Description - These rocks form when either magma or lava cools. Magma is melted rock inside the earth, which cools slowly allowing time for crystals to form. Lava is melted rock that erupts to the earth’s surface, which cools fast and doesn’t allow crystals to form. This can cause a glassy appearance. If there is gas in the lava, the gas escapes when the lava cools and holes are left in the rock.
What to look for – Look for rocks that appear speckled due to the light and dark crystals that formed when the rock cooled slowly. Also look for rocks that are glassy or full of holes and are light from fast cooling. Usually igneous rocks are shiny because they have crystals or appear glassy.
Description – These rocks form when other igneous, metamorphic, or sedimentary rocks are squeezed under high pressures or heated to high temperatures deep within the earth. The rocks are not melted, but they do change.
What to look for – Look for rocks that are shiny, but do not have nicely formed crystals. In fact, the crystals may appear kind of flat from the pressure on the rock. Also, these rocks often appear layered.
Description – These rocks are formed when sediments such as sand, clay, plants, or animals settle to the bottom of a body of water such as an ocean, lake, or river. After these sediments have been piled on top of each other, they get compacted and are turned into rock.
What to look for – Look for rocks that are not shiny, but look dirty, like sand or mud. These rocks can have fossils. Fossils are the remains of living things that are preserved in the rock. These rocks can also have small chucks of other rocks in them.
The type of sediment can change over time, so some rocks can be layered.
igneous – Any rock formed from the cooling and crystallization of magma or lava. metamorphic – Any rock type changed by high temperature or pressure. sedimentary – Any rock made of sediment such as gravel, sand, clay, plants, or animals.