Rock Detective!

HOVER OVER THE ROCKS TO LEARN WHAT TYPES THEY ARE

Rocks are made up of different minerals, which determine color and appearance. Geologists classify rocks into three groups—ingneous, sedimentary, and metamorphic—based on how they formed.

Igneous

These form from magma (or lava) that flows up from the deep within the Earth, cooling and solidifying at great depth or near/at the surface. There are two types of igneous rocks—intrusive and extrusive. Intrusive rocks, like granite, form when magma cools and hardens deep underground.

Slower cooling produces larger crystals; faster cooling produces small-to-medium size crystals. Extrusive (volcanic) igneous rocks, such as basalt, form when magma erupts from a volcano or solidifies in a volcanic chamber just beneath the Earth’s surface. The quickest cooling of all results in glass-like rocks, such as obsidian.

Sedimentary

These rocks form on the Earth’s surface when layers of sediment (sand, mud, gravel) accumulate and become cemented together over time to form rocks like sandstone or mudstone.

Sedimentary rocks resemble the places where they formed. A sandstone rock you find may once have been a sandy beach, the mudstone an ancient mudflat, and the limestone a muddy, shell-rich seafloor.

Metamorphic

These rocks start out as igneous or sedimentary rocks. When mountain-building or other geologic events bury these preexisting rocks deep within the Earth, they undergo complete metamorphosis (change) resulting from the extreme pressure and heat of deep burial.

Melting and recrystallizing at depth, the new rock has all the same minerals but looks entirely different from the original.