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?

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    $\begingroup$ I like this question - a geological chicken or egg question. It certainly is an interesting question. $\endgroup$
    – user889
    Mar 24 '15 at 6:02
  • $\begingroup$ I think is some half way igneous metamorphic rock, like some metamorphic rocks once is under pressure and heat they partially melt. $\endgroup$ Mar 24 '15 at 6:06
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    $\begingroup$ Whatever it was, right before it became a rock, ACDC saluted it. $\endgroup$
    – corsiKa
    Mar 24 '15 at 19:38
  • $\begingroup$ @corsiKa best comment ever! I am wondering when the rocks started to roll... $\endgroup$
    – user889
    Mar 25 '15 at 3:29
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    $\begingroup$ Thanks for the thoughtful dialogue! I love to pose open-ended questions to my 6th grade Earth Science students in order to get them thinking about specific characteristics of whatever it is we are studying. I've been struggling to find ways to make learning about the rock cycle meaningful (as we are studying plate tectonics next (another chicken and egg - believe me, I've thought about it...).) So - Thanks for the simple analysis of basic characteristics, and also thank you for the "bigger questions" that I can use to follow up on this one as we delve in to other earth systems. $\endgroup$
    – user4697
    Oct 25 '15 at 20:16

which came first

That's actually a very hard question. The most simple answer would indeed be igneous. Here's why:

  1. Sedimentary rocks (in the sense of rock cycle) comes from pre-existing igneous or metamorphic rocks, so you need to have had them first.

  2. Metamorphic rocks, by definition, are rocks that form from other kinds of rocks (be it igneous or metamorphic).

  3. 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.

  • $\begingroup$ The rocks that form the planetismals, what category would they fall under, if any? $\endgroup$
    – G. Gip
    Aug 3 '16 at 18:52
  • $\begingroup$ @G.Gip If they melted, then maybe igneous. Otherwise "space sediment"? It's hard to assign terrestrial classifications to space material. $\endgroup$
    – Gimelist
    Aug 3 '16 at 20:31
  • $\begingroup$ This is very interesting topic. Let me have some thinking there. Let´s say that comets are made of ice and dust. What kind of rock is this ? Europa, Enceladus and etc. have a crust composed mostly of ice. What kind of rock is this ? Io has so many volcanoes that the crust is composed mainly of igneous rocks. Rock cycle in space (asteroids, comets) and planetary bodies (+ some interesting moons) is very different from rock cycle on Earth (if there is any cycle at all). On Pluto there is probably completely different rock cycle. $\endgroup$
    – Petr Hykš
    Feb 7 '17 at 10:52
  • $\begingroup$ evaporites are sedimentary rock, they are chemical sediments. Sandstone is sedimentary regardless of where the sand came from, the rock cycle is more like a web, cotf.edu/ete/images/modules/msese/earthsysflr/EFCycleP2.gif $\endgroup$
    – John
    Apr 17 '19 at 20:20
  • $\begingroup$ Dust bunnies came first. $\endgroup$
    – Spencer
    Dec 4 '19 at 11:19

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:

  1. The existence of a crust (both a continental and oceanic crust actually.)
  2. Plate tectonics
  3. 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:

  1. When and how did the crust form?
  2. When and how did the oceans and atmosphere form?
  3. When and how did plate tectonics begin?
  • $\begingroup$ The first crust was surely basaltic ("ultramafic"). The first traces of silicate or continental crust may be as old as 4.2 to 4.4Gy pnas.org/content/115/25/6353, but this is debated nature.com/articles/ngeo2786. A "cycle" like in plate tectonics developed much later, like 2.5Gy, maybe a little earlier. $\endgroup$
    – user18411
    Dec 3 '19 at 21:14

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.

  • $\begingroup$ Is this a question or an answer? ;-) $\endgroup$
    – Alexander
    Mar 24 '15 at 10:34

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.


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