<span>If you see a very tall, rock feature that looks like a jagged, rectangular block sticking out of the ground, you're probably observing a volcanic neck. Also known as a lava neck or volcanic plug, a volcanic neck is formed when magma solidifies within a vent of an active volcano. This can harness great amounts of pressures which could lead to an explosive eruption.</span>
Answer:
b. flat rotation curves in spiral galaxies
Explanation:
Dark matter accounts for majority of the matter in the universe (about 85%). It is considered to be composed of subatomic particles which are yet to be discovered. Dark matter cannot be observed because it does not emit light that can be detected. Only its gravitational effect is visible.
Astronomers when calculated the mass of the visible matter and tried to account for the observed speed of stars in the galaxy, there was a mismatch. To understand what caused this mismatch, astronomers gave name to the invisible matter as the Dark Matter.
The expected velocity of stars should decrease away from the near the center of the galaxy but a constant velocity of stars was observed that is a relatively flat rotation curve was obtained.
Thus, flat rotation curves in spiral galaxies in an evidence of Dark Matter.
Most of those major cities are located near Petroleum Fields and Coal Mines.
<span>The four main forms of river erosion. Hydraulic action - the force of the river against the banks can cause air to be trapped in cracks and crevices. The pressure of the action weakens the banks and gradually wears it away.</span>
