circular depression. The caldera’s are
usually much larger than volcanic craters.
22. Most of the massive outpourings of lava on Earth erupt from long cracks, or fissures, in the crust.
23. A lava plateau is the resulting landform from repeated nonexplosive eruptions. Runny lava pours
from a series of cracks or fissures in the Earth’s crust. This lava can spread out evenly covering an
area thousands of square miles. The Columbia River Plateau in the northwest of the US is an
example of one.
24. Volcanoes actually form below the Earth’s surface in a region that includes the lower crust and part
of the upper mantle. When the magma collects in these deep regions, volcanoes form.
25. Even though the temperature is high enough to melt the rock within the mantle, the pressure is so
tremendous that the it remains a solid.
26. For the rock to melt and magma to form, either the temperature must increase, or the pressure on
the rock must decreases. Because the temperature of the mantle usually remains fairly constant,
form magma to form the pressure must decrease.
27. The magma rises to the surface of the Earth because it is less dense than the rock around it.
28. Together the tectonic plates surrounding the Pacific Ocean are called the “Ring of Fire,” because
75% of the active land volcanoes on Earth reside there.
29. A rift forms between tectonic plates at divergent boundaries.
30. The magma rises at divergent boundaries, cools, but still rises because the pressure on it
decreases. This causes the magma to be less dense then the surrounding rock.
31. Mid-Ocean ridges are the long mountain chains that form on the ocean floor at the divergent
boundaries, or where the tectonic plates pull apart.
32. Magma forms at convergent boundaries when water in the oceanic crust is released due to an
increase in temperature and pressure. The water mixes with the rock, lowering the mantle rocks’
melting point, causing it to melt.
33. Volcanoes form at “hot spots.” These are the places on the Earth directly above columns of rising
magma or mantle plumes.
34. The Hawaiian Islands are an example of volcanoes that have formed at hot spots.
35. The mantle plume of a hot spot stays in the same place while the tectonic plates are moving over
it. This movement causes the plume to create a chain of volcanoes.
36. Volcanologists classify volcanoes into 3 categories, extinct, dormant, or active.
Extinct – Extinct volcanoes are those that have not erupted in recorded history and probably never
will again.
Dormant – Dormant volcanoes are those that are not currently erupting, but have erupted in the
past.
Active – Active volcanoes are those that are in the process of erupting or that show signs of
erupting in the near future.
37. To determine if a volcano will erupt, scientists look at seismic activity, a volcano’s slope, the
outflow of volcanic gases, and the volcano’s temperature. (Create 5 flash cards for this)
Seismic Activity – Just before an eruption the size and number of small earthquakes increases.
Volcanic Slope – Scientists measure the slope with a tilt meter to detect small changes in the angle
of the slope. If there are changes, like bulges, this may indicate that magma is pushing up against
the inside of the volcano.
Volcanic Gases – Scientists know that when the ratios of the gases change, that things are
changing in the magma chamber.
Temperature – Scientists use satellite images to record infrared radiation which allows them to
measure the changes in temperature over time. Scientists can then determine if the site is getting
hotter.