The continents are indeed less dense than the oceans!
On average, the continental crust has a composition that is also seen in magmas that are produced at subduction zones (where denser oceanic crust is forced under continental crust) by the melting of the mantle. At the mid-ocean ridges, water is circulated through the newly produced oceanic crust, and the fresh basalt is metamorphosed, causing new minerals to grow which contain water as a part of their structure. Up to a few hundred million years later, this oceanic crust reaches a subduction zone, where it is pulled into the mantle. As it sinks, it is exposed to higher pressures and the water-bearing minerals become unstable. The water within them is driven off the crust and rises into the overlying mantle. At these pressures and temperatures, water is to rock what salt is to ice, and part of the mantle melts - think of it as a kind of 'slush'. As the magma (the liquid part of this slush) rises to the surface, it begins to crystallise, and the denser crystals (which form first) sink. Overall, this makes the magma less dense, continuing to drive it to the surface, where it may either eventually stall in the crust (in a pluton) or be erupted in a volcano. Now there is less dense material sitting on top of and within the oceanic crust and an island arc is born - an example today is the Aleutian Islands.
The magmas formed at subduction zones have a distinctive geochemical signature called the 'calc-alkaline' trend. Whereas magmas at mid-ocean ridges become enriched in iron because of the crystallisation of plagioclase feldspar, at subduction zones the presence of water suppresses feldspar crystallisation, instead producing 'wet' minerals like amphibole. As a result, these magmas do not become enriched in iron as they rise through the crust, and instead become rich in sodium and potassium. These magmas also have distinctive radiogenic isotope ratios and trace element contents. The continental crust (while highly compositionally varied) on average has similar signatures, suggesting that it was formed by this kind of activity.
The fun happens when two of these island arcs collide. They are both less dense than the underlying mantle, so neither will subduct easily. Instead, they coalesce into a single mass, and a continent is born. More common today is the collision of an island arc with a pre-existing continent. This happened before India collided with Asia to form the Himalayas, and the calc-alkaline plutonic rocks are visible at the surface in Tibet today. This is how the continents grow.
The continents are thought to have started to form during the Archaen Eon, starting at four billion years ago. The rate at which they formed is still very much up for debate, but it is thought that crustal growth was more rapid back then as compared to today, and was mostly complete by around 2.5 billion years ago. Today's tectonic plates are cored by ancient cratons, the oldest and most tectonically stable pieces of crust. Around these cratons are progressively younger strips of crust stuck on by colliding island arcs. Much of North America is made up of island arcs stuck to the Laurentian craton.
On average, the continental crust has a composition that is also seen in magmas that are produced at subduction zones (where denser oceanic crust is forced under continental crust) by the melting of the mantle. At the mid-ocean ridges, water is circulated through the newly produced oceanic crust, and the fresh basalt is metamorphosed, causing new minerals to grow which contain water as a part of their structure. Up to a few hundred million years later, this oceanic crust reaches a subduction zone, where it is pulled into the mantle. As it sinks, it is exposed to higher pressures and the water-bearing minerals become unstable. The water within them is driven off the crust and rises into the overlying mantle. At these pressures and temperatures, water is to rock what salt is to ice, and part of the mantle melts - think of it as a kind of 'slush'. As the magma (the liquid part of this slush) rises to the surface, it begins to crystallise, and the denser crystals (which form first) sink. Overall, this makes the magma less dense, continuing to drive it to the surface, where it may either eventually stall in the crust (in a pluton) or be erupted in a volcano. Now there is less dense material sitting on top of and within the oceanic crust and an island arc is born - an example today is the Aleutian Islands.
The magmas formed at subduction zones have a distinctive geochemical signature called the 'calc-alkaline' trend. Whereas magmas at mid-ocean ridges become enriched in iron because of the crystallisation of plagioclase feldspar, at subduction zones the presence of water suppresses feldspar crystallisation, instead producing 'wet' minerals like amphibole. As a result, these magmas do not become enriched in iron as they rise through the crust, and instead become rich in sodium and potassium. These magmas also have distinctive radiogenic isotope ratios and trace element contents. The continental crust (while highly compositionally varied) on average has similar signatures, suggesting that it was formed by this kind of activity.
The fun happens when two of these island arcs collide. They are both less dense than the underlying mantle, so neither will subduct easily. Instead, they coalesce into a single mass, and a continent is born. More common today is the collision of an island arc with a pre-existing continent. This happened before India collided with Asia to form the Himalayas, and the calc-alkaline plutonic rocks are visible at the surface in Tibet today. This is how the continents grow.
The continents are thought to have started to form during the Archaen Eon, starting at four billion years ago. The rate at which they formed is still very much up for debate, but it is thought that crustal growth was more rapid back then as compared to today, and was mostly complete by around 2.5 billion years ago. Today's tectonic plates are cored by ancient cratons, the oldest and most tectonically stable pieces of crust. Around these cratons are progressively younger strips of crust stuck on by colliding island arcs. Much of North America is made up of island arcs stuck to the Laurentian craton.