Chapter 7 - 5

05 Geology

listen to a discussion in a geology class. fill in the diagram with the information that you hear.

M: Continuing our conversation about volcanoes, today I want to talk about types of volcanoes. Geologist generally grouped them into four types: cinder cones, composite volcanoes, shield volcanoes, and lava domes.

Just to forewarn you: all of these have alternate names, but to avoid confusion, for this class, we’ll be using the above terminology. So, what was the first type I mentioned?

W: I think you said cinder cones.

M: That’s right! Thank you. OK, so anyway, cinder cones are thus named because they are composed of hardened pieces of lava called cinders. The cinder cone is formed when there is a violent eruption from a central vent: vent being the opening.

The lava solidifies when it contacts the air and it falls around the vent.

Over time, a circular cone builds up around the vent.

W: What are some other characteristics of cinder counts?

M: Well, for one thing, they’re usually very steep and they have a bowl, called the crater, at the very top. Another thing about cinder cones is that they don’t really get to be very high.

In fact, I would guess most don’t really get higher than a thousand feet. The next type of volcano is the composite volcano. These are very steep and symmetrical.

They have a central vent at top, and the eruptions through this vent are how the cone is initially built up.

But as the volcano gets larger, lava usually flows through little networks of cracks on the side of the cone.

When the lava hardens in the cracks, the entire structure is strengthened.

W: I’m sorry, but can give us some examples of competent volcanoes?

M: Of course. Composite volcanoes are much larger than cinder cones. In fact, a couple of the world’s biggest mountain, such as Mount Fuji and Mount Rainier are composite volcanoes. Does that help?

All right, the next type of volcano we’ll be talking about his shield volcanoes. This type of volcano gets its name from its appearance.

See, they’re really broad and kind of dome-shaped. I guess some geologist thought that from the side, they resemble a warrior’s shield.

Anyway, the reason shield volcanoes are shaped that way is because they are made when fluid lava eruptions spread over large distances and cool. Like composite volcanoes, lava often flows out of the flanks of the volcano, which also explains how it gets its gentle slopes.

W: Isn’t Hawaii completely made up of shield volcanoes?

M: That’s right. Hawaii is just a chain of shield volcanoes that rise up thousands of feet from the ocean floor. In fact, Mauna Loa, which also has the distinction of being the world’s most active volcano, has been found to project over 28,000 feet above the ocean floor.

Crazy, huh? Shield volcanoes are massive−they can be up to three or four miles wide.

There’s one more type that I’ll talk about. It’s called a lava dome. In a lava dome, the volcano doesn’t really erupt: lava just barely comes out, if at all.

Most of it piles up right around the vent. So, instead of growing from the buildup of love on the outside, it grows from the inside. It’s actually pretty common to see these on the side of a composite volcano.

1) What does the professor imply when he says this: Geologist generally grouped them into four types: cinder cones, composite volcanoes, shield volcanoes, and lava domes.

2) Which of the following can be inferred about volcanoes?

3) What can be inferred about how the types of volcanoes are named?