Inner Crater
Thickness
Lithofacies
Massive Deposits
0.2 – 0.5 m
MLT1
Massive
Lapilli Tuff
MLT2
Massive
Lapilli Tuff
1–4m
MLT3
Massive
Lapilli Tuff
5 – 30 cm
MPLT4-J
MassivePlanar Lapilli
Tuff - Juvenile
dominated
1 – 4 cm
Description- Section where found
This lithofacies is orange-tan in color, massive, and has normal grading. The beds tend to pinch and swale with
the cross-stratified lithofacies (XT1 and XT2). The base is grain supported with ~35% matrix, and fines upward
to matrix supported with ~50% matrix. The grain size at the bottom ranges from coarse ash to fine block with an
average grain size of coarse lapilli, and fines upward to an average grain size of fine-medium lapilli at the top.
The fine blocks compose 10% of the deposit, and as the deposit fines upward compose ~2%. The grains are
composed of ~92% accidental and <7% juvenile (larger than fine ash). The accidental fragments are composed
of xenolith peridotite, accidental minette fragments, Chuska sandstone, and white mudstone. Phlogopite
phenocrysts are common in the matrix. Inner Crater West
This is an orange-tan, thick, massive, coarse-grained lithofacies that alternates vertically from matrix supported
and grain supported. In some locations it is thick (up to 4 m) and laterally continuous, in others it is laterally
discontinuous and thickens down dip (~3 m in center of crater, and thins to less than a meter towards the crest of
the crater walls). The lithofacies consists of 30-40%, sometimes up to 70% fine ash matrix, ~10% angular
accidental minette, 15-30% juvenile pumicious clasts (variable throughout exposures), ~15% white mudstone,
~5-10% juvenile minette (variable throughout exposures), and <5% red and white sandstone and mantle
xenoliths. Inner Crater East (south of road); Inner Crater West
This lithofacies consists of an orange-tan, massive, matrix supported lapilli tuff. Diffuse planar stratification is
present, and very rarely some diffuse cross-stratification. The grain size alternates from beds with an average
grain size of coarse ash to fine lapilli, and beds with an average grain size of fine-medium lapilli. Rare beds have
an average grain size of coarse lapilli. Occasional fine to medium blocks are present, but have no associated
bomb sag underneath. Interbeds of fine-grained, cross-stratified ash are common. The matrix composes ~4070% (varies from bed to bed), 30-50% accidental grains, and ~10% juvenile scoriaceous grains (larger than
medium ash in size). The accidental grains consist of white mudstone, white sandstone, red sandstone, peridotite
xenoliths, and accidental minette clasts. No juvenile pumice was found in these deposits. Inner Crater East
(north of road)
Dark gray, alternating beds of coarse ash, fine lapilli, and medium lapilli. Grains within the individual layers are
well sorted but matrix supported. Close up the individual, alternating layers appear to be mostly internally
massive with no grading, but the deposit overall has planar stratification. The base of this unit has a
concentration of juvenile minette lapilli cinders to fine blocks (~80% juvenile, blocks composed ~10% of area)
that are elongated and appear to have welded together (fluidal when emplaced). They create a dense first meter
of glassy partially welded spatter. The rest of the deposit within this first meter are composed of ~10% matrix,
~4% accidental minette, <5% juvenile pumicious clasts, and <5% red and white sandstone and white mudstone.
Inner Crater East (south of road)
1–4m
This is a massive lithofacies with normal grading and no internal stratification. The base of the lithofacies tends
MTB1
Massive Tuff
to be grain supported with ~35% matrix, and fine upward to matrix supported with ~50% matrix. The grain size
Breccia
at the bottom ranges from coarse ash to fine block with an average grain size of coarse lapilli, and fines upward
to an average grain size of fine-medium lapilli at the top. The fine blocks make up 10-12% of the outcrop, and as
the deposit fines upward compose ~2% of the outcrop (largest block is 35 cm across). The uppermost part of the
lithofacies shows some faint planar stratification. The blocks consist mainly of angular accidental minette
fragments. The rest of the grains are dominantly a mixture of accidental fragments, white mudstone, white
sandstone, red sandstone, peridotite xenoliths, and accidental minette clasts. ~5% of the grains are juvenile
cinder grains (larger than medium ash in size). Inner Crater East (south of road)
2–6m
This is a gray, massive, poorly sorted lithofacies with mega-xenoliths of Chuska sandstone ranging from three to
MTB2
Massive Tuff
seven meters across. The contact with mega-xenoliths and pyroclastic deposits are highly irregular, and the
Breccia
sandstone edges are baked. The lithofacies is mostly massive and matrix supported, but some very crude planar
stratification exists due to coarse grain alignment. The deposit consists of ~50-60% matrix, and 40-50% grains
of medium ash to fine blocks. The average grain size is medium lapilli. Overall the grains are angular to
subangular, but some appear are more abraded than others. Lenticular interbeds are common and consist of
~40% matrix and 60% grains of coarse ash-fine lapilli. These beds often thin towards the outer crater, and
thicken towards the inner crater. The grains are composed of ~30% accidental minette, ~5% juvenile pumicious
clasts, ~6% red and white sandstones, ~2%white mudstone, ~2% mantle xenoliths, and ~10% juvenile minette.
