U.S. Department of the Interior USGS Fact Sheet 2006-3044

U.S. Geological Survey

Aulocoseira distans

Aulocoseira

 granulata

Pinnularia sp.

2 mm

Diatoms that formed 15 million years ago in a lake 

near Carlin, Nevada, as seen through a high-powered 

microscope. The barrel-shaped and circular diatoms 

are members of the Aulocoseira genus, and the more 

elongate diatom is part of the Pinnularia genus. The 

2-millimeter (mm) (~0.08 inches) bar in the lower left 

indicates the very small size of the diatoms.

Freshwater Diatomite Deposits in the Western United States

Diatoms are a type of single-celled algae 

found in lakes, streams, and oceans. They 

are abundant today and were common in 

many ancient environments. Diatom skeletons are a delicate lattice of silica (SiO2

), 

and each species has a distinctive skeleton. 

Diatom “blooms” can occur several times 

a year, and large blooms can produce billions of individual diatoms. The diatoms 

then die and their skeletons settle to the 

bottom of the water body. 

In the geologic past, diatom accumulations in some lakes occurred over thousands of years, producing a thick layer 

called diatomite. Diatomite is a soft, very 

fi ne grained sedimentary rock with a 

chalk-like appearance. These silica-rich 

rocks are very fi nely porous and low in 

density—they will fl oat on water until 

they become saturated. 

Diatomite is used principally as a fi lter 

aid (for example, in the clarifi cation of wine 

and beer) and also as a fi ller in a variety of 

products, insulation material, whitener in 

paint, abrasive in polishes, and silica additive 

in cement and various other compounds. It is 

also used as an absorbent for industrial spills 

and in pet litter. Because diatomite does not 

reshwater diatomite deposits in Fthe Western United States are 

found in lake beds that formed millions of years ago. These diatom-rich 

sediments are among the Nation’s 

largest commercial diatomite 

deposits. Each deposit contains 

billions of tiny diatom skeletons, 

which are widely used for fi ltration, absorption, and abrasives. 

New studies by the U.S. Geological 

Survey (USGS) are revealing how 

ancient lakes in the Western States 

produced such large numbers of 

diatoms. These fi ndings can be used 

by both land-use managers and 

mining companies to better evaluate 

diatomite resources in the region.

WASH

IDAHO

OREG

CALIF

UTAH

NEV

ARIZ

react with most chemicals, its usefulness is 

increased in many of these applications.

Diatomite is produced from large openpit mines. The commercial value and 

uses of diatomite depend on the purity 

and even the diatom species. In 2004, the 

United States accounted for approximately 30% of world diatomite production. 

This included 620,000 tons of diatomite 

valued at $177 million mined in California, Nevada, Oregon, and Washington. 

More than two-thirds of the production 

in these States came from deposits that 

formed in freshwater lakes. Other important diatomite deposits, such as those in 

southern California, formed in shallowmarine settings. 

Large deposits formed at the bottoms 

of many freshwater lakes in the Western 

United States during the Miocene and 

Pliocene epochs (24 to 2 million years 

ago). Many of these freshwater deposits are on public lands. To enhance the 

Nation’s understanding of industrial 

minerals and to aid in the resource evaluation of these deposits, U.S. Geological 

Survey (USGS) scientists are studying 

the reasons why many ancient lakes produced such large diatomite deposits and 

why others did not. 

How Did Diatomite Deposits Form?

Freshwater diatomites in the Western 

States formed over thousands to millions of 

years. The Miocene climate in the region was 

fairly mild and wet, but it gradually became 

drier in the late Miocene. Rain fell in the 

Locations of diatomite deposits and 

Miocene and Pliocene sedimentary 

rocks in the Western United States. 

These deposits and sedimentary 

rocks formed in a variety of geologic 

environments during the past 20 

million years. Most formed in lakes, 

but some sedimentary rocks and 

diatomite deposits along coastal 

southwestern California are related 

to shallow-marine environments.

Diatomite deposits

Miocene/Pliocene

sedimentary rocks

200 MILES

200 KILOMETERS

0

0

2006
Printed on recycled paper

Nutrient, SiO2

 inflow

Nutrient, Si

 inflow Lake surface

Rain in 

mountains

Sun provides light

for diatom growth

Shallow-water diatomite Deep-water diatomite

Volcanic ash cloud

Ash fall (SiO2) from eruption

This diagram shows the setting of diatomite deposits 

in a lake and some of the processes that aided their 

formation. Rainfall produces streams that carry silica 

(SiO2 ) and other nutrients, such as phosphorus (P), 

into the lake from nearby highlands. Silica also can 

enter the lake in air-fall ash that was erupted from volcanoes and deposited on the lake’s surface. Sunlight 

provides light for photosynthesis, which enables the 

diatoms to grow and bloom. After the diatoms bloom 

and die each year, their silica skeletons settle to the 

bottom of the lake and form a thin sedimentary layer. 

