Unit 6: Rocks and the Rock Cycle
The three rock families and the cycle that keeps shuffling material between them, melting, cooling, breaking, and squeezing it around forever.
5 topics · 15 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
The Rock Cycle as a System
- How it worksHow does the same material cycle between igneous, sedimentary, and metamorphic rock over millions of years?
- Common mistakeWhy isn't the rock cycle a fixed, one-way path that every rock has to follow in the same order?
- Going furtherWhat happens to a rock's path through the cycle if it gets subducted instead of just weathered at the surface?
Your turn1 of 2
Every igneous, sedimentary, and metamorphic rock formed from material that was already part of some earlier rock. Why is there no such thing as a 'first rock' that started off the entire rock cycle?Igneous Rocks: Cooling Rate and Texture
- How it worksHow does slow cooling underground grow large crystals while fast cooling at the surface freezes them small?
- Worked exampleFind whether a rock that cooled over 10,000 years underground or one that cooled in 2 days has larger crystals.
- Common mistakeWhy isn't obsidian's glassy texture a sign that it's chemically different from a rock like granite?
Your turn1 of 2
A batch of magma cools underground over 6995 years, forming large crystals, while a similar batch erupts and cools at the surface in 7 days. Converting the underground time to days at 365 days per year, roughly how many times longer does the underground batch spend cooling?Sedimentary Rocks: Compaction and Cementation
- How it worksHow does the weight of new sediment compact older layers while dissolved minerals cement the grains together?
- How it worksWalk me through sand turning into solid sandstone, from the moment it's deposited.
- Common mistakeWhy aren't all sedimentary rocks made from broken pieces of other rock the way sandstone is?
Your turn1 of 2
Freshly deposited sediment starts with about 50 percent open pore space. As it is buried and compacted, its pore space shrinks to about 18 percent. How many percentage points of pore space did compaction remove?Metamorphic Rocks: Heat and Pressure
- How it worksHow do heat and pressure rearrange a rock's minerals into new crystals without melting it?
- Common mistakeWhy isn't a metamorphic rock just an igneous rock that got a little bit hotter?
- Ties togetherHow does the amount of heat and pressure decide whether a rock ends up as slate, schist, or gneiss?
Your turn1 of 2
Marble forms from limestone that has recrystallized under heat and pressure deep underground, without ever fully melting. Why does limestone turn into marble without ever fully melting?How Rocks Change From One Type to Another
- How it worksWalk me through one rock's journey from melting into magma to cooling into granite to weathering into sandstone.
- Going furtherWhat happens to a metamorphic rock if it gets buried even deeper and finally melts completely?
- Ties togetherHow does plate tectonics supply the heat, pressure, and uplift that keep the whole rock cycle running?
Your turn1 of 2
A single quartz grain can begin inside a granite, weather free into a grain of sand, get cemented into sandstone, and later be metamorphosed into quartzite, all within one long geologic history. Why can the exact same rock material end up as igneous, then sedimentary, then metamorphic over its history?