Download Rocks and Minerals – CRCT Study Guide

Rocks and Minerals – Study Guide
All rocks are made up of materials called minerals. A mineral is a material that is found
in Earth. They are natural substances that were never alive. There are about 4,000
different minerals. Each mineral has its own set of properties. A property is something
you can observe, such as color or shape, which tells you about a mineral. Scientists
classify rocks and minerals by looking at their properties. Properties that can be used to
identify minerals are: hardness, texture, luster, cleavage, and color.
Hardness measures how hard mineral is. Scientists find hardness by rubbing one
mineral against another. Scientists use Mohs’ Hardness Scale to find the harness of a
mineral. The harder mineral will always scratch the softer. The softest mineral is talc
and the hardest is diamond.
Texture is how a rock or mineral feels. Some might be smooth, while others are bumpy.
Some might have small particles in them and some much larger particles. Some rocks
might have sparkles in them which would mean they might be quartz.
A mineral’s luster is how it reflects light. Some minerals have a metallic luster which
means they shine like metal. Others might look dull (non-metallic). Gold is metallic and
quartz is non-metallic.
Cleavage is a property of some minerals that causes them to break apart along flat
surfaces.
Color can also help identify a mineral. A mineral leaves a color when it is scratched
against a special plate or tile. The color it leaves is called its streak.
Earth is made up of rock. The top part of Earth’s land is mostly covered with a material
called soil. Soil is filled with the remains of dead plants and animals. Over time, these
remains break down into small bits called humus. Sand, silt, and clay are three kinds of
soil. The difference among them is the size of the particles in the soil. Sand has the
largest particles. The mineral quartz is found in sand. Sand that makes up beaches might
have tiny bits of broken shells in it. The large spaces of sand allow water to pass through
it easily. Clay has the smallest particles. If you squeeze it, it will feel slick and smooth.
The small spaces between the particles make it hard for water to pass through. The
particles in silt are larger than clay, but smaller than sand. Silt feels smooth when you
touch it. Silt holds little water, Rivers carry silt. When water floods the land, silt is left
behind. Many soils are made up of a mixture of sand, silt, slay, and other materials. This
type of soil is called loam. Loam is used as potting soil. It is very good soil for growing
plants.
The soil that covers the Earth is made up of different layers. The top layer of soil is
called topsoil and is the layer in which plants grow. This layer has lots of humus.
Animals, insects, and worms make their homes in this layer.
The next layer of soil is subsoil. Subsoil is made up of broken rocks. It has very little
humus. Roots from trees grow deep into this layer. The layer below subsoil is called
bedrock. This layer is made up of solid rock. Very little water gets through this layer.
Earth’s surface changes over time. Many of Earth’s landforms are slowly changed by
wind and weather. Weathering happens when large pieces of rock are broken into
smaller pieces. Rocks are carved into many different shapes by wind. The wind changes
the shape of the rocks and the way they look. Some shapes look like arches, while others
may look like tall columns. This kind of weathering takes place over hundreds or
thousands of years. Wind can change the land too. If soil becomes dry, wind can pick up
the soil and carry it to other places. This happens in soil that has few plants in it. The
roots of the plant hold the soil in place. Without roots, soil is easily blown away by the
wind. When rock or soil is moved from one place to another, it is called erosion.
Moving water can cause erosion too. Water that flows along a river or stream picks up
small particles of soil and silt. The moving water carries away the soil and silt. Soon, the
soil and silt are deposited, or dropped, where the river flows into the ocean. This forms a
fan-shaped delta. The movement of sand from one place to another often causes sand
dunes to form. Blowing winds cause the sand to pile up. Waves washing sand up onto
the beach also help build huge sand dunes.
Water can cause weathering too. Water can seep inside the cracks of rocks. When the
temperature gets very cold, the water freezes. When water freezes, it expands, or gets
larger. The ice stuck in the cracks of a rock pushes against the sides of the rock. This
causes the rock to split apart.
There are three major types of rock: igneous, metamorphic, and sedimentary. Igneous
rock is rock that forms when melted rock from inside Earth cools and hardens.
Metamorphic rock is rock that forms when two other kinds of rock are changed by heat
and pressure. Sedimentary rock is rock that forms when sediment, or small bits of rock
and stone, are cemented, or glued together. Each type of rock will eventually become
each of the other types by going through the rock cycle. The rock cycle is the continuous
series of changes that rocks undergo. A geologist is a scientist who studies rocks.
FOSSILS:
Extinct means that all of the members of a certain group of living things have died. No
people today have ever seen a living mammoth or dinosaur or trilobite. So how do
scientists know about these extinct animals? Scientists have found fossils. A fossil is the
mark or remains of a plant or animal that lived long ago. Many kinds of fossils are
found in rocks. Although it is rare, whole animals have been found in ice or in tar.
Insects have been found in hardened tree sap called amber.
Fossils of animal bones or teeth are often found in rocks. Millions of years ago, animal
remains became trapped between layers of sediment. Sediment is bits of rock, animal,
and plant materials. Over time, flowing water carries the sediments. They are deposited
in layers. The plant and animal deposits are squeezed between the layers. Heat and
pressure cause the sediments to turn into rock.
Fossils are not just bones and teeth. Many fossils show the traces, or impressions, of a
plant or animal. If you have every pressed your hand into a flat piece of clay, you have
made an impression.
Scientists who study fossils are called paleontologists.