Great questions!
As to why Earth is layered, early in its history it got hot enough (from gravitational compression, the energy of impacting bodies, and radiation, to have partially melted. When this happened, the densest material sank toward Earth's center, while the less dense stuff rose to the top. It's a little like a hot chocolate-- foam floats to the surface, and the heavy syrup settles to the bottom.

And about dinosaurs: thousands of books and articles have been written about this controversial subject. One important thing to keep in mind is that not all dinosaurs became extinct--many thousands of species are still very much with us. They happen to be covered with feathers. We call them birds, of course. Although most people don't appreciate this idea yet, birds are as much dinosaurs, as poodles are dogs. Birds have many unique qualities relative to other dinosaurs, just as poodles are special compared to other dogs, but birds are dinosaurs and poodles are dogs just the same. So, what killed off the dinosaurs that weren't birds? Most scientists now agree that a catastrophic collision with an asteroid at the end of the Mesozoic is what did them in. Why non-bird- dinosaurs and many other kinds of organisms, disappeared at this time, while many other groups survived, is what we're having a difficult time explaining.

The major layers of the Earth, starting from its center, are the inner core, the outer core, the mantle, and the crust. These layers formed as the building blocks of Earth, known as planetesimals, collided and collapsed under their own gravity around 4.5 billion years ago. At that time, the heaviest elements (like iron and nickel) sank to the core, while the lighter elements (like silicon, oxygen, and carbon), rose to form the mantle and crust.

The crust itself is composed of many layers, which are the ones we can see at the surface. Most continents have a crystalline core, known as a 'kraton'. Every continental kraton is then draped in many layers of sedimentary, igneous, and metamorphic rock. The most dramatic layers, like those in the mountain ranges above Santa Barbara, are often sedimentary rocks, which formed as long-gone mountain ranges eroded away and the resulting sediments were deposited somewhere else on the continent. A change in tectonic plate motion then caused those sedimentary layers to be brought up to where we can see them today.

When volcanoes erupt, the lava that comes out can spread across broad areas, forming a layer of igneous rock within the sedimentary layers. Thus, most of the layering on the Earth's surface is due to erosion and deposition of mountain ranges, and lava flowing out on the surface.

Most scientists believe the Dinosaurs became extinct at the end of the Cretaceous period (around 65 million years ago) when a large asteroid impacted the Earth, probably causing global fires, giant tsunamis, and major climatic change. In addition to Dinosaurs, nearly 75% of all plant and animal species on Earth went extinct around that time. This event is known as a "mass extinction". It is possible that more than one asteroid hit the Earth, as scientists have found multiple large impact craters that date to the same period.

The other leading theory for the mass extinctions is the eruption of huge volumes of basaltic lava in India, an event known as the Deccan Traps. This enormous amount of lava was erupted in a relatively short time period, and could have drastically altered climate, changing habitat zones faster than the plants and animals could adjust to them.

It is likely that both the Deccan Traps and the impact of multiple asteroids contributed to the demise of the Dinosaurs and so many other species at the end of the Cretaceous.

Layers of rocks are formed when sediment is laid down, and flattens out to be as close to the center of the Earth as possible. Then, a new layer is laid down, which has to fall onto the older layer, and thus lies above it.

Dinosaurs are not extinct - birds are living dinosaurs. The cause of the extinction of the non-bird dinosaurs is not completely known, although there is reason to think that it had something to do with an asteroid impact in Mexico about 65 million years ago.