Rocks and Rock Cycle Mrs. Reese

Do rocks change over time? The Earth is 4 billion years old and people only live for 100 years so most people do not get to see very many dramatic changes to their surroundings. The evidence of change is found in many of our landforms made from rock. Change happens over many years (hundreds, thousands, and even millions of years.)

WATER EROSION

WIND EROSION

VOLCANIC ACTIVITY

What is a Rock? A rock is a solid material made up of one or more minerals or other substances such as once-living organisms that occurs naturally.

Why are rocks important?

Glimpse at the past… Rocks are a fascinating way to discover information about the history of the Earth. The Cambrian period (490 million years ago) was characterized by an interesting diversity of life in the seas. Life had not yet evolved upon land. The highest forms of life existing at this time were the trilobites (marine arthropod), which reached lengths up to two feet.

How do geologist classify rocks? Geologist can classify rocks based on various characteristics such as color, texture, arrangement of grains, composition (what minerals it has), density, and how the rock forms.

How many groups of rocks are there? There are THREE main groups: Igneous, Metamorphic, and Sedimentary.

How are igneous rocks formed? Igneous rocks are formed from molten magma or lava cooling and hardening. The word, igneous means "fire".

What happens when the magma cools slowly or quickly? If the magma cools and hardens inside the earth it is called "intrusive" igneous rock. These rocks cool slowly and have large crystals. When the lava comes out of the earth's crust through a volcano, it is called "extrusive“ igneous rock. If it cools off quickly, the crystals that form are very small, the rock will take a glassy appearance, or the rock will be porous.

PUMICE (EXTRUSIVE)

OBSIDIAN (EXTRUSIVE)

GRANITE (INTRUSIVE)

Weathering All rocks are subject to weathering. Weathering is anything that breaks the rocks into smaller pieces or sediments. This can happen by the forces of wind, rain, chemical reactions, and freezing water.

Sediment Sizes

Erosion The combination of weathering and movement of the resulting sediments is called erosion.

Deposition The sediments that form from these actions are often carried to other places by the wind, running water, and gravity. As these forces lose energy the sediments settle out of the air or water. As the settling takes place the rock fragments are graded by size. The larger heavier pieces settle out first. The smallest fragments travel farther and settle out last. This process of settling out is called deposition.

Compaction and Cementation Compaction occurs after the sediments have been deposited. The weight of the sediments squeezes the particles together. As more and more sediments are deposited the weight on the sediments below increases. Waterborne sediments become so tightly squeezed together that most of the water is pushed out. Cementation happens as dissolved minerals become deposited in the spaces between the sediments. These minerals act as glue or cement to bind the sediments together.

Clastic Sedimentary Rocks The most common set of sedimentary rocks consist of the granular materials that occur in sediment: mud and sand and gravel and clay. Sand is made of quartz, and mud is made of clay minerals. As these sediments are steadily buried over geologic time, they get packed together under pressure and low heat, not much more than 100°C. In these conditions the sediment is cemented into rock: sand becomes sandstone and clay becomes shale. If gravel or pebbles are part of the sediment, the rock that forms is conglomerate.

Organic Sedimentary Rocks Sediment can form in the sea as microscopic organisms. Dead plankton steadily shower their dust-sized shells onto the seafloor, where they accumulate in thick layers. That material turns to two more rock types, limestone (carbonate) and chert (silica). Another type of sediment forms where dead plant material builds up into thick layers. With a small degree of compaction, this becomes peat; after much longer and deeper burial, it becomes coal.

Chemical Sedimentary Rocks Ancient shallow seas sometimes allowed large areas to become isolated and begin drying up. In that setting, as the seawater grows more concentrated, minerals begin to come out of solution (precipitate), starting with calcite, then gypsum, then halite. The resulting rocks are limestone, dolomites, gypsum, and rock salt.

SHALE (Sedimentary) FORMED FROM CLAY-IT HAS VERY FINE GRAINS.

CONGLOMERATE (Sedimentary) COARSE GRAINS

SILTSTONE (Sedimentary) MADE FROM SILT AND HAS VERY FINE GRAINS.

SANDSTONE (Sedimentary) MADE FROM SAND AND HAS MEDIUN COARSE GRAINS.

LIMESTONE (Sedimentary) MADE FROM CALCITE(CHALK) AND HAS VERY FINE GRAINS.

COAL (Sedimentary) MADE FROM DEAD PLANT SEDIMENTS AND HAD FINE GRAIN.

GYPSUM (Sedimentary) MADE FROM EVAPORATION OF WATER WITH CONCENTRATED MINERALS.

How are metamorphic rocks formed? Metamorphic rocks are formed when other kinds of rocks are changed by great heat and pressure inside the earth. The word "metamorphic" means changed. Think of metamorphic rocks as recycled rocks. When igneous, sedimentary or even metamorphic rocks get buried deep beneath the surface of the earth, over millions of years the heat and pressure inside the earth change them into another rock.

CHANGED SLATE PHYOLLITE

CHANGED GRANITE GNEISS

CHANGED PERIDIOTITE SOAPSTONE

Rock Cycle The rock cycle is a continuous cycle in which forces inside the Earth and at the surface build, destroy, and change the rocks in the crust.

Rock Cycle