5. During a geology field trip you observe an exposure of rock with a basalt lay

Question

5. During a geology field trip you observe an exposure of rock with a basalt layer sandwiched between a sandstone and a shale bed (both sedimentary rocks). The basalt is fine-grained with abundant vesicles along its upper margin. A fellow student suggests that the basalt layer is a fine-grained intrusive body of igneous rock called a sill. You disagree and argue that it is actually a lava flow. What allowed you to distinguish between a basaltic flow and a basaltic intrusion?

6. Describe the formation of a Crater Lake, a volcanic landform found in Oregon.

7. Name two composite cones. How would you characterize a typical eruption for one of these volcanoes - would it be explosive or quiescent? Why do these volcanoes erupt in this fashion?

Explanation / Answer

the fine grained texture and abundant vescicles suggest that it is a lava flow. if it is an intrusion then we may not find those vesicles , because the vescicles form in conditions where there is a low pressure and temperature conditions which are available only on the surfce of the earth

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