For nearly 4,600 million years, our
planet has been keeping a record of events. The geological time scale is
the history of the Earth written in the rocks of the Earth’s crust. By
studying rocks and fossils, scientists have been able to determine that
dinosaurs were numerous at one time, that Iowa was once located on the Equator,
and that swampy forests grew in Antarctica. Evidence of the Earth’s
history is all around us in the rocks.
Rocks Made By Fire
Igneous rocks are formed by the
cooling of molten rock. Igneous rocks come from deep inside the Earth’s
mantle, between 100 and 200 km down. This molten material is called
magma. When it comes to the surface through a volcanic opening, it is
called lava. Very fluid lava may spread over hundreds of square kilometers.
Stiffer lava may build steep-sided volcanoes. Eruptions may be explosive
and cause a great deal of damage, or they may be quite gentle. Igneous
rocks are exceedingly hard and difficult to erode. Granite, obsidian, and
basalt are all examples of igneous rocks.
Rocks Made Underwater
Most of the Earth’s surface is
covered with loose material called sediment. Sediment may be the soil in
your garden, the sand and pebbles on the beach, or the mud in rivers.
Geological processes turn this sediment into rocks called sedimentary
rocks. Over many years, layers of this sediment are laid down. Because of
the pressure of the top layers, they eventually turn into rock.
Sedimentary rocks show distinctive patterns as each of the sediments has a
Sedimentary rocks were often formed
under shallow seas. As a consequence, many of these rocks contain small
animal bodies which have been fossilized.
Examples of sedimentary rocks
include sandstone, chalk, and limestone.
New Rocks From Old
Metamorphic rocks are
sedimentary. Igneous rocks have been changed by high pressure and high
temperatures. When these rocks were heated and squeezed inside the
Earth’s crust, they grew new minerals inside the rocks.
Examples of metamorphic rocks include
marble, shale, and quartzite.
Mineral or Rock - What Is The
Minerals are the building blocks
from which rock is made. Some minerals such as gold, copper, or sulfur
consist entirely of one chemical element. Others minerals are a mixture
of elements. The important property of minerals is that their composition
is always the same.
Rocks, on the other hand, are nearly
always a mixture of minerals. Because they are formed in different ways, their
Examples of minerals are quartz,
agate, amethyst, pyrite, and malachite.
Crystals and Gems
Minerals are made from atoms of
different elements. These atoms are packed together in simple pattern.
The pattern is always the same in any particular mineral. It is these
patterns that give mineral crystals their characteristic shape. Minerals
only form crystals when they have room to grow freely. That is why the
best crystals grow in geodes where the space in the rock is caused by a bubble
To be classified as a gemstone, a
crystal must be rare, beautiful, and very hard. Diamonds, emeralds, and
rubies are all gemstones.
Every animal and plant that dies
could be fossilized, but it has to die in the right place. If its body is
eaten, rots away, or is broken up, it probably will never turn into a
fossil. But if it is quickly covered by mud and sand, the chance of
becoming a fossil increases.
The most likely place to become
fossilized is in the sea where sediments are constantly being deposited. Many
of the fossils we find today have come from ancient sea beds. Land animal
fossils are less likely to be found.
A creature is much more likely to
become fossilized if it has some hard parts, such as bones and teeth.
These hard parts survive being knocked or jolted over time as the sediment
covers them and they do not rot as quickly as softer muscles and tissue.
There are several different ways in which fossils are formed. Many are
turned to stone where the substance of which the bone or shell is made is
replaced by new minerals. These fossils may be brought to the surface by
erosion. They may be whole skeletons, casts, or imprints of the
animal. Fossil tracks, imprints of an animal’s footprint, have also been
found. Fossils may be found preserved in amber, natural tar pits, or
frozen in ice packs.
From Nature Boxes for Early
Childhood Educators, Debbi Williams, Story County Conservation Board
Story County Conservation
Linda R. F. Zaletel
56461 180th St.
Ames, IA 50010
go to “Conservation and Parks”