Such a profound question! We tend to take the shape of our planet for granted. The reason why Earth is round is because a sphere is the easiest 3D shape that nature can form. In other words, creating a sphere takes the least amount of energy to make. That's why many natural objects in nature are spherical (round) in shape, such as our Sun, water droplets, pearls, and many more.

Now for your second question. Volcanoes are formed when magma (or molten rock) from inside the Earth reaches the surface. As more and more magma pile up, the bigger a volcano gets. I hope I answered your questions. Good luck with studying!

The earth is round (not flat) because of gravity. If you drop a ball, we see it falling "down" but it's actually falling towards the center of the earth. The center of the earth is the center of its gravity, so everything on our earth is drawn to the center of the earth -- that's why when you jump, you're immediately drawn down to the ground again. As everything is being pulled to the center of the earth it makes a round shape.

We stand on the surface of the earth on what we call the crust. Beneath the crust is the mantle, which is semi-liquid magma. Sometimes the magma leaks up to the surface, and it erupts out as a volcano!

The Earth actually isn't round - it's what's called an "oblate spheroid". This means that it's mostly spherical, but has a bit of a bulge around the Equator. Nevertheless, it is round-ish instead of flat because of the way the Earth formed - through a process called "gravitational accretion".

Our solar system formed from bits of rock and dust (produced by the explosion of a star). Pieces of rock and dust clumped together until a rock was formed that was large enough to cause its own gravitational field (called a proto-planet). Once a gravitational field was established, pretty much everything else close to the proto-Earth got drawn into the gravitational field, until the orbit around our Earth was cleared (free of any rocks/dust hanging out in space). The reason why the Earth formed in a spherical shape (instead of being flat) is because gravity works in all directions, so bits of rock/dust were clumped onto the Earth on all sides. For example, three people standing at the North Pole, the South Pole, and the Equator can all jump up, and still be brought back down to Earth because they are all under the influence of gravity. If gravity only operated on one plane, then only the person standing at the Equator would be affected by gravity, and the people at the poles would fly away.

How are volcanoes formed?
Lucky for you, I am actually a volcanologist!! (This is a real job - I study volcanoes, how they form, and what makes them erupt - and I get paid for it!)

For starters, there are three main types of volcanoes (and a few other types of volcanoes as well): spreading ridges (like the Mid-Oceanic Ridge that runs through Iceland), "hotspot" volcanoes (like Hawaii and Yellowstone), and arc volcanoes (like Mt. St. Helens). Each of these three types of volcanoes form in very different ways.

Spreading ridges (which are located where two tectonic plates spread apart) produce eruptions of basalt. This happens because room is made in between the two tectonic plates, and the Earth's mantle "upwells" (meaning that it rises and expands and melts at the same time). This melted upper mantle actually cools as it melts (which doesn't make any sense, but take my word for it). This type of volcanism accounts for about 90% of all of the volcanic eruptions on Earth every year - but, because the majority of spreading ridge volcanoes are underwater (in the ocean), most people never hear about them. A well-known spreading ridge eruption would be the 2010 eruption of Eyjafjallajökull.

Hotspot volcanoes are perhaps the most well-known type of volcano (think Hawaii and Yellowstone). Hotspot volcanoes occur when an extremely large and hot parcel of melt from the Earth's lower mantle (right next to the liquid outer core, so it's REALLY hot) upwells extremely fast, traveling through the mantle to the Earth's surface (in sort of an upside-down teardrop shape). This is called a "mantle plume". Hotspot volcanoes can behave differently depending on the type of crust (oceanic vs. continental crust) the melt parcel has to travel through before it can erupt. The mantle plume that has formed the Hawaiian islands travels through very thin oceanic crust before it is erupted, so Hawaiian eruptions are not very violent, and happen a lot. The mantle plume that produced Yellowstone, on the other hand, has had to travel through about 18.5 miles of continental crust before it can be erupted, so along the way it melts part of the continental crust as it travels to the surface. This contaminates the mantle plume, causing it to have a different chemistry than a regular mantle plume, making the Yellowstone hotspot volcano extremely violent and dangerous (and also not erupting very often, only once every few million years). Hotspot volcanoes make up only 1-2% of all volcanoes on Earth.

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Arc volcanoes are what most people think of when they think of a volcano (like Calbuco volcano in Chile, which has been erupting for the last 5 days). Arc volcanoes form where two tectonic plates meet, and one of the plates (the oceanic plate) gets pushed underneath the other plate (either another oceanic plate or a continental plate). As the plate gets pushed down, along with sediments from the ocean floor and a lot of water (a process called "subduction"), the temperature within the Earth heats up the plate, and melts part of it (a process called "partial melting"). These partial melts (and water vapor) rise through the crust and melt part of it to form the initial magma chamber. Later partial melts may come into the chamber and mix with the magma that is already there, triggering a volcanic eruption! (This is actually what I study). Arc volcanoes make up about 7-8% of all volcanoes on Earth.

Also, if you want to know something really cool, there is a special type of volcano that is my favorite - called a"carbonatite". Do you remember ever making a baking soda volcano for a science fair? Did you know that THERE ARE REALLY VOLCANOES THAT ERUPT BAKING SODA?!?!? There is only one in the whole world that is currently active - Ol Doinyo Lengai, in Tanzania. Look it up - it's REALLY cool!

I hope this has answered your questions adequately - always feel free to ask more volcano questions!

The Earth is round because its gravity is too strong for it to be in any other shape. It tries to bunch up into the smallest thing it can closest to its center of mass, which is a ball.

Volcanoes happen when molten rock under the surface erupts onto the surface or above it. They are driven by the expansion of gasses dissolved in the magma, much the way a soda can will spout if you shake it up before opening it. Volcanic eruptions are basically steam explosions.