I am an undergraduate student but I am a tutor at a High School, and one student asked me. Attempting to explain the rock cycle "if one rock turns into the other then which came first" my gut is telling me probably igneous (given Earth geologic past.) Resources online have different opinions. Am I right in my assumption?
which came first
That's actually a very hard question. The most simple answer would indeed be igneous. Here's why:
Sedimentary rocks (in the sense of rock cycle) comes from pre-existing igneous or metamorphic rocks, so you need to have had them first.
Metamorphic rocks, by definition, are rocks that form from other kinds of rocks (be it igneous or metamorphic).
Igneous rocks form by melting of other rocks. Now here is the catch - what do you consider "other rocks"? The first igneous rocks on Earth (or to be exact - the proto-Earth) likely formed by impact melting of various small planetary bodies hitting each other and coalescing to form the Earth. These planetesimals formed by condensation of gas from the solar nebula, before Earth even existed, and before the rock cycle began.
So yes, igneous rocks were here first. The rocks (or material, if you wish) that existed before the igneous rocks do not fit into the traditional igneous/sedimentary/metamorphic definitions.
However, note that the rules of the rock cycle aren't written in stone (pun intended). What about evaporites? Rocks such as halite or gypsum beds form on the surface on the Earth, and their constituents aren't derived directly from either igneous or metamorphic rocks. What about sandstones that form by weathering of former sandstones? What about igneous rocks that form by partial melting of metamorphic rocks, and then melt again without ever reaching the surface to become sedimentary rocks? And my favourite one: pyroclastic rocks. Are they sedimentary or igneous? One textbook quips that they're igneous going up, and sedimentary going down.
The Earth existed long before there were crustal rocks or a "rock cycle."
The idea of the "rock cycle" has prerequisites to even be meaningful. These include:
- The existence of a crust (both a continental and oceanic crust actually.)
- Plate tectonics
- The existence of an atmosphere and ocean.
Without all of these, the concept of the rock cycle (as introduced in textbooks) really is not applicable. (You might suppose there were other 'rock cycles' in the very distant past but that is not what is discussed in undergraduate textbooks.)
The Earth is older than (1) (2) or (3) - so when these preconditions were not in place there was no rock cycle. The three preconditions also did not "turn on" at exactly the same time. It took a long time for the Earth's crust to form (When and how did continental crust form?). Oceans and lakes appeared later (When did oceans form on Earth?). Plate tectonics may have began early (When did plate tectonics begin?).
The point is that the planetary conditions necessary for the rock cycle we see evidence for in the geology of the last billion years or so evolved over much time. Earth was very different in the distant past.
This is the reason why there are no scientists working on the question: "What was the first rock in the rock cycle?" Scientists would like to answer big questions, such as:
- When and how did the crust form?
- When and how did the oceans and atmosphere form?
- When and how did plate tectonics begin?
I suppose it also depends on how far back we go.
The Earth is a rocky planet formed by the accretion/agglomeration of cosmic dust and ice. One might argue that the first rock of Earth was an agglomeration, and once a critical mass had been reached, the agglomerated core heated and began to melt creating igneous rock, assisted by the heat of bombardment from more cosmic rocks and of radioactive decay.
The Earth has a 4.6 billion year history involving proto-planet collision, meteorite bombardment during the Hadean Era, and an early molten phase when massive scale differentiation took place. We have good reason to believe that the earliest rocks, from which the Earth was formed by accretion, were meteorite in composition . Many of these are stony meteorites containing 'chondrules', i.e. millimetre-sized spherical droplets which were flash melted to more than 1500 deg C, probably by being 'cooked' in close proximity to a supernova explosion. These chondrules were probably formed one or two million years before being incorporated into the rocky meteorites where we now find them. Therefore, I would nominate these as being the first known rocks of the terrestrial rock cycle.