Hey there!
First of all, before we get into this, it's important to note that there are two types of crust: <u>oceanic crust</u>, which is located under the oceans of the world and is much thinner than the other type of crust, which is <u>continental crust</u>, located on the continents and is much thicker.
---
The first type of boundary is a convergent boundary.
At a convergent boundary, the plates are moving toward each other and crashing into each other, or colliding.
At a convergent boundary between one continental plate and one oceanic plate (continent-ocean) and a boundary between two oceanic plates (ocean-ocean), one plate will <u>subduct</u> under the other, always the oceanic plate in a continent-ocean boundary. The crust that subducts under the other melts into magma.
In the case of two continental crust pieces converging (continent-continent), they don't really subduct under each other, they just shove up into each other.
Main effects of a convergent boundary include volcanoes, mountain ranges, and earthquakes/seismic activity.
---
The second type of boundary is a divergent boundary.
At a divergent boundary, the plates are moving away from each other.
At an oceanic divergent boundary, the separating allows a fissure to be opened and, in many cases, it acts like a cut that's healing. New magma comes up, which creates new crust.
At a continental divergent boundary, it pulls apart, creating faults and earthquakes. This forms a rift. Water flows into the rift as well. If you've ever heard of the Great Rift Valley in East Africa, that's a great example of this type of boundary.
Sometimes there is volcanic activity along rifts made by divergent boundaries.
---
The third type of boundary is a transform boundary.
At a transform boundary, the plates are sliding past each other.
One main effect of a transform boundary is earthquakes. Tsunamis are also an issue. Tsunamis are sometimes caused by an earthquake.
---
Hope this helps!
