Tutor profile: Dan G.
What is the difference between an island arc, like the Aleutian Islands and an island chain, like the Hawaiian Islands?
An island arc is created by volcanic eruptions on an overriding oceanic plate. When oceanic plates collide, one plate subducts under the other. This subducting plate goes down toward the Earth's mantle where it is heated and melted. This molten material, magma, rises up into the overriding plate causing parts of it to melt, rise to the surface, and erupt as volcanic islands. This produces a long line of active volcanoes that parallels the boundary between two oceanic plates. An island chain is produced when an oceanic plate moves over a "hot spot." A hot spot is a location on the Earth's surface above a plume of heat and heated material rising up from deep in the earth's interior. The hot spot heats and melts a spot on the oceanic plate producing an active volcanic island. As the plate moves across the surface of the Earth, it carries the volcanic island off of the hot spot, rendering it inactive and creating an opportunity for the hot spot to make a new volcanic island. The result over time is a line of islands parallel to the direction of the plate's movement, but with only one active volcanic island.
What is dark matter and what does it do?
The moon is held in its orbit by the Earth's gravitational force. The centripetal force required for the moon's orbit is supplied by the Earth's gravity. Fc = Fg. We see this same relationship for the planets orbiting the sun, and their moons orbiting the planets. But when observing rotating galaxies and galaxies that have an orbiting companion galaxy, we see that the circling motion is far too fast! That is, if you add all the masses of all the objects in a galaxy and calculate the force of gravity, Fg we still find that Fc > Fg. So there must be something in a galaxy that also exerts gravitational force. we call that "something" DARK MATTER. Dark matter has never been directly observed so we know very little about it. We do know that it is not made of familiar particles like atoms, etc because it gives off absolutely no EM energy, and we do know that it exerts gravitational forces.
What are decibels and how does the decibel scale work?
Decibels (abbreviated dB) are units of relative intensity, that is loudness, of sound. It starts with zero at the point where we can't quite hear and range up to dangerously loud sounds at 100 dB and above. The variable for relative intensity is B. The scale of relative intensity is a logrithmic scale so that each increase of ten dB corresponds to a sound that is ten times louder. To calculate the decibels of a sound, you first find the Intensity by dividing the power (P) of the source (in Watts) by the surface area of the sound wave at a distance, r, in meters squared. (Area = 4 pi r^2). Then you take the log of the Intensity.... then add 12 (this adjustment makes zero at the threshold of hearing)... then multiply the result by 10 (this puts the "deci" in decibels.) Here is the equation: B = 10 x (12 + Log[P/(4 x pi x r^2)})