A Geologist's Discovery: Interpreting an Ancient Outcrop
A geologist is working in an area of old volcanic rocks exposed by erosion.
He finds a large rock outcrop that contains flat lying gravels in the base of the outcrop. These gravels are over-lain by basalt that forms a series of layers representing flows.
The uppermost basalt lava flows have a dip of about 10 degrees to the west.
What does this outcrop record?
A) Basalt flows formed above a series of gravels, filling a river valley.
B) Basalt flows formed after gravels were deposited, covered the local terrain, and built a shield volcano with dips of about 10 degrees.
C) Gravels were initially deposited in a river valley and a cinder cone was erupted, damming the valley, before a cinder cone formed.
D) You can't tell anything about the history of this region from this outcrop.
Answer: B) Basalt flows formed after gravels were deposited, covered the local terrain, and built a shield volcano with dips of about 10 degrees.
Explanation:
If the viscosity is low, non-explosive eruptions usually begin with fire fountains due to release of dissolved gases. When magma reaches the surface of the earth, it is called lava. Because the basaltic magma contain the least silica, as a result, they flow most easily.
What type of eruptions typically occur when the viscosity of magma is low? Non-explosive eruptions usually occur when the viscosity of magma is low. This can result in fire fountains due to the release of dissolved gases. When the magma reaches the surface, it becomes lava.