Inner Crater East (south of road)
1–4m
This is a thick, massive, poorly sorted tuff breccia with no internal stratification. It consists of ~20% fine ash
MTB3-J
Massive Tuff
matrix, ~70-80% juvenile clasts, and <10% accidental clasts. The grains range from medium ash to coarse
Breccia blocks in size. The average grain size at the bottom of this unit is fine blocks, decreases to coarse lapilli-fine
Juvenile
blocks towards the middle, then again increases to fine-medium blocks towards the top. The base of the deposit
dominated
consists of ~80% blocks, the middles 50% blocks, and back towards 80% blocks towards the top. Some of the
juvenile grains appear to have been partially solidified before impact (angular and non-vesiculated), while the
majority have irregular shapes and sizes, are typically vesiculated, and show evidence of being fluidal upon
deposition due to their flattened, irregular nature. All accidental clasts have a thin glassy rind around them (cord
bombs). Inner Crater East (north of road)
Planar and Cross-stratified Deposits
0.5 – 1 m
This lithofacies consists of low-angle, symmetrical, wavy cross-strata. These cross strata commonly truncate
XT1
Crossunderlying strata and show both lee and stoss side accretion. The cross strata also show pinch and swale features
stratified Ash
Tuff
XT2
Crossstratified Ash
Tuff
PXT1
Planar-Crossstratified Ash
Tuff
PT1
Planar Ash
tuff
5 cm
PLT1-R
Planar Lapilli
Tuff- Reverse
grading
0.4 – 1.5 m
and have an average wavelengths between 3-5 m, and amplitudes between 0.5-1 m. The flow direction is coming
from the vent direction upward and out of the crater. The stoss side of the cross strata are commonly elongated
to around 3 m, and the lee sides are also elongated to around 2-3 m. The cross strata are composed mainly of fine
to medium ash with <10% grains of coarse ash. Massive lenticular lenses (3 – 10 cm thick) are common and
consist of coarse ash and fine lapilli grains. Inner Crater West
This lithofacies consists of thin beds of orange-tan, cross-stratified, matrix supported, fine-grained ash. These
beds are typically only interbeds found within the massive deposits of MLT1. Inner Crater Southwest
3 – 25 cm
This lithofacies alternates from faintly planar and cross stratified. It is composed of nearly 100% fine-medium
grained ash, with occasional floating coarse ash grains. Fine quartz sand (Chuska sand?) is abundant in matrix.
Typically found in the stratigraphically higher inner crater sections. Inner Crater Southwest
0.03 – 1 m
This lithofacies is comprised of grain supported, coarse ash beds with occasional reverse grading. The
composition is ~15-20% sandy matrix (quartz sand) and fine to medium ash. Grains consist of ~65% juvenile
pumicious clasts which contain abundant phlogopite phenocrysts ~5% red sandstone, ~5% accidental minette
clasts, and 25% sand. Typically found in the stratigraphically higher inner crater sections. Inner Crater
Southwest
This lithofacies is reversely graded and consists of diffuse planar-stratified beds. The average grain size varies
from fine lapilli to coarse lapilli, and the deposits vary from matrix supported to grain supported. The clasts are
composed of ~20-40% fine to medium ash sandy matrix (variable), 10% red sandstone, ~10% white sandstone,
~15% juvenile pumicious clasts, ~40-45% angular minette clasts (not glassy), <3% mudstone, and <2% purple
sandstone. Typically found in the stratigraphically higher inner crater sections. Inner Crater Southwest
This is a fine-grained, matrix supported, fairly massive lithofacies with faint planar and cross stratification. It is
composed of ~70% fine-medium ash, sandy matrix, and ~30% grains of coarse ash and fine lapilli. Typically
found in the stratigraphically higher inner crater sections. Inner Crater Southwest
0.25 – 1 m
XLT1
Crossstratified
Lapilli Tuff
Scoriaceous – Magmatic Deposits
2–3m
Large blocks and juvenile magma characterize this lithofacies. It begins with ~20-30% fine ash matrix, 70-80%
MAG1
Magmatic
juvenile fragments, and <10% accidental fragments (white mudstone, white sandstone, red sandstone, accidental
minette, and peridotite xenoliths). Several fine- to coarse blocks (up to 2 x 2 m) are present. The bases of these
blocks are broken and fractured, and a small sag structure is common. The blocks are surrounded by a glassy
juvenile rind and/or chunks of darker gray scoriaceous-pumicious juvenile material (cord bombs). It appears as if
part of they were partially molten when emplaced, while part was solidified (angular around some edges).
Occasionally the pyroclastic rock and juvenile fluidal-block size clasts appear to have intermixed. This
lithofacies pinches out towards the outside of the crater (north) and appears to thicken towards the inner crater.
Inner Crater East (north of road)