Over thousands of years, these layers accumulate to 

form a diatomite deposit. Diatomite deposits can form 

both in the shallow (littoral) and deeper (benthic) parts 

of the lake. Different species of diatoms prefer each 

environment, and the resulting diatomite deposits in 

the two locations can differ as a result.

FORMATION OF A FRESHWATER DIATOMITE DEPOSIT

mountains, and streams carried the water 

down into lowlands, some of which had 

natural dams and thus contained lakes. The 

streams also carried important nutrients, such 

as phosphorus, from the hills into the lakes. 

Miocene and Pliocene lakes in the Western States formed in a variety of ways. 

Very commonly, eruption of lava fl ows 

dammed streams and produced lakes. This 

happened in east-central Washington, 

where extensive fl ows of the middle Miocene Columbia River Basalt were erupted 

over a span of several millions years, producing lakes behind the volcanic dams. 

Other lakes in the Western States formed 

when fault movements blocked stream 

fl ow. The various lakes were present for 

a few thousand to several million years 

before the dams failed or were breached, 

the basin was fi lled by additional volcanic 

fl ows, or the lake simply dried up.

Diatoms thrive and bloom in many different types of water and at different water 

depths. Diatoms in these ancient freshwater lakes bloomed both near shorelines and 

in the middle of the lakes, many of which 

were neither acidic nor alkaline, very 

much like tap water. Some lakes in the 

region were shallow and alkaline and did 

not produce signifi cant diatomite deposits, 

although zeolites, another important group 

of industrial minerals, formed in the sediments at the bottoms of these alkaline lakes.

Diatom activity relies on an infl ux of 

silica and nutrients. Silica is needed to form 

their skeletons, and it can come from several sources. Volcanic ash, produced from 

nearby and distant eruptions, lands on and 

settles through a lake and can react with 

the water, releasing silica. Dissolved silica 

also is washed into lakes by streams, either 

from weathering rocks or breakdown of 

silica-rich grasses in stream headwaters. All 

of these sources contributed needed silica 

to Miocene and Pliocene lakes in the Western States. However, ash from hundreds of 

eruptions spaced closely in time landed on 

some lakes, thereby limiting the time needed to form a thick diatomite deposit. 

On the basis of studies of modern diatom growth, the most important nutrient 

is phosphorus (P). Most P is derived from 

weathered rocks in nearby highlands and 

is carried into a lake by streams. Deposition of P-rich ash also can contribute P to 

a lake. Current research for this project 

indicates a strong correlation between Prich basaltic to andesitic volcanic rocks in 

the nearby highlands and the presence of a 

large diatomite deposit in the adjacent lake. 

Other P-rich rocks, such as shale and some 

limestone, also may have contributed P to 

some lakes. Lakes that did not have P-rich 

rocks in the surrounding highlands generally did not produce diatomite deposits.

Resource Summary

The formation of a diatomite deposit in a 

freshwater lake requires that many processes 

take place in and adjacent to the lake. The absence of one or more of these processes can 

limit or prevent the growth of diatoms and 

thus the formation of a thick diatomite deposit. Indeed, far more lacustrine sediments in 

the Western States do not contain diatomite 

deposits, or contain only small deposits, than 

contain deposits of commercial value.

Identifying the processes that formed 

economically large diatomite deposits is an 

important goal of the Sources of Industrial 

Minerals in the Western States Project. These 

results can be used by the diatomite industry 

to search for and evaluate new diatomite 

deposits, many of which may be present on 

public lands, and by land-use managers to 

plan for diatomite-mining-related activities. 

The results also further knowledge about ancient climate (“paleoclimate”) and landscape 

evolution in the Western States and have 

important applications to studies of ground 

water and of other types of mineral deposits.

For further information contact:

Alan R. Wallace

U.S. Geological Survey, MS 176

University of Nevada, Reno

Reno, NV 89557-0047

(775) 784-5789

http://minerals.usgs.gov/west/

This Fact Sheet and any updates to it are available 

online at

http://pubs.usgs.gov/fs/2006/3044

Alan R. Wallace, David G. Frank, and Alan Founie

Edited by James W. Hendley II

Graphic design by Judy Weathers

The Clark Station diatomite mine east of Reno, 

Nevada. The white areas are a 200-foot-thick pure 

diatomite that formed in a Miocene lake about 10 

million years ago. Gray bands through the diatomite 

are beds of ash that formed when ash from volcanic 

eruptions was deposited onto the lake’s surface and 

settled to the bottom. The lake beds originally were 

horizontal, but later fault movement tilted the beds.